摘要:

JANUARY 1937 THE ANALYST Vol. 62 No. 730 PROCEEDINGS OF THE SOCIETY OF PUBLIC ANALYSTS AND OTHER ANALYTICAL CHEMISTS AN Ordinary Meeting of the Society was held on Wednesday December 2nd 1936, at the Chemical Society’s Rooms Burlington House the President Dr. G. Roche Lynch in the chair. Certificates were read in favour of :-Granville Hubert Clarke Leonard Cartlidge Dutton A.I.C. John Charles Giblin BSc. A.I.C. Ronald William Gillham Ph.C. Ronald Murray Hamilton Walter Thomas Lunt B.Sc. A.I.C., William Douglas McFarlane M.A. Ph.D. William Stewart Patterson Ph.D., M.Sc. F.I.C. Thomas Charles Williams BSc. A.I.C. George Henry Wray. The following were elected members of the Society :-Frederick Brown, Alan James Cavell A.R.C.S. D.I.C. A.I.C. James Pettigrew Ogilvie F.I.C., Richard Kenneth Sanders B.A.Alexander Martin Smith Ph.D. D.Sc. A.I.C., George Edward Speight A.Met. B.Sc. The following were elected Honorary Members of the Society :-Bernard Cracroft Aston F.I.C. and Robert Howson Pickard D.Sc. Ph.D. F.I.C. F.R.S. The following papers were read and discussed :-“ The Detection of Arachis Oil in Olive Oil,” by Norman Evers B.Sc. F.I.C.; “The Enzymes of Milk I Notes on Kay and Graham’s Phosphatase Test,” by E. B. Anderson M.Sc. F.I.C., 2. Herschdorfer Ph.D. and F. K. Neave; “The Determination of Cyanide in Aqueous Extracts of Road Tars,” by W. G. Moffitt Ph.D. A.I.C. and E. H. Williams B.Sc. A.I.C.; and “A Rapid Method for the Determination of Tri-ethanolamine,” by C. J. Eastland F.I.C. Norman Evers BSc. F.I.C. and T. F.West B.Sc. A.I.C. NORTH OF ENGLAND SECTION A MEETING of the Section was held in Manchester on December 12th. There was an attendance of twenty-nine; in the absence of the Chairman and Vice-chairman, Prof. W. H. Roberts was elected to the chair. The following paper was read and discussed :-(‘Sussex Ground Oats,” by F. Robertson Dodd F.I.C. and A. Pattinson Telford. A discussion took place on the desirability of changing the title of the parent Society. 2 OBITUARY SCOTTISH SECTION AN ordinary meeting of the Section was held in the Central ‘Hotel Glasgow on November 19th 1936. Professor G. G. Henderson gave a short informal address and the following papers were read and discussed:-“Note on the Estimation of Lead in Drinking Water,” by J. W. Hawley B.Sc. F.I.C. and W. Wilson F.I.C. ; and ‘‘ The Analysis of Peaty Waters,” by R. T. Thomson F.I.C

ISSN:0003-2654    

DOI:10.1039/AN9376200001

出版商:RSC    

年代:1937

数据来源: RSC

摘要:

2 OBITUARY Obituary JOHN MAY HERBERT MUNRO BY the passing of Dr. Munro on November 6th 1936 those who knew him have lost a great friend and the Society of Public Analysts one of its oldest members. John May Herbert Munro was born on December 23rd 1854 at Kingswood, near Bristol. He was educated first at a preparatory school in Painswick then at Taunton School and later attended the Trades School Bristol where he had his life-long friend Sir Ernest Cook as his fellow pupil. In those days it was the policy to encourage the study of science by offering Science Exhibitions for competition among the schools of the country. In the examination of 1872 there were only six Exhibitions available and although Munro did not secure one of them he was so nearly successful that he was offered a modified Exhibition which enabled him to enter the Royal College of Science, Dublin.After the usual three-year course he took the Associateship of the College and in 1876 returned as an assistant master to his old school in Bristol, and while there he took his D.Sc. at London University. He next competed for, and obtained an examinership in the Patent Office London where he remained until 1880 when he joined Messrs. Wrightson and Fream in founding the Agricul-tural College at Downton. I t was not until middle life that he turned his attention to medicine-in fact, he was still a professor at Downton while taking the preliminary medical course at Brist ol University. Dr. Munro who had been elected a Fellow of the Institute of Chemistry in 1888 was appointed Agricultural Analyst to the Wiltshire County Council in 1897 ; he was also Agricultural Analyst to the City of Bath and held both appointments until his death.From 1925 to 1928 he was Honorary Analyst to the Baths Committee at Bath. He published the results of a number of investigations on the biochemical aspects of medical problems (cf. Lancet December 3rd 1932 p 1211). Although he had a great gift of imparting knowledge and was a born teacher, yet he himself never ceased to be a student. He was quiet and reserved in his manner but when once drawn into conversation he held the interest of his listeners. His unfailing kindness and understanding endeared him to his patients, many of whom he kept as close friends until his death BOLTON AND WILLIAMS COLOUR MEASUREMENT OF OPAQUE SURFACES 3 In his younger days Dr. Munro was fond of golf and until the end he was a lover of the country; after tea from early spring to late autumn; he would drive (always taking friends) to see some interesting place such as Spye Park in the blue-bell season. Every house of note and beauty spot in Somerset and Wiltshire was familiar to him. He died in full harness just as he had wished. S. RANDS Deaths WITH deep regret we announce the death of Sir Herbert Jackson on December 10th and of Mr. P. A. Ellis Richards Past-President of the Society on December 22nd. Obituary notices will be published later

ISSN:0003-2654    

DOI:10.1039/AN9376200002

出版商:RSC    

年代:1937

数据来源: RSC

摘要:

OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix. about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order. It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary. Elsdon and Stubbs (Eoc.it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained. In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents.To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'. It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions.The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned.In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary. It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix.about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order. It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary.Elsdon and Stubbs (Eoc. it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained. In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents.To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'. It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions.The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned. In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary.It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix. about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order.It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary. Elsdon and Stubbs (Eoc. it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained.In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents. To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'.It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions. The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air.There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned. In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary.It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix. about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order. It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined.It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary. Elsdon and Stubbs (Eoc. it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained. In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents.To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'.It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions. The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place.The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned. In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary. It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix. about 1.5, 1.0,0.8, and 0.5" C.It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order. It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined.It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary. Elsdon and Stubbs (Eoc. it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained. In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents.To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'.It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions. The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place.The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned. In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary. It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix.about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order. It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary.Elsdon and Stubbs (Eoc. it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained. In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents.To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'. It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions. The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air.There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned.In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary. It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix. about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order.It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary.Elsdon and Stubbs (Eoc. it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained. In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents.To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'. It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions.The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned. In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary.It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix. about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order.It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary. Elsdon and Stubbs (Eoc. it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained.In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents. To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'. It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions. The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned. In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary. It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,

ISSN:0003-2654    

DOI:10.1039/AN9376200003

出版商:RSC    

年代:1937

数据来源: RSC

摘要:

MORSE THE TURBIDIMETRIC TITRATION OF GELATIN SOLUTIONS 11 The Turbidimetric Titration of Gelatin Solutions BY J. F. MORSE (Read at the Meeting November 4th 1936) A REVIEW of the literature1 concerning the group of degradation products termed gelatin and its properties reveals the great complexity of this substance and it is not surprising therefore that conflicting statements as to its physical properties exist. Gelatin may be prepared by hydrolysis of collagen contained in the skin, sinews etc. of animals or of ossein the protein of bones. The user of gelatin is chiefly concerned with two of its physical properties viz. gel strength and clarity. It would appear that the clarity of a gelatin solution is influenced by:-The present communication deals with the latter.The ash-content of the gelatin. While in technical practice the nature and amount of ash present in a gelatin may cause turbidity due to the formation of calcium salts in laboratory technique turbidity is not usually associated with the ash-content. A r k 2 and Dh& and Gorgolewski3 found that gelatin solutions turbid at a temperature below 30” C. became clear when the temperature was raised above 30” C. this effect increasing with decreas-ing ash-content. Such turbidity however may be due to the colloidal behaviour of the gelatin itself or it may be connected with change in pH value as influenced by the inorganic ions present. The $H of the solution the maximum turbidity being at the iso-electric point. Iso-electric values ranging from pH 4.0 to 8.0 have been published and serve to show the variable and complex nature of gelatin.Various workers have suggested that gelatin consists of fractions differing in properties. Thus Nagorny4 obtained two fractions by means of ammonium sulphate Northrop and Kunitz5 isolated two fractions by alcohol precipitation at different temperatures but Sheppard Hudson and Houck6 are of the opinion that the “insoluble” fraction is identical with the heat-coagulable proteins derived from the skin which they isolated in small amounts-less than 1 per cent. Straup’ flocculated material from a gelatin solution over a temperature range and suggests that the phenomenon is due to fractions. Duclaux and Hiratas regard gelatin as existing in two forms, one of which (less than 2 per cent.) scatters white light strongly with a maximum at pH 5.Provided the pre-treatment of the raw material containing collagen or ossein is carried out in the same way there appears to be no appreciable difference in the gelatin obtained from the two types of material. Acid and alkali pre-treatment, however certainly produce gelatins of differing properties and Briefers has shown that the difference is so characteristic as to be uninfluenced by subsequent removal of mineral matter 12 MORSE THE TURBIDIMETRIC TITRATION OF GELATIN SOLUTIONS Kraemer and Dexter,lo Briefer9 and Sheppard and Houckll have shown that gelatins from acid-treated raw material have a maximum zone of turbidity in the region of pH 7 to 8 whilst lime-treated stock gave values in the neighbourhood of pH 4.7 to 5.0.Sheppard and Houck however found their material to give maximum alcohol precipitation at pH 4.9 indicating that the substance or sub-stances giving turbidity at pH 7 to 8 did not constitute the major portion of the sample. The fact that acid-treated raw material yields a gelatin differing in properties from that obtained from lime-treated stock and that subsequent purification does not alter the properties conferred by these treatments may in part account for the different results recorded by various workers using gelatins from raw material whose origin and pre-treatment were unknown however thorough the laboratory purification may have been. Regarding turbidity simply as a physico-chemical property and not necessarily as an indication of iso-electric point agreement exists that for acid-treated raw material it is at a maximum in the neighbourhood of pH 8 and that lime-treated stock yields a gelatin with a maximum turbidity about pH 5.Various methods for comparing the turbidities of gelatin solutions have been employed e.g. visual inspection intensity of scattered light at right angles to the Tyndall beam a photo-electric cell being used and nephelometric determinations. Such measurements require the preparation of a number of samples of differing pH values and strictly comparable thermal histories. Moreover the preparation of a sufficiently large number of samples accurately to cover the pH range 3 to 9 is a laborious task. The method here described is rapid and the pH of maximum turbidity of a gelatin as determined by it is unaltered by the differences in technique so far explored.Fig. 1 shows the apparatus employed to determine the “turbidity” curve of gelatin (and other proteins). Fittings from a Leitz Large “Uma” Photomicro-graphic apparatus formed the basis of the arrangement. The optical bench was the auxiliary fitting supplied on which to clamp accessory apparatus. The source of illumination A was a 6-volt 5-amp. projection lamp run from a 10-volt car accumulator. Under these conditions 6 amps. were passed through the lamp and absolutely steady illumination was obtained during the course of a titration. Fluctuations in current of the mains supply rendered it impossible to run the lamp by means of resistances. The aspherical focussing condenser B and iris diaphragm, C served to obtain the optimum condition of the light beam.Behind the con-denser a filter-carrier and its accessories were constructed and fitted (D). The carrier received two glass strips containing Wratten filters in the form of gelatin films 1-inch square. A clicking arrangement allowed the filters to be adjusted easily and correctly. The titration was carried out in the glass cell E having parallel sides and a curved bottom. The capacity was 450ml. and the stratum 6cm. The light beam after leaving the cell was received by the Weston Photronic photo-electric cell F. A “2” type glass stirrer G gave rapid and effective mixing without aeration or undue surface agitation. Additions of acid and alkali were made by means of the burette H MORSE THE TURBIDIMETRIC TITRATION OF GELATIN SOLUTIONS 13 Measurement of pH was by means of an antimony electrode in conjunction with a saturated KC1 calomel cell such an arrangement h‘aving proved most satisfactory in these laboratories over a period of eight years.Other electrode systems could of course be used. A Cambridge slide-wire potentiometer together with a Weston normal cell and a 2-volt accumulator were used for the deter-minations. A Cambridge micro-ammeter reading to 6 micro-amperes was employed as a null-point galvanometer and after the pH value had been deter-mined this meter was switched over to the photo-electric cell and the current output read directly to 0-05 and by estimation to 0.01 micro-ampere. FIG. 1 The stirrer calomel and antimony electrodes and burette were so arranged that no interruption was offered to the beam of light and screens (not shown in Fig.1) were fitted so that no extraneous light reached the photo-electric cell. The method adopted was as follows:-Six g. of gelatin were soaked in 100ml. of distilled water until thoroughly hydrated and the mixture was then warmed to about 40” C. care being taken to avoid local over-heating. When solution had been effected 200ml. of cold water were added giving 300 ml. of 2 per cent. gelatin solution. The light with the selected filter was adjusted to give a convenient deflection of the meter and the highest reading recorded during an estimation was regarded as representing no light scattered. The following factors which might influence the results were then investi-gated :-(a) Wave-length of light employed; (b) temperature of the solution; (c) type of acid employed for adjustment; (d) “direction” of titration.(a) WAVE-LENGTH OF LIGHT EMPLoYED.-The following Wratten filters were used:-49 (C4) 75,62,73,72,70. The predominant wave-lengths transmitted (as stated by the makers) were 460mp 490mp 530mp 570mp 610mp and 700mp 14 MORSE THE TURBIDIMETRIC TITRATION OF GELATIN SOLUTIONS respectively. An Ilford infra-red filter transmitting at about SOOmp was also employed. Filters transmitting infra-red light (apart from the Ilford filter) viz. 73 72 and 70 were used in conjunction with an aqueous copper sulphate filter inserted between the filter and containing cell (cf. Bolton and Williams ANALYST, 1935 60 449).fiH FIG. 2. EFFECTS OF TEMPERATURE ON TURBIDITY A. 17.3 'C b. 30 'C PH FIG. 3. For these and the following experiments a high-grade commercial powdered gelatin was used. Fig. 2 illustrates the results titration being from @H 3 to 9. It will be noted that the light scattered increases as the wave-length decreases the difference between the infra-red and blue light being very noticeable. This increased scattering with short wave-lengths was to be expected and Rawling12 has published photographs of the phenomenon. I t will be seen that the pH of maximum turbidity is independent of the wave-length of light employed. The Wratten No. 49 (C4) blue filter was selected for subsequent work as the most accurate results were to be expected with light of short wave-length.~ E C T OF ACID EMPLOYED ON ~ ~ I D I T Y Cuqvc :I A. WIP~OCWLOU4 9. ClTPlC ACID A C . LACTIC ACID 1-I 4C1. E ~ C T or ' DIRECTION OF TTTPATIW' ON TUp WmTY CUcV€ *I as. a 7 m FIG. 6 MORSE THE TURBIDIMETRIC TITRATION OF GELATIN SOLUTIONS 15 (b) THE EFFECT OF TEMPERATURE.-Kraemer and DexteP investigated the effect of temperature on gelatin solutions. Fig. 3 shows the effect on the “turbidity curve.” Although the general contour is somewhat modified the point of maximum turbidity is unchanged. (c) THE TYPE OF ACID EMPLOYED.-In all work hydrochloric acid and caustic soda were employed to adjust the reaction. Fig. 4 shows the effects of hydrochloric citric and lactic acids the last two being tried because they are frequently employed with gelatin in various food products.*I #H FIG. 7. (d) THE DIRECTION OF TITRATIoN.-Fig. 5 compares the results obtained by the three methods available i.e. from PH 3 to 9 from PH 9 to 3 and from the PH of the freshly prepared gelatin solution in each direction. From the foregoing results it will be seen that in every instance the PH of maximum turbidity is the same. Minor differences in the form of the curves exist, particularly noticeable being the sharp maximum turbidity when the direction of titration is j5H 9 to 3. If maximum accuracy be desired the titration should be carried out with hydrochloric acid from pH 9 to 3 with the use of blue light and at a low temperature (but not low enough to cause gelling of the solution). Very dark solutions may necessitate the use of a yellow or orange filter.As far as possible conditions similar to those to be encountered by the gelatin should be arranged and what-ever method be employed the point of maxim tumurbidity should be the same 16 MORSE THE TURBIDIMETRIC TITRATION OF GELATIN SOLUTIONS EXAMINATION OF COMMERCIAL GELATINS .-A series of commercial gelatins was examined by the technique outlined above. Gelatins from lime-treated stock gave points of maximum turbidity ranging from pH 4.2 to 5.1. Good-grade gelatins were usually at their maximum turbidity in the neighbourhood of 4.9 and there appeared to be little connection between the grade of gelatin and the amount of light scattered. Fig. 6 shows the curves obtained with commercial gelatinsof varying grades. A Scotch glue solution 0.1 per cent.concentration had maximum TUI(BID1TY CUcVfS OF ACID rP,EAl‘ED LIMP I‘qBATED GELATIYS 6 -EQUAL PART8 OF EACH. Y -PH 4 . s 7 b 0 FIG. 8. turbidity at pH 4.65 and scattered 95 per cent. of the original incident light. Three samples of gelatin from acid-treated stock gave the curves illustrated in Fig. 7. As noted by other worker~,~,~OJ1 a sharp maximum was not obtained but a zone between pH 7 to 9 with a maximum at $H 8. A sample of each type of gelatin was dissolved in water and the turbidity curve of the mixture was obtained. Fig. 8 shows the curves of the two gelatins and of the mixture. It will be noted that two turbidity maxima are not apparent, but a new one between those of the two gelatins occurs. This phenomenon has been observed by Kraemer,lP and is receiving detailed investigation in this laboratory.Preliminary work with proteins other than gelatin viz. peptone white-of-egg, and isinglass indicates that the method has wide application. SUMMARY.-(~) A method for the rapid determination of the “turbidity” curve of a gelatin solution over the pH range 3 to 9 has been described. (2) With commercial gelatins prepared from lime- and acid-treated stocks, the generally accepted @H 4.9 and pH 7 to 8 as regions of maximum turbidity, respectively were obtained. Two distinct maxima were not obtained with mixtures of the two types of gelatin. (3) The application of the method to other water-soluble proteins has been indicated. In conclusion grateful acknowledgment is made to Dr.William Clayton for suggesting this method and for his advice and criticism during the investigation. REFERENCES 1. 2. 3. 4. W. Clayton Colloid Aspects of Food Chemistry apzd Technology London 1932. L. Arisz PYOC. Acad. Sci. Amsterdam 1913 26 331; Kolloid Chem. Beihefte 1915 7 22. C. DhCr6 and M. Gorgolewski J . Physiol. Path. Ge‘n. 1910 12 645; 1911 13 157 167. A. Nagorny Kolloid-Z. 1927 41 123 MORSE THE TURBIDIMETRIC TITRATION OF GELATIN SOLUTIONS 17 5 . 6. 7. 8. 9. 10. 11. 12. 13. 14. J . H. Northrop and M. Kunitz J. Gen. Physiol. 1926,10 161; J . Phys. Chem. 1931,35, S. E. Sheppard J. H. Hudson and R. C. Houck J . Amer. Chem. Soc. 1931 53 760. D. Straup J. Gen. Physiol. 1931 14 643. J . Duclaux and F. Hirata J. Chirn.phys. 1933 30 213. M. Briefer Ind. Eng. Chem. 1929 21 270. E. 0. Kraemer and S. T. Dexter J. Phys. Chem. 1927 31 764; E. 0. Kraemer Colloid Symposium 1926 Monograph 4 107. S. E. Sheppard and R. C. Houck J. Phys. Chem. 1930 34 2187. S. Rawling Infra Red Photography London 1933 p. 40. E. 0. Kraemer and S. T. Dexter J. Phys. Chem. 1927 31 764. E. 0. Kraemer Colloid Symfiosdum 1926 Monograph 4 102. 162. RESEARCH LABORATORY 36 CRIMSCOTT STREET BERMONDSEY LONDON S.E.l DISCUSSION Mr. D. M. FREELAND asked the author what point of the maximum turbidity was in his opinion the iso-electric point. A great deal of work had been published by the Adhesives Research Committee of the Department of Scientific and Industrial Research (1922 1926 1932) who endeavoured to prepare a very pure gelatin by dialysis and other means.They found an iso-electric point of $H 4.7. He wondered whether in this connection the author had dialysed any gelatin, and whether the mineral matter present affected the turbidity to any large extent. Mr. J. F. MORSE replied that in his experience the $H of maximum turbidity depended on the degree of hydrolysis and in laboratory experiments values from $H 4-2 to 5.2 had been obtained. A value of pH 5.7 seemed intermediate between the products of acid and alkali hydrolysis. Maximum turbidity at pH 8 to 9 did not necessarily indicate the iso-electric point although it might possibly be that of a fraction of the sample. He had not dialysed gelatin but the literature was full of such processes and the different iso-electric points found for the purified material were doubtless due to the various gelatins used.Dr. W. CLAYTON said that this work was commenced after the reading of various papers which included findings for the iso-electric point of gelatin ranging from pH 4 to pH 8 and the opinion moreover that gelatin possessed two iso-electric points. Such contradictory data warranted fuller inquiry. Within the last eighteen months there had appeared a number of papers particularly from Russia which seemed to point to the presence in gelatin of at least five fractions, characterised so far simply as different micelles of gelatin. The relation of maximum turbidity and the iso-electric point was an important matter for in-vestigation especially concerning their coincidence or not.He concluded by saying that gelatin was the most nondescript material that the food chemist was called upon to examine from a physico-chemical standpoint. The difference that had been shown between lime-treated gelatin and acid-treated gelatin in respect of clarity would be of importance in the use of gelatin in foods where clarity was required. He fully agreed that commercial gelatin was a heterogeneous material and that there were always degradation products present to some extent a small amount of such appreciably reducing the gold number of the gelatin which was not however further reduced by larger amounts of the degradation products. Rendering the gelatin “ash-free” did not affect the gold number but did greatly affect solubility and the character of the gel.He found the results and conclusions reported in this paper to be particularly interesting in that they were in accordance with Loeb’s theory of which he also was a supporter. Mr. T. MCLACHLAN pointed out that the material employed in the manufacture of gelatin was not a pure product and that treatment with acid or alkali gave hydrolysis products. According to the type of hydrolysis used so one obtained different end-products. Dr. E.-B. HUGHES remarked that he was very interested in this paper 18 HAMENCE A SCHEME FOR THE SEPARATION AND THE Professor HUGGETT said that he was extremely interested in the physiological aspect of this paper. Most of the speakers had emphasised the significance of the origin of the gelatin. It was essentially a heterogeneous product the properties of which depended on its original source. For all practical purposes it contained, from a nutritional point of view little or nothing of importance. It contained none of the essential acids and no vitamins and was characterised as “a second-rate protein.” Its physico-chemical properties were variable in the sense that they depended upon the fact that the gelatin was extracted from different types of animal material. Dr. J. H. HAMENCE asked what type of curve was obtained when the gelatin solution had in it a small amount of insoluble mineral matter in suspension and whether such an impurity had any influence on the maximum value given by that particular gelatin. Mr. MORSE replied that so far as he could see it would have no effect whatever

ISSN:0003-2654    

DOI:10.1039/AN9376200011

出版商:RSC    

年代:1937

数据来源: RSC

摘要:

18 HAMENCE A SCHEME FOR THE SEPARATION AND THE A Scheme for the Separation and the Determination of Metallic Impurities in Foodstuffs BY J. HUBERT HAMENCE M.Sc. PH.D. F.I.C. WITI- the increasing attention now given to the presence of traces of metallic impu.-ties such as tin and lead etc. in foodstuffs it was thought desirable to produce a scheme whereby all the heavy metals which commonly occur as impurities might be determined in the same solution. The general tendency to-day is to test separate portions of the sample each for a different impurity. This procedure is a perfectly good one when the substance is readily obtained in solution and a colorimetric test can be applied directly to that solution without any previous separations. In the majority of instances, however this is not the case and when complex organic bodies such as foodstuffs, are being dealt with the conversion of three or four portions of the same sample into a condition suitable for applying the colorimetric tests involves much time and labour.Many of the tests for traces of heavy metals necessitate the absence of inter-fering bodies so that in these instances some form of separation becomes necessary before the final test can be applied. ANALYTICAL DETAILS PREPARATION OF THE SOLUTION AND SEPARATION STAGE 1.-In dealing with organic compounds 20 g. are destroyed by the wet oxidation process and the final colourless sulphuric acid solution is diluted with 50 ml. of water and evaporated once more to the fuming stage or if a portion of this solution is to be employed for the determination of arsenic 25 ml.of saturated ammonium oxalate solution are added instead of the water DETERMINATION O F METALLIC IMPURITIES I N FOODSTUFFS 19 The sulphuric acid solution is again diluted with water and the contents of the Kjeldahl flask are filtered into a beaker any insoluble matter left on the filter being washed back into the flask and boiled with conc. hydrochloric acid. After dilution this solution is also filtered into the beaker and the flask is finally rinsed out with distilled water. In the resulting solution (which should now amount to about 50 ml.) is dissolved 2 g. of citric acid of analytical quality and 10 mg. of crystalline ferrous sulphate. If however ashing be preferred to wet oxidation 20 g. of the sample are incinerated at a low temperature in a silica basin.To the ash 5 ml. of conc. hydrochloric acid are added and the liquid is evaporated to dryness on a water-bath. The residue is dissolved in a mixture of 5 ml. of conc. hydrochloric acid and 20ml. of water and the solution is filtered. The citric acid and ferrous sulphate are added to this solution. The solution thus obtained is cooled and after the addition of three drops of universal indicator solution is treated with ammonia until a green colour is obtained indicating a fiH of about 8 the solution being cooled during this addition. Hydrogen sulphide is then passed through the solution for 20 minutes and the precipitate is filtered off on a small No. 2 Whatman paper. The precipitate is washed well with dilute ammonium sulphide solution.This solution is prepared by adding ammonia (sp.gr. 0.880) to water saturated with hydrogen sulphide until the liquid smells distinctly of ammonia. It is important that the precipitate should be washed free from ammonium citrate. Tin.-The tin can now be determined in the filtrate. One method of achieving this and the one that has been employed to obtain the results given at the end of this paper is to render the filtrate acid with hydrochloric acid after the addition of 2 ma. of yellow ammonium sulphide solution when the tin will be precipitated with the sulphur. After standing for some time the precipitate is filtered off washed with hydrogen sulphide water and burnt to tin oxide. Alternatively the filtrate and washings may be made up to 100 ml.and the tin in an aliquot part determined by Clark’s 4-methyl-1.2-dimercaptobenzene reagent1 in the following manner : To 10 ml. of the solution are added 2 drops of thioglycollic acid the solution is neutralised to litmus and 10 drops excess of conc. hydrochloric acid followed by 0.5 ml. of reagent,l are added. The red colour developed on warming is matched against standards containing known amounts of tin and the same quantities of reagents. SEPARATION STAGE 11.-The black sulphide precipitate is dissolved by gently warming the filter-paper containing the precipitate in a mixture of 20ml. of water and 2 ml. of conc. nitric acid. When the sulphides have completely dissolved the filter-paper is picked out with a glass rod and replaced in the funnel.The solution is filtered through the same filter which is subsequently washed with hot water. The filtrate and washing should amount to about 50 ml. To the filtrate is added 1 g. of ammonium sulphate of analytical quality and the solution is heated until near the boiling-point; the iron is precipitated by the addition of 15 ml. of 15 per cent. w/v ammonia solution and the heating is con-tinued until the ferric hydroxide has coagulated. It is important that the solutio 20 HAMENCE A SCHEME FOR THE SEPARATION AND THE should not be boiled for long during this precipitation because if most of the ammonia is boiled off material adsorption of the copper may result.2 The pre-cipitate is filtered off and washed with hot water and 2 per cent. ammonia solution, and the filtrate is reserved for the determination of zinc and copper.SEPARATION STAGE 111.-The ammonia precipitate from the Separation Stage I1 is dissolved by warming the filter-paper and precipitate with a mixture of 2 ml. of conc. nitric acid and 20 ml. of water; the solution is filtered through the original filter and the filter is washed with hot water. The filtrate is made up to 50ml. Bismzdh.-To 15 ml. of the filtrate are added 5 ml. of saturated ammonium thiocyanate solution and 5ml. of water and the solution is extracted with suc-cessive quantities of 10 ml. of ether until the last ethereal wash is colourless; four washes are usually necessary. To the aqueous solution 5ml. of the ammonium thiocyanate reagent are then added (for preparation of the reagent see end of paper).The bismuth if present may be determined by matching the yellow colour against standards pre-pared by adding known amounts of bismuth nitrate solution to 15 ml. of water and 15 ml. of reagent and extracting the mixture once with 10 ml. of ether. Lead.-If no bismuth is found the lead is determined in 25 ml. of the filtrate, after removing the iron by adding 5 ml. of saturated ammonium thiocyanate and extracting the ferric thiocyanate with a mixture of amyl alcohol and ether.3 If bismuth is present the lead is determined after the iron and bismuth have been extracted by the pyridine thiocyanate process.’ (See also the paper by Roche Lynch and his co-workers.6) SEPARATION STAGE 1V.-Co$$er.-The filtrate from the ammonia pre-cipitation of Separation Stage I1 is made up to l O O m l .and the copper in a portion of it is determined by means of the diethyl dithiocarbamate reagent.6 Zinc.-If copper is present it must be removed before testing for zinc; this is achieved as follows:-The remainder of the solution is evaporated to 20 ml. and then warmed with 5 ml. of 4 N hydrochloric acid to dissolve any traces of zinc that have become adsorbed by the glass during the evaporation. The solution is cooled 1 mg. of copper (as copper sulphate) is added and the whole of the copper is precipitated with hydrogen sulphide. The addition of copper enables traces of copper to be removed more effectively. The precipitated cupric sulphide is filtered off the precipitate is washed with hydrogen sulphide water and the filtrate is evaporated to 20ml.The zinc is determined with 8-hydroxyquinoline in the following manner:-To the concentrated filtrate are added 5 ml. of 20 per cent. ammonium acetate solution and the j5H of the solution is adjusted to about 7.5 by the addition of ammonia two drops of universal indicator being added for this purpose. Two ml. of a 2 per cent. solution of 8-hydroxyquinoline in N acetic acid are then added and the solution is boiled for 5 minutes in a covered beaker. The “oxine” precipitate which is obtained in a crystalline form is filtered off after standing for seveial minutes and washed with hot water. The filter is then washed twice with 3 N hydrochloric acid and twice with water the washings which should amount to 30 ml. being collected in a stoppered bottle.To this N/50 bromide and bromate solution (see end of paper) is added until a drop of the solution yields a A yellow colour indicates the presence of bismuth DETERMINATION OF METALLIC IMPURITIES I N FOODSTUFFS 21 blue reaction when tested with starch iodide paper 2 ml. excess being then added and the solution well shaken. After standing for several minutes in the dark potassium iodide solution is added and the excess of bromine is determined by titration with N/50 thiosulphate solution. One ml. of bromine solution absorbed zz 0.163 mg. of zinc. The scheme just described by which tin lead bismuth copper and zinc may be determined quantitatively all in the same solution is summarised below. GENERAL SCHEME OF THE SEPARATIONS 1. 2. Precipitate k- 1 4-methyl- 1.2-dimercaptobenzene reagent.This is dissolved in dilute nitric acid and the iron precipitated by an excess of ammonia in the presence of ammonium sulphate. The precipitate is washed with ammonia and hot water. Hydrogen sulphide is passed into the solution to which a trace of ferrous sulphate has been added and the pH is adjusted to 8; the precipitate is washed with dilute ammonium sulpliide solution. Fdtrate TIN 1s determined in this solution either by precipitation as sulphide effected by addition of ammonium sulphide followed by excess of hydrochloric acid ; or colorimetrically by 3 I \ v 3. Preczpztate \ \\ 3 This precipitate will contain the hydroxides of le&d and bismuth adsorbed on the ferric hydroxide. BISMUTH is tested for and deter-mined as the yellow thiocyanate and LEAD is determined after the removal of the iron and bismuth if any by the thiocyanate process.4. Filtrate COPPER is determined directly with diethyl dithiocarbamate in a portion of the filtrate. ZINC is determined in the remainder by 8-hydrosyquinoline after the removal of the copper if any by hydrogen sulphide precipi-tation in the acid solution. Part of this scheme-namely that part embodying the separation of lead, copper and zinc-has been adopted by the “Investigatory Panel on Methods of testing Gelatin ” of the British Standards Institution for the determination of heavy metals in gelatin. Bismuth although not a common impurity in foodstuffs has been included in this scheme owing to its significance in toxicological work in which it is sometimes necessary to determine traces of bismuth and also to determine traces of lead etc., in the presence of bismuth.EXPERIMENTAL SEPARATIONS.-The efficacy of the scheme was tested in the following manner :-A solution containing 0.10 g. of calcium carbonate 0.10 g. of sodium phosphate and 5ml. of conc. sulphuric acid in 50 ml. of water was prepared to simulate in some respects the solution that is obtained after the destruction of an organic material by wet oxidation. To this solution wer 22 HAMENCE A SCHEME FOR THE SEPARATION AND THE added known amounts of the various metals and the resulting mixtures were subjected to the separations described in this paper. Expt. A. Added Found B. Added Found C. Added Found D.Added Found E. Added Found F. Added Found Tin mg-5.0 4.8 0.0 0.0 30.0 30.4 1-0 1.2 Lead mg. 0.04 0.04 0.50 0.48 0.15 0.13 0-25 0.24 0.20 0.20 Bismuth mg. 0.0 0.0 0.50 0.47 1.0 * 0.0 0-0 Copper mg. 0.06 0.07 0.25 0.25 0.15 0.16 0.05 0-06 0-22 0.24 Zinc mg. 1 -00 0.95 0.22 0.23 0.20 0.30 0.26 o-ost * Bismuth detected but not determined. See p. 23. Other experiments have been made with a variety of foodstuffs and organic materials and the results of the determination of heavy metals obtained by these separations have been checked by other methods which have been in use for some years with satisfactory agreements. It is important that blank experi-ments should be made on each new batch of reagents.DIscussIoN.-Tia.-Only a limited number of experiments have been made with Clark's 4-methyl-1.2-dimercaptobenzene reagent but these indicate that the reagent is best employed when the quantity of tin is of the order of 50 parts per million or less. These experiments appear to indicate that the standards should be prepared from water saturated with hydrogen sulphide and should contain the same amounts of ammonium sulphate and ammonium citrate as are in the solution that is being tested. The concentration of hydrochloric acid is also important in the reaction since a large excess of hydrochloric acid inhibits the production of the red coloration. Also by the preliminary separation of the tin as described in Separation Stage I the interference produced by other metals which yield coloured mercaptides, notably bismuth giving a brick red mercaptide is avoided.Bismuth.-The method described for the detection and determination of traces of bismuth is a modification of that first described by Heinrichs and Hertrich.' These workers made no attempt to avoid interference due to traces of iron in the reagents. In the procedure here described iron is extracted as the thiocyanate with ether and copper the only other metal besides bismuth which yields a yellow thiocyanate is removed during the ammonia precipitation prior t o the test. This colour test for bismuth can be applied only after the metal has been separated from large amounts of mineral salts in the manner indicated because the presence of these would destroy the yellow colour.Traces of lead do not interfere with this method of determining bismuth and by it 0.02 mg. of bismuth may be detected DETERMINATION O F METALLIC IMPURITIES I N FOODSTUFFS 23 The thiocyanate reagent is prepared by adding 25 ml. of saturated ammonium thiocyanate solution to a mixture of 10ml. of water and 1.5ml. of conc. nitric acid and diluting the whole to 50ml. This solution is extracted with 10ml. of ether and 10 ml. of amyl alcohol which removes any iron that may be present. Zinc.-The use of yellow ammonium sulphide for washing the hydrogen sulphide precipitate in Separation Stage I is to be avoided. Low results for the zinc have been obtained when this procedure is adopted as is shown by experiment E. But the use of dilute yellow ammonium sulphide for this washing may become necessary when large quantities of tin (400p.p.m.and over) are present. This procedure has no effect on the determination of the lead bismuth or copper but it may produce low results for zinc. The N/50 bromide and bromate solution is prepared by dissolving 0454g. of potassium bromate and 2.24 g. of potassium bromide in 1 litre of water and is st andar dised ag dinst N/50 t hiosulphate. Should the filter-paper become “pulped” during the boiling with dilute nitric acid in Separation Stages I1 and I11 the solution together with the pulp must be filtered through a new “well washed’ ’ filter-paper. I wish to thank Dr. Bernard Dyer and Mr. George Taylor for their interest in this work. REFERENCES 1. 2. 4. __ i d . 1934 59 274. 5. 6 . 7. R. E. D. Clark ANALYST 1936 61 242. J . H. Hamence Trans. Faraday SOC. 1934 30 299. 3. - ,4NALYST 1932 57 622. G. Roche Lynch R. Slater and T. G. Osler id. 1934 59 787. T. Callan and J. A. R. Henderson id. 1929 54 650. H. Heinrichs and M. Hertrich Glas. Ber. 1924 2 112. 17 GREAT TOWER STREET LONDON E.C.3 DR. DYER’S LABORATOR

ISSN:0003-2654    

DOI:10.1039/AN9376200018

出版商:RSC    

年代:1937

数据来源: RSC

摘要:

OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix. about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order. It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary. Elsdon and Stubbs (Eoc.it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained. In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents.To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'. It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions.The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned.In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary. It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix.about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order. It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary.Elsdon and Stubbs (Eoc. it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained. In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents.To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'. It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions.The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned. In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary.It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix. about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order.It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary. Elsdon and Stubbs (Eoc. it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained.In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents. To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'.It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions. The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air.There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned. In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary.It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix. about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order. It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined.It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary. Elsdon and Stubbs (Eoc. it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained. In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents.To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'.It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions. The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place.The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned. In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary. It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix. about 1.5, 1.0,0.8, and 0.5" C.It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order. It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined.It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary. Elsdon and Stubbs (Eoc. it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained. In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents.To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'.It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions. The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place.The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned. In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary. It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix.about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order. It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary.Elsdon and Stubbs (Eoc. it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained. In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents.To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'. It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions. The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned. In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary. It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,

ISSN:0003-2654    

DOI:10.1039/AN9376200023

出版商:RSC    

年代:1937

数据来源: RSC

摘要:

OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix. about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order. It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary. Elsdon and Stubbs (Eoc.it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained. In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents.To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'. It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions.The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned.In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary. It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix.about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order. It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary.Elsdon and Stubbs (Eoc. it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained. In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents.To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'. It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions.The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned. In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary.It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix. about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order.It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary. Elsdon and Stubbs (Eoc. it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained.In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents. To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'.It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions. The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned. In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary. It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,

ISSN:0003-2654    

DOI:10.1039/AN9376200029

出版商:RSC    

年代:1937

数据来源: RSC

摘要:

OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix. about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order. It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary. Elsdon and Stubbs (Eoc.it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained. In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents.To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'. It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions.The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned.In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary. It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix.about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order. It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary.Elsdon and Stubbs (Eoc. it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained. In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents.To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'. It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions.The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned. In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary.It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix. about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order.It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary. Elsdon and Stubbs (Eoc. it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained.In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents. To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'.It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions. The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned. In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary. It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,

ISSN:0003-2654    

DOI:10.1039/AN9376200032

出版商:RSC    

年代:1937

数据来源: RSC

摘要:

OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix. about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order. It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary. Elsdon and Stubbs (Eoc.it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained. In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents.To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'. It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions.The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned.In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary. It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix.about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order. It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary.Elsdon and Stubbs (Eoc. it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained. In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents.To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'. It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions.The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned. In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary. It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,

ISSN:0003-2654    

DOI:10.1039/AN9376200034

出版商:RSC    

年代:1937

数据来源: RSC

摘要:

OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix. about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order. It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary. Elsdon and Stubbs (Eoc.it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained. In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents.To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'. It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions.The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned.In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary. It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,OF MILK: CORRECTION FACTORS AND THE INFLUENCE OF STIRRING: I1 h I11 225 It was originally intended to employ four different amounts of supercooling for each sample of milk, vix.about 1.5, 1.0,0.8, and 0.5" C. It was found, however, that the differences in the readings of the thermometer corresponding to these different amounts of supercooling were in the neighbourhood of 0.002"-an amount so small that the unavoidable errors of observation might possibly approach the same order. It was, therefore, decided to employ only two different amounts of supercooling, as widely apart as practicable, say about 1.5" and 04", and to increase the number of samples examined. It was stated by Hortvet that, unless a much greater amount of supercooling than 0.5" is employed in the use of his cryoscope and technique, the rise of the mercury column is not sufficiently pronounced, and that there is more or less wavering, so that difficulty arises in deciding on the exact point at which the top of the column becomes stationary.Elsdon and Stubbs (Eoc. it.)^ found the same result when using a supercooling of less than about 0.8"; the mercury rose very slowly, and did not maintain a steady position for any appreciable time, and the proper freezing-point might not, under these circumstances, be attained. In the first experiments, made with the object of ascertaining the super- cooling correction, it was observed that where the amount of supercooling was small, that is, less than say 0-75", even in the absence of alcohol in the jacket surrounding the freezing-tube, the rise of temperature, when freezing occurred, was very slow-so slow, indeed, as to suggest doubts whether the thermometer would indicate the freezing-point of the milk, influenced only by supercooling, owing to the reading being affected by the length of time which elapses and the possibility of imperfect thermal insulation, causing a nett loss of heat from the freezing tube and contents.To give an instance; in an experiment when the supercooling was 0-71" the time required for the mercury column to rise until, on observation through the telescope with the aid of the horizontal cross-wire, the ascent became imperceptible, was 92 minutes, as compared with about 3& minutes when the same milk was super- cooled 1-49'. It will be seen later that, in the absence of alcohol in the space surrounding the freezing-tube, the heat insulation of the milk is not perfect; a nett loss of heat occurs, for it is possible t o carry through a freezing-point determination under such conditions.The difference between the two methods of working- with and without alcohol-results in a longer time being required for the cooling of the milk when the space around the freezing-tube is occupied by air. There were also the difficulties of judging when the rising column of mercury had reached the highest point, due to the very slow movement which occurred in that region, and of getting satisfactory readings, owing to the short time that elapsed before a fall took place. The idea of following strictly the Hortvet technique, as regards stirring, for these particular experiments was therefore abandoned. In the first series of experiments, the results of which are recorded in Table 111, four or five stirrings of three strokes each were employed, and in the second series the mechanical stirrer, working at the rate of 40 strokes per minute, was in operation all the time, the thermometer rising until tapping became necessary. It does not appear that these variations in stirring caused any significant differences in the results obtained, but the times of rising of the mercury, after two different extents, large and small,

ISSN:0003-2654    

DOI:10.1039/AN9376200035

出版商:RSC    

年代:1937

数据来源: RSC