- --- 96 THE ANALYST. ON LOGWOOD AS A RE-AGENT. [BY A. ASEBY, N.B., F.R.C.S.] LOGTVOOD, as is well known, has long been used as an indicator in alkalimetry, but I am not aware that the peculiar action upon it of many acids to be described in this paper has been previously observed. Alterations in the colour of logwood extract and of hsmatoxjlin, under the influence of various agents, are, of course, well-known. In Watt's Dictionary of Chemistry, V'ol. iii, p. 732, it is stated tbat acids turn the colour of logwood dye to yellow ; alkalies deepen its colour, and give it a purple or violet hue. Again, on page 662, of the first supplement of the same work, we read : a solution of hsematoxylin, or paper saturated with it, is recommended by Wildenstein (Zeitschr Anal. Chem., ii.9) as a test paper, especially for the detection of ammonia, the fixed alkalies, alkaline earths, and certain metals. Swedish filtering paper thus prepared has a yellowish colour when dry, and is coloured red, violet, or violet blue, by the smallest trace of an alkali. And on page 920, Pt. 2, Vol. viii. of the Dictionary, hmmatoxylin is recommended as an indicator in acidemetry, especially for the estimation of non-volatileTHE ANALYST. 97 --_- acids, which, by ittj aid, may be directly titrated with alkaline bicarbonates, and, accord- ing to Frbbault, it may be used for the estimation of iodine. I find that logwood, or hzematoxylin, is capable of being put to many more uses than the foregoing. The re-agent may be used as a test paper, but it allows of greater delicacy when the method, to be presently described, is put in practice.The paper may be made from au alcoholic or aqueous extract of logwood. To pre- pare the latter, pour 100 C.C. of boiling water on to about 2 grammes of logwood chips, and allow it to extract for an hour or so. Then draw pieces of filter paper through the solution one or more timee, according to the depth of tint desired. When dried they should have a uniform pale buff colour. The paper should not be fingered or touched with metallic substances whilst wet, as a blue colour is thus readily imparted to it. HEematoxylin paper may be made in the same may, using about a 0-1 per cent. aqueous solution, In use, the paper is moistened with the solution to be tested and dried in a current of heated air.I find about 180" G. a convenient temperature, and it is desirable that it r;;hotdd have been subjected to this heat before being used. A piece of copperpipe, an inch or so in diameter and about a foot long, placed in a slanting direction over a burner, answer8 the purpose, and it may be so arranged as to fix on to an ordinary Bunsen's burner. The test paper must not be dried over 'a naked gas flame, on account of the acid products of combustion. It gives a purple colour with alkalies, and a rose red colour with mineral and some fixed organic acids, this particular reaction only taking place on evaporation, but with a comparatively strong solution of the acids it takes place at once. When moistened with weak solutions and dried as described, a beautiful.evanescent rose-coloured blush, commencing at the edges, traverses the surface of the paper. On the other hand,:volatile and some fixed organic acids either give no re-action with logwood, or else impart to it a more intensified yellow tint. With extremely weak solutions it is necessary to repeat the moistening and drying several times, adding dropa of the solution to the paper rather than re-dipping the latter into the solution, so as to concentrate the acid on the paper ; the sensitiveness of the re-action being thus considerably enhanced. The surface of the paper must be closely watched during the drying, as the colour is exceedingly fugitive with very weak solutions. Perhaps a better way to use logwood as an indicator, is to evaporate its extract on white porcelain over a water bath or argand burner.SeveraI very small drops of it should be evaporated at the same time, to one of them should be added a drop of the solution to be tested, the others serving for subsequent use and for comparhon of colours. When dry, if there is no distinct reaction, the former may be re-moistened with a drop of the solution under examination and again evaporated. This process may be repeated if necessary, and thus the re-action becomes extremely delicate. In the following table are embodied the re-actions of various acida and acid d t s with logwood so far as I have observed them :-98 THE ANALYST. ~ o ~ o w r reaction on evaporating ~~~~~& Albliies Name of Acid. Name of Acid. Subsequent c o l ~ ~ ~ ~ ~ g On reaction with Alkahes b.Oraasic acids: a. iKineraZacds. Arsenic . . Arsenious . . Boracic . . Carbonic . . Hydrobromic., Hydrochloric. . HydriOdiC .. Hydrocyanic . , Hydroflouric . , Hydrosulphurio Iodic .. .. Nitric .. .. Xitrous , , Molybdic . . Osmic . . , , Phosphoric . . Phosphorous , . Sulphuric . . Sulphurous , . Tit&& . . Tungstic . . Vanadic . . Acetic , . Benzoic Butyric Cinnamio Citric .. Formic Gallic .. Hippurio Lactic . . Mali0 ., Meconic Oxalic . , Picric ,. Salioylic succinic Tannic Tartaric uric .* Valerianic Rose red, charring . . .. Grey . . . . . . Purple Rose red .. .. Nd Nzl . . . . . . Purple Rose red . . . . Slight bluish { Rose red, slight] purple Rose red . , . . Purple f RzLr&!t@g] Ditto Xd . . . , , . Ditto Colour destroyed, } lybz f nored Rose red, fugi- f tive,notrepro- Ditto ducible Colour destroyed ., Ditto Purple grey . . , , Purple Blue . . . . . . .. Rose red . . . . .. Rose red, charring,. Purple Rose red . . . Ditto Rose red , . . . Ditto Purple grey . , . , Bluish Purplkh grey , , Purple { f Rosered, charring . . .. .. .. .. .. .. .. .. .. a . .. . I .. . I .. * . .. .. . I . I 3right. Yellow . . Vd .. .. .. 3ligbt yellow . . i?ellow . . . . Red .. .. .. c. Acad salts : Acid phos- phate of aoda NaHpO, .. Bisulphate of potash KHSO, Bitartrate bi potash .. ] Purple Ditto Ditto Ditto Ditto Bright yellow . . Nzl .. .. Orange red . . , , Yellow ,. ,. Orange red , , , , Rose red ., .. Rove red .. .. Reddish .. I Ditto ..Ditto Ditto Ditto Ditto Ditto Ditto ,.Green Orange .. ,. Red .. .. .. Nbl ... . 3lightly yellAw . . Orange .. .. Rose red .. . . N d .. .. .. Ditto Ditto Ditto Ditto Ditto Ditto Ditto orange The carbonic acid appears to affect the colour of logwood in a very small degree, turning it slightly to a reddish grey, but as it is not sufficient to be of any practical use, I have put the re-action in the table as nil. With logwood or hmmatoxylin paper, the rose red re-action is ear;ily perceptible with an acidity equal to 0.2 SO, per 1000, and by concentrating on the paper a few times, 0.1 SO3 per 1,000 gives a distinct re-action. With n dried spot of logwood extract on white porcelpin 0.05 SO, per 1,000 gives a slight re-action the first time of evaporating it. A re-action can be obtained with a solution containing only 0.025 SO3 per 1,000, if a drop of it is added to the logwood spot several times in succeesion, adding the drops before the remainder has had time to evaporate to dryness.Logwood affords a mean8 of distinguishing between nitric and other mineral acids, such as the sulphuric or hydrochloric, for when logwood paper is moistened with a solition containing nitric acid and dried, a rose red blush traverses the paper, but is not re-produced on repeating the operation, and it will be found that the paper is then no longer turned purple by alkalies, as the haematoxylin has been destrayed. It must be borne in mind, however, that the re-action with alkalies cannot be obtained after the rose red colour has been produced with logwood through the agency of boracic acid. The colouring matter of the paper is more readily charred by sulphuric acid and a few others than is the paper itself.It is, therefore, possible with the aid of logwood, to detect the presence of free mineral and some fixed organic acids when mixed with volatile organic acids.THE ANALYST. 99 An admixture of a minerd acid with a caloured vinegar to the extent of one part H2S04 per 1,000 can be readily detected by the use of logwood paper, and an admix- ture of 0.25 per 1,000 can be observed when using a spot of logwood extract dried on white porcelain. I presume that notice would not be taken of less quantities than that, but the limits of sensibility may be pushed still further with colourless vinegars. If, therefore, R spot of logwood extraot on white porcelain, on being moistened with a drop of a sample of vinegar, and dried, gives no red colour, then the article under examinam tionmay safely be declared to be free from adulteration with mineral acids.If, on the other hand, there should be a somewhat indefinite reaction, or a distincti rose-red colour, then the sample should be analysed by Hdmer’s method, since the colour might be due to the presence of tartaric acid. Logwood cannot be employed for the detection of mineral acids when mixed with lime or lemon juice, because citric acid also gives the re-action. 1 find that when nitric acid is in a vinegar its presence may be detected by logwood paper, which will assume at first the characteristic rose-red colour, vaniehing on drying, and after, if necessary, repeated applications of the vinegar, the red colour will no longer be observed, and a purple colour will not be given to the paper on the addition of an alkali, the haematoxylin having been destroyed by the nitric acid.I have observed that when a vinsgar containing free nitric acid is evaporated and incinerated, the residue is, nevertheless, alkaline ; therefore, logwood paper goes still further than Hehner’s qualitative test for the admixture of mineral acids with vinegar, since that isnot applicable to the detection of nitric acid, moreover, it is not available with distilled vinegars, whereas the logwood re-action is. I have made Borne experiments which show that logwood may be employed as an indicator in the direct titration of acetic acid in acetates, and no doubt in the titration of other organio acids, which do not give the rose-red colour with it, by adding standard sulphuric acid to a, known quantity of the salt in solution, until the free mineral acid re-action with logwood is just perceptible.I have not yet had time to pursue this branch of the subject further, but propose to take it up on a future occasion with special refer- ence to the assay of crude commercial acetates. Logwood does not readily lose its sensitiveness. I happened to leave some logwood solution in an open vessel in my laboratory in December last. In the middle of March I made some logwood paper from this, purpoaely leaving it exposed, and it still remains sensitive. These re-actions of logwood may be used conversely for the detection of logwood when employed as a colouring matter in wines, &c.I coloured some sherry slightly withit, and on dipping a piece of filter paper into it and drying, I found that ammonia gave a purple colour to it ; then, on moistening another piece of the paper with nitric acid, and drying, a fugitive rose-red c~olour was produced; but ammonia would then no longer give to it a purple colour. The charackkristic reactions of hydrochloric and sul- phuric acid, using the latter about deci-normal strength, were &en. In short, logwood paper had been made from the artificially coloured wine. I next tried the behaviour of the natural colouring matter of port wine, and for that purpose I dipped some Glter paper into the wine and dried it. Ammonia gave it a dirty100 THE ANALYST. green colour. Moistened with nitric acid and dried, a rose-red colour was not produced, but in place of it a yellow, which was not afterwards changed by ammonia. The colouring matter of claret behaves in precisely the same way. I have not yet had an opportunity of observing the behaviour of other colouring matters which are occasionally used for colouring wines, when treated in a similar manner, tosee if logwood may be distinguished from all of them, but hope to be able to do so before long. I think, however, that if a wine should be found to yield on paper a residue behaving after the fashion of logwood, it may fairly be considered to have a colouring matter other than its own natural one.