Nov., 19501 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS 577 The Standardisation of Volumetric Solutions* BY THE ANALYTICAL CHEMISTS' COMMITTEE OF IMPERIAL CHEMICAL INDUSTRIES LIMITED THE methods employed by Imperial Chemical Industries to standardise volumetric solutions have been in use for a number of years, and it is thought that it is now opportune to publish a full description of them. An outline of the scheme was given by Edward Hinksl in 1930, but this did little more than mention the relationships that are employed. It is hoped that the following detailed description of Imperial Chemical Industries' practice will be of service to all analysts. SYSTEM OF STANDARDISATION The standardisation system is based on pure electrolytic silver as ultimate standard, with iodine, sodium carbonate and sodium chloride as working standards and arsenious oxide and potassium dichromate as secondary working standards.The principle underlying the system is the adoption of a single ultimate standard of reference upon which the titre of all the principal laboratory standard solutions is based. The working standards are used as intermediaries between the ultimate standard and the volumetric solutions, but where the latter are not directly referable to the working standards, reference is made through the secondary working standards. The whole scheme is given in outline in Fig. 1. ULTlMATe STANDAR- SILVER t N HQ F H 001 N AgNOi t 001 N NHiCNS I ODINE I OlCHf r-t-l I o i N )4)2 Cel 0 Kl' SECONDARY :KGE -WORKING STANDARDS N 201 1 0.2 N Tic11 0.1 NITI2(SO4, Fig.1. Scheme of standardisation It should be mentioned that the scheme outlined in Fig. 1 does not include some of the less frequently used secondary working standards, such as antimony (pure metal), copper foil (pure electrolytic), $-nitroaniline and picric acid. * The bulk of the work involved in devising the system described in this paper was carried out in the laboratories of the Nobel Division under the direction of the late Mr. W. Rintoul.578 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS [Vol. 75 INDICATORS AND OTHER REAGENTS WATER- (a) Where reference is made to water, ordinary distilled water is intended. (b) "Boiled-out water'' is defined as distilled water that has been boiled until it is free from ammonia and carbonate, and cooled out of contact with the air.(c) The specially purified, or specially pure, water that is used in the reference of working standards to ultimate standard silver is prepared as follows- Place 2 litres of distilled water in a quartz or hard-glass flask and add 2 to 3 g. of A.R. quality sodium carbonate and 2 to 3g. of A.R. quality potassium permanganate. Distil the water through a quartz or block tin condenser and reject the first 500 ml. If the distillate no longer contains chlorides or ammonia, collect and store the next 1OOOml. in a quartz or copper flask. INDICATORS- to a litre. Methyl red-Dissolve 0.5 g. of water-soluble methyl red in water and dilute the solution Methyl orange-Dissolve 0-5 g. of the dye in water and dilute the solution to a litre. Bromophenol blue-Triturate 0-4 g.of the dye in an agate mortar with 6.0 ml. of 0.1 N sodium hydroxide and dilute the resulting solution to a litre with 20 per cent. v/v alcohol prepared by the appropriate dilution of industrial methylated spirit. Phenolphthaleilz-Dissolve 0.5 g. of pure phenolphthalein in a litre of 50 per cent. v/v alcohol that has previously been neutralised to the indicator. Industrial methylated spirit may be used for the preparation of this reagent. Starch-Triturate 5 g. of pure starch and 0.01 g. of mercuric iodide with 30 ml. of water in a mortar. Pour the resulting cream into 1 litre of boiling water, boil for 3 minutes, allow the solution to cool and, when the sediment has settled, decant off the clear liquid. Ferroin (0-phenaathroline ferrous suZ*hate)-Dissolve 0.695 g.of A.R. grade ferrous sulphate, FeS0,.7H20, in 100 ml. of warn water, then add 1.48 g. of o-phenanthroline and shake the vessel until the reagent dissolves. Immediately before use, transfer 10ml. to a 400-ml. beaker and titrate it, drop by drop, with 0.1 N ceric sulphate solution. As the oxidised form of the indicator in concentrated solution is blue-green in colour, the solution darkens and becomes brown. Titrate just to the disappearance of the red colour; the end- point is sharp to 1 or 2 drops. Diphenylamine-Dissolve 5 g. of diphenylamine in a litre of A.R. grade concentrated sulphuric acid, of density 1.836 at 20" C. Ferric ammonium alum-Dissolve 140g. of the A.R. grade salt in water. Slowly add sufficient concentrated nitric acid to change the colour of the solution from red-brown to yellow.Dilute to 1 litre with water. Potassizcm chromate-Dissolve 100 g. of chloride-free A.R. quality potassium chromate in water and dilute the solution to 1 litre. Potassium thiocyanate-Dissolve 20 g. of the A.R. quality salt in water and dilute the solution to 100ml. Ammonium thiocyanate-Dissolve 20g. of the A.R. quality salt in water and dilute the solution to 100ml. The oxidised indicator does not keep well. REAGENTS- Osmic acid solution-Dissolve 0.25 g. of osmium tetroxide, OsO,, in 0-1 N sulphuric acid and dilute the solution to 100 ml. with 0.1 N sulphuric acid. Sulphuric - phosphoric acid reagent-Mix 150 ml. of A.R. grade concentrated sulphuric acid, density 1.84 at 20" C., with 150 ml. of A.R. grade phosphoric acid, density 1-70 at 20" C.; add the mixture cautiously to 500 ml.of water. Cool the solution and dilute it to 1 litre. ULTIMATE STANDARD The ultimate standard silver is prepared from fine silver by electrolytic deposition. It The purity is kept in sealed tubes in the form of buttons weighing from 9 to log. each. of the ultimate standard silver is 99.995 per cent.Nov., 19501 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS WORKING STANDARDS 579 SODIUM CARBONATE The standard sodium carbonate is prepared by heating sodium sesquicarbonate to 270" & 10" C. until it is constant in weight. The sesquicarbonate is prepared from a selected sample of sodium bicarbonate which has been shown to be of very high purity by exhaustive qualitative tests. PREPARATION OF SODIUM SESQUICARBONATE- Add 1025 g.of the selected sodium bicarbonate gradually and with frequent stirring to 4000 ml. of hot water in a hard-glass beaker, the mixture being heated to 86" & 1" C. and maintained at this temperature until the whole of the bicarbonate has been added. Filter the resulting solution while still hot through a close-grained filter-paper and cool the filtrate quickly. When the separation of crystals of sesquicarbonate is complete, pour off the mother liquor and break up the crystalline mass. Transfer the wet crystals to a large funnel fitted with a perforated porcelain disc and remove as much as possible of the mother liquor by suction. Transfer the crystals to a porcelain basin, dry them in an electrically heated boiling water oven, grind them to a po'wder and store in a glass-stoppered bottle.Repeat this process until the desired quantity of sesquicarbonate has been obtained; then blend the various batches together thoroughly, weigh off 100-g. portions of the product into hard-glass bottles and seal the bottles by drawing out the necks in a blowpipe flame. Use a glass funnel in filling the bottle to avoid contamination of the neck and consequent difficulties in the sealing operation. Suitable tests are given by Rosin.2 METHOD FOR REFERRING THE WORKING STANDARD SODIUM CARBONATE TO THE ULTIMATE Accurately weighed quantities of the working standard sodium carbonate are titrated from a Ripper's weighing burette with N hydrochloric acid that has been previously stan- dardised gravimetrically with the ultimate standard silver.The procedure involves the conversion of sodium sesquicarbonate to sodium carbonate, and for this purpose a temperature of 270" -j= 10" C. is maintained and crucibles of platinum, or preferably silver, are used. Because of the ease with which sodium carbonate absorbs traces of sulphur oxides, precautions must be taken to avoid its exposure to the products of combustion of coal gas. An electrically heated oven should, therefore, be used. The time required for the conversion may be as little as 2 hours, provided that the material is stirred from time to time and that the oven is well ventilated. However, 6 t o 8 hours may be necessary before constant weight is obtained if the oven is badly ventilated and the material is not stirred.Fit a thermometer in the oven with its bulb in the vicinity of the crucible. It is advisable to determine the time required for the conversion with the particular apparatus employed and under the conditions of heating and stirring that are used. By heating for half an hour longer in subsequent experiments with the same apparatus, confidence may be placed in the success of the operation. The details are given below. Preparation u f pure hydrochZoric acid-Prepare an aqueous solution of hydrogen chloride, free from other halogen acids and free chlorine, by dropping concentrated sulphuric acid into hydrochloric acid in the apparatus shown in Fig. 2. The sulphuric acid used for this purpose must be free from nitrogen and halogens, and the hydrochloric acid must have been boiled thoroughly after addition of a small quantity of potassium permanganate.Wash the gas with a solution of cuprous chloride in hydrochloric acid and with boiled-out water. Con- duct the washed gas through a hard-glass tube into 400 ml. of boiled-out water contained in a 600-ml. silica flask covered with a piece of platinum foil. Boil the resulting hydrochloric acid, dilute it with boiled-out water until it is of approximately normal strength and then mix. Preserve the solution in the dark in a hard-glass bottle that has been well steamed out and washed with water. Standardisation of hydrochloric acid with the ultimate standard silver-Weigh accurately a button (9 to 10 g.) of the standard silver, which has previously been scrubbed with sand, boiled successively in dilute hydrochloric acid, dilute ammonium hydroxide and water, and finally dried.Place the button in a 2-litre hard-glass conical flask and add 20 ml. of chloride- free 40 per cent. nitric acid. Remove the nitrous fumes by boiling, then cool and dilute the solution to 900ml. with specially pure water. STANDARD SILVER-580 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS [Vol. 75 Weigh from a weighing burette a quantity of the normal hydrochloric acid slightly less than that required to precipitate the whole of the silver, and dilute it to 900 ml. with water. Add the silver nitrate solution slowly to the diluted hydrochloric acid and wash the flask which contained the silver nitrate with water, adding the washings to the united solution. Mix the liquid well, but do not shake it violently, and set it aside overnight in a dark place.Filter the liquid through a filter-paper that has previously been washed free from chloride, and wash the precipitated silver chloride six times by decantation with specially pure water, shaking vigorously each time, and pour the washings through the filter. Transfer the precipitate to the filter and wash it thoroughly. Reject the precipitate. Evaporate the Fig. 2. Apparatus for preparation of pure hydrochloric acid filtrate and washings to dryness in a porcelain basin. Dissolve the residue in a small amount of hot water and, after the addition of 1 ml. of 3 M* nitric acid, allow the mixture to stand in the dark for at least 12 hours. Filter the solution through a close-grained filter-paper previously washed with specially pure water, and wash the precipitate of silver chloride with 0.01 M nitric acid.Add an excess of 0.1 M hydrochloric acid to the combined filtrate and washings and allow the mixture to stand in the dark for 12 hours. Collect the precipitated silver chloride in a tared Gooch - Munroe crucible and wash it with 0.1 M hydrochloric acid. Dry the crucible and its contents to constant weight at 130" C. Calculate the concentration of the hydro- chloric acid from the weight of the acid corrected for displacement of air, the weight of the silver and the excess of silver weighed as silver chloride, the weight of silver being corrected for air displacement (see Table I, p. 587). Note on the use of weighing burettes-Where a weighing burette is used in standardisation work i t should be allowed to remain in the balance case for 20 minutes before each weighing.Standardisation of sodium carbonate with the standardised hydrochloric acid-Weigh from a weighing burette a quantity of the standardised hydrochloric acid in the same way as for the standardisation of the acid. Weigh in a tared silver crucible a quantity of the sodium sesquicarbonate slightly less than that required to neutralise the weighed quantity of the acid. Heat the sesquicarbonate in an electrically heated air-bath maintained at 270" 3 10" C. until the salt is constant in weight; 8 hours is usually sufficient. During weighing operations place the crucible containing the carbonate in a well stoppered weighing bottle. Transfer the anhydrous sodium carbonate to a silica flask and reweigh the empty crucible; then dissolve the salt in ammonia-free water.To this solution slowly add the hydrochloric acid, pouring it through a funnel which has been drawn out to a capillary. After the acid has been added and washed into the flask, boil the solution until all carbon dioxide is expelled, and then cool. Titrate the excess of acid with 0.01 N barium hydroxide, methyl red being used as indicator. Calculate the result as a percentage of sodium carbonate in the heated salt, the weight of sodium carbonate being corrected for air displacement. Reject the precipitate. * In this paper the designation molar ( M ) , with or without modifying figures, signifies that the solution need not be accurately made up or standardised, in contrast to those designated by N.Nov., 19501 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS 581 The 0.01 N barium hydroxide used in the above procedure should be standardised by means of 0.01 N hydrochloric acid, which has been prepred by dilution from the standardised N hydrochloric acid, with methyl red as indicator.IODINE PREPARATION OF IODINE- Iodine is purified by sublimation from potassium iodide and calcium oxide. The potassium iodide retains any chlorine or bromine that may be present and the lime combines with water and with any acid such as hydriodic acid. Reduce some re-sublimed iodine to powder and mix it intimately with 10 per cent. of its weight of powdered potassium iodide and 5 per cent. of ground calcium oxide. Sublime the mixture slowly in an apparatus of the form shown in Fig.3. Place the mixture in the beaker and heat by an electric lamp placed below, the whole being surrounded by an aluminium sheath covered with asbestos mill-board. The iodine sublimes in a thick crust and adheres to the projecting part of the covering flask, which is kept cool by allowing tap water to flow through it. Detach the iodine by filling the flask with ice-cold water, break up the iodine and re-sublime it in the same apparatus after removal of the residue from the first operation. This second sublimation, which should be carried out at as low a temperature as possible, separates any small traces of potassium iodide or lime that may have been carried up mechanically in the first operation. Powder the purified iodine and dry it over phosphorus pentoxide in a desiccator, the rim of which should be lubricated with the minimum amount of grease, which should be unreactive in nature.* Transfer 10-g.portions to hard amber glass tubes and seal the latter in a blow- pipe flame. Use a glass funnel to obviate contamination of the upper part of the tube and consequent difficulties in the sealing operation. STANDARDISATION OF IODINE WITH THE ULTIMATE STANDARD SILVER- Weigh accurately about 3 g. of the ultimate standard silver that has been previously scrubbed with sand, boiled successively in dilute hydrochloric acid, dilute ammonium hydroxide and water, and finally dried. Transfer the button to a 1-litre conical flask; add 6 ml. of chloride-free nitric acid (density 1-415 at 20" C.) and 5 ml.of specially pure water, and evaporate the solution to dryness on a water-bath in order to expel the excess.of acid. Dissolve the residue in 750 ml. of specially pure water. Weigh accurately in a small well-stoppered weighing bottle a quantity of the iodine half a milligram less than that theoretically required to react with the whole of the silver. Withdraw the stopper from the weighing bottle and immediately drop both stopper and bottle into a mixture of 3 ml. of 1 + 1 solution of hydrazine hydrate in water and 15 ml. of 3 M ammonium hydroxide dissolved in 250ml. of specially pure water contained in a 2-litre hard-glass flask fitted with a reflux condenser ground into the neck; the condenser prevents mechanical loss arising from the effervescence which occurs at this stage.When the reduction of the iodine is complete, dilute the solution to 750 ml. with specially pure water added through the condenser, and acidify with dilute nitric acid which has been well boiled to remove nitrous acid. Cover the flask with brown paper to protect the contents from light and slowly add the solution of silver nitrate through a funnel with occasional gentle agitation. When all the solution has been added, wash the last traces of silver nitrate into the iodide solution with several small portions of specially pure water. Mix the liquid thoroughly and allow it to settle in the dark overnight, or until the supernatant liquid is clear. Filter the solution through a close-grained filter-paper that has been well washed with specially pure water to remove chloride.Wash the precipitated silver iodide by decantation with pure water. Boil Fig. 3. Apparatus for Of iodine * The following composition has been found to be efficacious: Dissolve 35 g. of ceresine wax in 65 g. Stir the of B.P. quality liquid paraffin by heating the mixture in a water-bath until it is homogeneous. mixture until it cools to a uniform mass.582 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS [Vol. 75 the precipitate for 5 minutes with water containing 1 per cent. of ammonia, cool, acidify with dilute nitric acid and filter it. Finally wash the silver iodide thoroughly with boiling water. Discard the residue and evaporate the combined filtrate and washings to dryness in a porcelain basin. Extract the residue with 0.03 M nitric acid, filter if necessary, and add 10 ml.of 0.1 M hydrochloric acid. Allow the mixture to stand in the dark overnight. Collect the precipitated silver chloride in a tared Gooch - Munroe crucible, wash with 0.1 M hydro- chloric acid and dry to constant weight in an air oven at 130” C. Calculate the purity of the iodine from the weight of silver combined with the iodine, the necessary corrections for air displacement being applied. It is of the utmost importance that the distilled water, the reagents and the apparatus shall be free from traces of chloride. Specially pure water should be used throughout, and the filter-papers and apparatus should be washed with it before they are used. METHOD FOR THE COMPARISON OF DIFFERENT BATCHES OF WORKING STANDARD IODINE- Different batches of working standard iodine are compared by weight titration with 0.5 N sodium thiosulphate that has been prepared by the dilution of a well-matured strong solution of the pure salt with freshly boiled-out water.A small excess of the sodium thio- sulphate is used and the excess titrated with 0.01 N iodine, starch solution being used as indicator. The details of the procedure are as follows- Place 5 g. of potassium iodide and 0-5 ml. of water in a small well-stoppered weighing bottle. Put the whole inside the balance case for half an hour and then weigh it accurately. Add 2 to 3 g. of the iodine, again place the vessel inside the balance case for half an hour and weigh it accurately. Rotate the bottle to ensure that all the iodine is dissolved. Withdraw the stopper and immediately drop the bottle and the stopper into 200 ml.of water contained in a 1-litre conical flask. From a weighing burette add a quantity Qf the 0.5 N sodium thiosulphate slightly in excess of that required to reduce the whole of the iodine, and dilute the clear solution to 750ml. with water. Titrate the small excess of sodium thiosulphate with 0.01 N iodine, starch solution being used as indicator, until a pale purple-pink colour is obtained. Prepare the 0.01 N iodine by accurately weighing about 0.64 g. of working standard iodine in a small stoppered weighing bottle containing 2 g. of moistened iodate-free potassium iodide; take care to ensure that the weighing bottle and its contents are at the same temperature as the air inside the balance case.Dissolve the iodine and the potassium iodide in boiled-out water, dilute the solution to 500 ml. and calculate the normality factor. In this comparison there is no need to correct the weighings for air displacement. SODIUM CHLORIDE THE STATUS OF SODIUM CHLORIDE AS A STANDARD CHEMICAL- Richards and Wells, in their classical work “A Revision of the Atomic Weights of Sodium and Chl~rine,”~ showed the identity of many specimens of sodium chloride prepared in various ways from materials of widely different origin. The method of purification adopted by them was the familiar one of precipitation by hydrochloric acid gas followed by recrystallisation from water. It is very doubtful if sodium chloride can be completely dried at any temperature below its melting-point, but the authors show (ibid.) that the expulsion of water upon fusion of sodium chloride is attained without change in its composition and that “probably not enough gas is absorbed during fusion of the salt in air sensibly to alter the apparent combining weight.” They state “that sodic chloride is among the substances whose preparation in a pure state is an easy problem.” In view of the foregoing, and in view of the fact that the technique of the method of referring sodium chloride to silver has been elaborated to the last detail by many notable workers in the field of atomic weight determination, it is concluded that sodium chloride satisfies all the criteria usually demanded of a working standard chemical.It is superior to sodium carbonate in that it can be directly referred to silver, and superior to iodine in that the technique of its reference to silver is not attended by the difficulties that accompany the reference of iodine to silver.PREPARATION OF THE SODIUM CHLORIDE- The working standard sodium chloride is prepared from purified brine containing about 30g. of sodium chloride per 100 ml. and not more than traces of calcium and magnesium. Alternatively, a solution of pure sodium chloride, A.R. quality, may be used.Nov., 19501 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS 583 Into a convenient volume of the brine, pass hydrogen chloride gas until the acidity is about 5 N . The hydrogen chloride gas used for this purpose should be prepared by the slow addition of sulphuric acid to fragments of clean rock salt, the gas being scrubbed by passage through a gas-washing bottle containing concentrated hydrochloric acid.Filter the precipitated salt by suction through prepared "Tarantulle"* cloth in a porcelain Biichner funnel. The Tarantulle is prepared by steeping it in concentrated hydrochloric acid and washing it with specially pure water. Wash the salt on the filter with a little specially pure water, then mix it with a little of the water into a slurry and again filter by suction through Tarantulle. Recrystallise the washed salt twice from pure water in porcelain vessels, filtering the solutions through Tarantulle, and concentrating the filtrates by evaporation so that a yield of about 60 per cent. is obtained in each crystallisation.Collect the final crystals on Tarantulle and dry them as completely as possible by suction. Reject the layer of crystals in contact with the cloth. Dry the remainder at 105" C. in an electrically heated air oven and, after cooling, mix well. During the preparation care must be taken to exclude dust, and vessels of porcelain should be used throughout. Before proceeding with the standardisation, verify the purity of the salt according to the directions given by Rosin.2 DETERMINATION OF MOISTURE IN SODIUM CHLORIDE- Grind about 5 g . of the sodium chloride to fine powder in an agate mortar, transfer it to a tared platinum crucible fitted with a lid and weigh the whole accurately. Dry to constant weight in an electrically heated air oven at 270" & 10" C., the lid of the crucible being removed during the drying.Note the loss of weight, Transfer the crucible with the lid in position to an electrically heated muffle furnace, gradually raise the temperature to 820" to 830" C., and maintain the furnace at a temperature not exceeding 830" C. for 10 minutes. Remove the crucible from the furnace, cool it in a desiccator and weigh it. The fused salt should be free from colour, and the difference between the total percentage loss of weight and the percentage loss of weight at 270" C. should not exceed 0.1. STANDARDISATION OF SODIUM CHLORIDE WITH THE ULTIMATE STANDARD SILVER- Weigh accurately a button (9 to 10 g.) of the standard silver, which has previously been scrubbed with sand, boiled successively in dilute hydrochloric acid, dilute ammonium hydroxide and water, and finally dried.Place the button in a hard-glass flask and add 20ml. of chloride-free 40 per cent. nitric acid; insert a funnel in the neck of the flask and warm the vessel on a water-bath to aid solution. Boil carefully to expel nitrous acid and add 500 ml. of pure water, washing the funnel inside and out with the water. Grind 6 to 7 g. of the standard sodium chloride to a fine powder in an agate mortar and dry it to constant weight in an electrically-heated air oven at 270" & 10" C. Weigh accurately a quantity of the dried salt sufficient to precipitate all but a milligram or so of the silver, and dissolve it in 500 ml. of specially pure water in a hard-glass flask. Add the silver nitrate solution slowly to the salt solution with gentle rotatory agitation and wash the last traces of the silver solution into the salt solution with several small portions of specially pure water.Stopper the flask, mix the contents thoroughly by gentle rotation and set aside in the dark overnight, or until the supernatant liquid is clear. Decant the clear liquid through a filter- paper previously washed free from chloride with specially pure water. Wash the silver chloride six times by decantation with 100ml. of specially pure water, shaking vigorously each time, transfer it to the filter and wash it with specially pure water, filling the filter six times. Evaporate the combined filtrate and washings to dryness in a porcelain basin over a water-bath in a dust-free atmosphere. To the residue add 5 ml.of 0.1 M nitric acid and 50 ml. of specially pure water, and filter through a small filter-paper previously washed with specially pure water, collecting the filtrate and washings in a hard- glass flask. Add 10 ml. of 0.1 M hydrochloric acid and set the mixture aside in the dark overnight or until the supernatant liquid is clear. Collect the precipitate of silver chloride in a prepared Gooch - Munroe crucible and wash it with 0.1 M hydrochloric acid. Finally, dry the crucible to constant weight at 130" C. Reject the precipitate. * Tarantulle is a high grade cotton cloth manufactured by Tootal Broadhurst Lee Co., Ltd. It is free €rom filling materials and has a very low ash content.584 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS [Vol. 75 From the weight of silver chloride thus obtained calculate the corresponding weight of Deduct this from the weight of silver taken and calculate from the difference the Correct the various weighings for air displacement, silver.titre of the standard sodium chloride. by means of the values given in Table I (p. 587). SECONDARY WORKING STANDARDS ARSENIOUS OXIDE THE STATUS OF ARSENIOUS OXIDE AS A STANDARD CHEMICAL- Arsenious acid was first used as a volumetric standard by Gay Lussac in chlorimetry. Penet improved the original method and later Mohr adapted it to iodimetry. The first critical study of the theory and practice of the method, however, was made by Wa~hburn,~ who clearly explained the part played by the sodium bicarbonate in the reaction, determined the upper and lower limits of the pH range permissible and showed that sodium phosphate or borax - boric acid could replace the sodium bicarbonate buffer with advantage for certain purposes. He also showed that, if a high degree of accuracy was to be attained, the pH value of the reaction mixture at the end of the titration must be about 7, Le., the solution must be neutral.His final conclusion was that “arsenious acid is the most convenient and accurate standard for iodimetry.” Menzies and McCarthy6 advocate the use of arsenious acid as an alkalimetric standard. For this purpose a known weight of arsenious oxide is oxidised with nitric acid, the excess of the latter is removed by evaporation and the residue of arsenic acid is titrated with the alkaline volumetric solution. Chapin6 elaborated a laboratory method for the preparation of pure arsenious acid and methods for detecting likely impurities, e.g., antimonious oxide. The same author’ established the reliability of properly purified arsenious oxide as a standard and, with borax - boric acid as a buffer in a series of weight titrations, he obtained a mean figure for the iodine to arsenious acid ratio that differed from the theoretical by less than 1 part in 3600. Gooch* recommends the use of arsenious acid as an iodimetric and as an oxidimetric standard. For the latter a standard solution of arsenite is prepared and used for titrating the iodine liberated from an iodide by a known volume of permanganate solution in presence of acid. Thus, there is a large body of evidence to support the use of arsenious oxide as a standard in volumetric analysis.SELECTION OF MATERIAL- The standard arsenious oxide is selected from the purest available material of A.R. quality, the selection being based on the results of tests for purity applied according to the specification of the Committee on Analytical Reagents of the American Chemical Society.9 Place 5 g. of pure potassium iodide and 0-5 ml. of water in a small glass-stoppered weighing bottle, allow the mixture to stand inside the balance case for half .an hour and then weigh accurately. Add about 3 g. of the working standard iodine to the mixture, allow the whole to stand for half an hour in the balance case and weigh accurately. Rotate the bottle gently to ensure that all the iodine is dissolved. Weigh accurately a quantity of the arsenious oxide, previously dried to constant weight at 105” C., that would reduce 1 mg.more than the weight of iodine taken. Transfer the arsenious oxide to a l-litre conical flask and dissolve it in 50 ml. of N sodium hydroxide, warming the flask to aid solution. When solution is complete, cool the liquid to room temperature, add 50 ml. of N hydrochloric acid to neutralise the alkali, dilute the solution to 650 ml. and add sufficient of a 5 per cent. solution of sodium bicarbonate to give 2 g. of the bicarbonate for every gram of iodine taken. Withdraw the stopper from the weighing bottle and gently drop both stopper and bottle into the flask. Shake the flask gently and, as soon as the whole of the iodine is reduced, add 5 ml. of starch solution and titrate the mixture at once with 0.01 N iodine until a pale purple-pink colour is obtained.Perform a blank titration with the 0.01 N iodine on a solution containing the following, mixed in the order given- STANDARDISATION OF ARSENIOUS OXIDE WITH WORKING STANDARD IODINE- 50ml. of N sodium hydroxide 55 ml. of N hydrochloric acid 560ml. of water 100ml. of 5 per cent. sodium bicarbonate solution 9 g . of potassium iodide 5 ml. of starch solution,Nov., 19501 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS 585 Deduct the volume of 0.01 N iodine required to reproduce the end-point in the blank, usually about 0.2ml., from the volume used in the titration of the arsenious oxide. Correct the weights of arsenious oxide and iodine for air displacement and calculate the titre of the arsenious oxide.POTASSIUM DICHROMATE THE STATUS OF POTASSIUM DICHROMATE AS A STANDARD CHEMICAL- The chief advantages of potassium dichromate as a standard chemical are the ease with which it can be obtained in a high state of purity and the permanence of its aqueous solution. Its use as a standard for oxidation processes was placed on a firm basis by 131iihns.l~ For a discussion on the reliability of potassium dichromate as a primary standard, reference may be made to Kolthoff and Furman.ll One of the difficulties of its use in the standardisation of solutions of sodium thiosulphate is the colour imparted by the chromium salt to the solution. Thus the advantage of the use of starch solution as indicator is somewhat impaired. These difficulties are overcome if the liberated iodine is distilled into a solution of potassium iodide in a stream of carbon dioxide.The titration with sodium thiosulphate may then be carried out on the distillate with the high degree of accuracy associated with this titration. PREPARATION- Recrystallise A.R. quality material from water until the product after drying at 150" C. complies with the specification of the Committee on Guaranteed Reagents of the American Chemical Society.12 STANDARDISATION OF POTASSIUM DICHROMATE WITH WORKING STANDARD IODINE- The reference of potassium dichromate to iodine involves an accurate determination of two ratios, (1) potassium dichromate to 0.5 N sodium thiosulphate, and (2) 0.5 N sodium thiosulphate to working standard iodine. STEP 1.DETERMINATION OF THE RATIO POTASSIUM DICHROMATE TO 0-5N SODIUM THIOSULPHATE-The following reagents and apparatus are required- Sodium thiosulfihate-45 N solution, prepared by dissolving 313 g. of the pure salt in 24 litres of freshly boiled-out water. Allow the solution to stand for at least 14 days, then decant or siphon off the clear liquid into an amber glass bottle fitted with a siphon and soda-lime tube. Boiled-out water-Oxygen free. Potassium iodide-20 per cent. solution, freshly prepared from pure iodate-free potassium iodide of A.R. quality and boiled-out water. Hydrochloric acid-Hydrochloric acid of density 1.1 at 20" C., chlorine-free, freshly prepared by boiling 50 g. of pure concentrated hydrochloric acid in a small open hard-glass flask until only 25 g.remain. Starch solution. Iodine solution-0.01 N , prepared as described above (p. 582). A Gooch distillation apparatus (Fig. 4)-The tap and joints of this should be lubricated with syrupy phosphoric acid. A Kip$', or other suitable carbon dioxide generator. Procedure-Weigh accurately about 1 g. of the potassium dichromate, previously dried over phosphorus pentoxide, and dissolve it in 50 ml. of boiled-out water. Transfer the solution to flask A of the Gooch distillation apparatus (Fig. 4) through the tap funnel; wash the whole of the solution into the flask with several small portions of boiled-out water, using 20ml. in all. Place 100ml. of the potassium iodide solution in the receiver, B, and a few ml. in the trap, C. Loosely close the exit tube of the trap with a plug of glass wool, D, to prevent loss of spray.Dilute the solution in the receiver, B, with sufficient boiled-out water to trap gases entering from A. Remove air from the apparatus by passing a stream of carbon dioxide for 20 minutes. Add 15ml. of the chlorine-free hydrochloric acid to the flask, followed by sufficient of the 20 per cent. solution of potassium iodide to give 3-75 g. of potassium iodide for each gram of potassium dichromate taken. Wash down the tap funnel with 20 ml. of oxygen-free water, running the washings into the flask and taking care to exclude air.586 THF STANDARDISATION OF VOLUMETRIC SOLUTIONS [Vol. 75 Distil the liberated iodine in a slow stream of carbon dioxide; continue the heating until the whole of the iodine has passed over into the receiver, which should be cooled by immersion in a constant-level water-bath through which cold water is circulated.At the end of the distillation allow the apparatus to cool, but maintain the stream of carbon dioxide; then disconnect the distilling flask. Transfer the potassium iodide solution from the trap to the receiver, wash the trap and glass wool plug with boiled-out water and add the washings to the receiver. CARBON DIOXIDE Fig. 4. Gooch distillation apparatus Add from a weighing burette a slight excess of 0-5 N sodium thiosulphate. Transfer the decolorised solution to a 1-litre conical flask, wash the receiver with several small portions of boiled-out water and add the washings to the flask. Add 5 ml. of starch solution, dilute to 750ml.with boiled-out water and titrate with 0.01 N iodine until a pale purple-pink colour is obtained. Correct the volume of 0.01 N iodine used for that required to produce the end-point tint in 750 ml. of boiled-out water containing 14.6 g. of potassium iodide and 5 ml. of starch solution. Calculate the weight of potassium dichromate equivalent to 100 g. of 0.5 N sodium thiosulphate. STEP 2. DETERMINATION OF THE RATIO 0.5 N SODIUM THIOSULPHATE TO WORKING STANDARD IoDINE-Place 5g. of potassium iodide and 0.5 ml. of water in a small well- stoppered weighing bottle, allow the bottle and contents to remain inside the balance case for half an hour and then weigh it accurately. Add 2 to 3 g. of the working standard iodine, again place the vessel inside the balance case for half an hour and weigh it accurately.Rotate the bottle gently to ensure that all the iodine is dissolved. Withdraw the stopper and immediately drop the bottle and the stopper into 200ml. of boiled-out water contained in a 1-litre conical flask. From a weighing burette add a quantity of the 0-5 N sodium thio- sulphate slightly in excess of that required to reduce the whole of the iodine, and dilute the clear liquid to 750 ml. with boiled-out water. Add 5 ml. of starch solution and titrate the excess of sodium thiosulphate with 0.01 N iodine until a pale purple-pink colour is obtained. Correct the volume of 0.01 N iodine used for that required to produce the end-point tint n 750 ml. of boiled-out water containing 5 ml. of starch solution and 9 g. of pure potassiumNov., 19501 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS 587 iodide.Calculate the weight of iodine equivalent to 100 g. of 0.5 N sodium thiosulphate and, from the result so obtained and that obtained in Step 1, calculate the titre of the potassium dichromate in terms of working standard iodine. In this calculation corrections for air displacement, shown in Table I, should be applied to the weighings of iodine and potassium dichromate, but they need not be applied to the weighings of the sodium thiosulphate solution. TABLE I CORRECTIONS FOR AIR DISPLACEMENT IN GRAMS PER GRAM OF SUBSTANCE WEIGHED g. per ml. 0.0012 - Density of air, assumed . . .. - Density of aluminium, assumed . . - Density of brass, assumed . . - 8-4 Density of platinum, assumed . . = 21.55 2.67 - - Assumed Substance weighed density Arsenious oxide .. .. .. 3.7 Iodine . . .. . . .. . . 4.93 Potassium dichromate . . . . 2.69 Silver . . . . .. . . . . 10.49 Sodium carbonate . . . . .. 2.533 Sodium chloride . . . . .. 2.161 Brass weights + 0*00018 + 0~00010 + 0.00030 + 0.00033 + 0.00041 - 0.00003 Platinum weights + 0.00027 + 0.00019 + 0.00039 + 0.00006 + 0.00043 + 0*00050 Aluminium weights - 0~00012 - 0.00021 + 0~00000 - 0.00033 + 0~00002 + 0~00010 Note-For ordinary sets of analytical weights calibrated on a “weight in air” basis, use the correction given in the column headed “Brass weights” for all the weights in the set. For special sets of weights calibrated on a “mass” basis, the corrections given in the columns headed “Platinum weights” and “Aluminium weights” must be used for the corresponding fractional weights.STANDARD SOLUTIONS A standard solution is one that contains an exactly known quantity of any reagent Sometimes a standard solution is designated by its percentage content, in unit volume. but more often in terms of its relation to a normal solution, NORMAL SOLUTION- A solution is stated to be normal when it is of such strength that 1 litre of it contains, in available form, 1 gram-equivaknt of the essential component. The gram-equivalent is that quantity which can theoretically combine with, or replace, 8.000 g. of oxygen or 1.008 g. of hydrogen. A solution which is weaker or stronger than normal is designated by the fraction, or multiple, of its normality; for instance, a solution of one-tenth normal strength is designated as 0.1 N and a solution twice normal strength as 2 N.FACTOR OF A STANDARD SOLUTION- Standard solutions are made as nearly the required normality as is conveniently possible, and the strength is accurately determined. The variation from the normality is expressed in the form of a factor by which the volume of the solution actually used must be multiplied in order to ascertain the equivalent volume of a solution of exact normality. The following example shows how this factor is determined. Assuming that it has been found that 2400g. of pure sodium carbonate require 4840ml. of approximately normal hydrochloric acid for neutralisation, the factor will have to be such that when 48-00ml. is multiplied by it, the result will be 47.17 ml., the exact quantity which would have been required had the hydrochloric acid been strictly normal.In other words, the factor would be in this case- 47.17 48.00 -= 0.9827 This hydrochloric acid would be labelled thus- N Hydrochloric acid, factor 0.9827. In the same way an 0.1 N potassium permanganate solution having a factor of 1.023 would be labelled- 0.1 N Potassium permanganate, factor (acid solution) 1.023.588 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS [Vol. 75 STORAGE AND PROTECTION OF SOLUTIONS Glass bottles for the storage of standard solutions should be thoroughly cleaned with chromic acid and then well washed with tap water and distilled water. Solutions must be protected from contamination, and on no account should an unused portion be returned from the burette to the stock bottle.The stock bottle should not be exposed to direct sunlight and the solution should be well mixed before portions are with-drawn. The stopper and neck of the bottle should be wiped with a clean cloth immediately after use. STANDARD TEMPERATURE As the temperature of reference for graduated volumetric glassware is 20" C., standard volumetric solutions should be made up at this temperature. If work at this temperature is impossible, an allowance must be made in the final adjustment of the volume of the solution so that when the solution is used at 20" C. the normality is correct. Table IV may be used for calculating the necessary corrections; for 0.1 N solutions not mentioned in the table the corrections for 0.1 N hydrochloric acid should be used.STANDARDISATION OF SOLUTIONS ACIDIMETRY AND ALKALIMETRY The scheme for referring standard solutions of acids and alkalis to the working standard sodium carbonate is outlined below- OUTLINE OF STANDARDISATION SCHEME FOR ACIDS AND ALKALIS I Working Standard Sodium Carbonate ' _ J- J J 4 + t J- N HC1 0-5 N HC1 0.1 N HC1 N HNO, 0.1 N HNO, N H$O, N NaOH J- I 0.5 N NaOH 5. .1 4 J. 0.1 N NaOH 0.1 N Ba(OH), 0.01 N HC1 0.01 N Ba(OH), STANDARD ACIDS THE PREPARATION OF STANDARD ACIDS- Only pure concentrated acids of reagent quality should be used in the preparation of standard acids, and the dilution should be made with distilled water of good quality that has been well boiled to free it from carbon dioxide. N acids from concentrated acids-Hydrochloric acid is recommended in preference to sulphuric acid because with certain indicators the end-point is more sharply defined.The approximate quantities of concentrated acids required for the preparation of various normal standard acids are given in Table 11. By means of a British Standard density hydrometer, determine the density at 20" C. of the concentrated acid to be used and calculate from the above data the quantity of acid required to produce the desired volume of normal solution. Standard solutions of nitric acid should be protected from light; they are best stored in bottles of amber glass. 0.1 N acids-Prepare 0.1 N acids by dilution of accurately known amounts of N acids so that the calculated value of the factor may serve as a check upon the determined value. Large batches are best prepared by dilution on a weight basis, use being made of the expression V x F x b -v __I_- a 1.00283 W =Nov., 19503 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS 589 where- W = weight of water in grams, weighed in air, to be added to Vml.of the given 'c' = volume in ml. at 20" C. of the given standardised solution to be diluted F = factor of N/a a = value of a in the expression of the normality of the given standardised solution, standardised solution as N / a b = value of b in the expression of the normality of the required diluted solution, as 1.00283 = volume in ml. at 20" C. of 1 g. of water weighed in air. N / b The following example will make the use of this formula quite clear. It is required to prepare 0.05 N hydrochloric acid from 2000 ml.of 0.5 N hydrochloric acid of factor 0.9965. The weight of water W to be added to the 2000 ml. of 0-5 N acid is as follows- 2000 x 0.9965 x 20 - 2ooo 0 L 1.00283 W = = 17879.4g. 0.01 N acids-Prepare 0.01 N acids by dilution from accurately known amounts of 0.1 N In this case the purity of the distilled water is of particular importance and it is essential acids. that ammonia and carbon dioxide should be absent. TABLE 11 DATA FOR USE IN THE PREPARATION OF NORMAL SOLUTIONS OF ACIDS Acid HCl . . * . .. . Density of acid at 20" C., g. per ml. 1.140 1.145 1.150 1.155 1.160 1.165 1.170 1.175 1.180 (D) Concentration of acid corresponding to D, 28.2 29-1 30.1 31.1 32.1 33.2 34.2 35.2 36.2 % HNO, .. .. . . H,SO, . . . . .. 1.405 68.1 1.410 69.1 1.415 70.4 1.420 71.6 1.425 72.8 1.395 1.520 1.834 1.834 1-835 1.835 1.836 1.836 50.0 62.0 95-1 99.1 95.6 98-7 96-4 98.1 Quantity of acid required for dilution to 1 litre of approximately normal strength r- g.129.5 125.1 121.0 117.1 113.5 110.0 106.7 103.6 100.7 92.6 91.2 89.5 88.1 - 86.6 98.0 79.0 51.5 49.4 51.3 49-6 50.8 50-0 ml. 113.6 109.3 105.2 101.4 97.8 94.4 91.2 88.1 85.3 65.9 64-7 63.3 62.0 60.8 70.3 52.0 28-09 26.96 27-93 27-05 27-68 27-21 STANDARDISATION OF NORMAL AND DECINORMAL ACIDS- Several methods axe available for the standardisation of acids with sodium carbonate, but the conversion of the sesquicarbonate into anhydrous sodium carbonate is the first step and is common to all methods (see p. 579). Transfer the crucible containing the anhydrous carbonate while still hot to a desiccator containing a stoppered weighing bottle of suitable size.When it is cold, place the crucible in the bottle and transfer the latter to the balance case and weigh it. In order to obtain equilibrium between the air in the balance case and the surface of the bottle, half an hour should elapse before weighing.590 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS Wol. 75 SoEution of the carbonate-Place the crucible well inside the neck of a dry conical flask of hard glass and empty the carbonate into the flask, care being taken to avoid loss of fine particles of the carbonate dust. Wear finger-stalls of thin rubber whilst handling the crucible. Immediately return the empty crucible to the weighing bottle and weigh it after an interval of 15 minutes in the balance case. Add about 50 ml.of water to the flask, taking care to avoid loss of fine particles of the carbonate, and promote solution by swirling the liquid gently. The titration-The choice of weight or volume titration depends chiefly upon the precision sought. Where a high degree of precision is desired, it is imperative to employ weight titration. The density of the acid must then be known and it is necessary to correct the weighings for air displacement. The error of the weight titration method for the standardisa- tion of N hydrochloric acid in experienced hands may be as low as &3 in 10,000 for a single titration. Under the best conditions and with a 50-ml. burette the error of the volumetric titration method is at least three times as great as this, i.e., the error of a single titration is of the order of 1 in 1000.This error is unavoidable and is due to uncertainty in the calibration and reading of the burette. It may be reduced considerably, however, by using the 105-ml. bulb burette referred to below in place of the 50-ml. burette. If an accuracy of not less than 99.9 per cent. is required in the determination of alkalis by volumetric titration with N acid, the acid must be standardised by weight titration, or by titration from the 105-ml. bulb burette. If a lower degree of accuracy suffices, the standardisation may be carried out volumetrically with a 50-ml. burette. Care must be taken to avoid loss of sodium carbonate due to spraying, e.g., by placing a funnel in the neck of the titration flask; if a funnel is used, it will be necessary to rinse it inside and out with water before completing the titration. Only burettes that have been carefully calibrated should be used in standardisation work, and titrations should be performed at or near the temperature of reference, i.e., 20" C.Greater accuracy is obtained in the volumetric titration by the employment of a 105-ml. bulb burette, the stem of which is calibrated from the 80-ml. to the 105-ml. mark in ml. and twentieths. These burettes must be fitted with jets such as would be fitted to the corre- sponding 25-ml. burettes if drainage errors are to be avoided. See the British Standard Specification for bulb burettes.13 Choice of conditions-The conditions of standardisation, including the choice of an indicator, are almost invariably governed by the rule that the method of use of a volumetric solution should be comparable in all respects with the method of standardisation.PROCEDURE-For the standardisation of N , 0.5 N and 0.1 N acids the quantities of sodium sesquicarbonate shown in Table I11 are recommended. Weigh the quantity of working TABLE I11 WEIGHT OF SODIUM SESQUICARBONATE FOR USE IN THE STANDARDISATION OF N , 0.5 N AND 0.I N ACID Sodium sesquicarbonate required for r A 1 Volumetric titration using a 105-ml. bulb burette graduated from 80 ml. to 104 ml. in 0.05 ml., Weight titration using a 60-g. Ripper's burette, g- g- 1-ON . . . , 5.0 (= 3.4 g. of Na,CO,) 7-1 (= 4.8 g. of Na,CO,) 0.5 N . . .. 2-5 (= 1.7 7) ) 3.6 (= 2.4 39 ) 0.1 N . . . . 0.50 (= 0.34 n ) 0.71 (= 0.48 33 ) standard indicated in the table into a silver or platinum crucible and heat at 270" f 10" C.in an electrically heated air oven until it is constant in weight, stirring the sodium carbonate occasionally with a stout silver or platinum wire. Transfer the crucible to a desiccator containing a stoppered weighing bottle of suitable size and, when cold, place the crucible in the bottle. Transfer the bottle to the balance case and weigh accurately after an interval of half an hour. Empty the contents of the crucible into a dry 500-ml. conical flask of hard glass, placing the crucible well inside the neck of the flask in order to avoid loss of material. Immediately replace the crucible in the bottle, transfer the whole to the balance case and weigh after half an hour.Nov., 19501 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS 591 Dissolve the sodium carbonate in 50 ml.of water that has previously been neutralised to pH 3.9," washing down any carbonate adhering to the neck and side of the flask, and add 4 to 5 drops of bromophenol blue solution. Weight titration-Put into a 60-g. weighing burette a quantity of the standard acid slightly greater than that required to neutralise the sodium carbonate, and weigh accurately. Titrate the solution until a greenish-blue colour is obtained, or until the colour of the solution matches that of a standard buffer solution of pH 3.9 containing the same relative amount of the indicator. At the end of the titration again weigh the burette. Determine the density of the acid solution in grams per ml.at 20" C. by means of a Regnaul t pyknome ter . From the weight of acid required in the titration calculate the normality factor, x , of the acid at 20" C. by means of the expression- B A X - X C 100 D x E x F x = where A = weight in grams of sodium carbonate taken B = percentage purity of the sodium carbonate C = density of the hydrochloric acid in grams per ml. at 20" C., i.e., C = 1.0165 for N HC1 = 1.0004 for 0-1 N HCl = 1.0072 for 0.5 N HC1 D = weight in grams of hydrochloric acid used in the titration E = grams of Na,CO, equivalent to 1 ml. of solution i.e., E = 0.053 for N solution = 0.0265 for 0-5 N solution = 0.0053 for 0.1 N solution and so on F = correction factor for air displacement of the hydrochloric acid, i.e., 1.00105.Volumetric titration-Fill a standardised 105-ml. bulb burette (see p. 590) with the acid, set the meniscus accurately on the zero of the scale and titrate the sodium carbonate solution until the colour changes to greenish-blue, or until it matches a standard buffer solution of pH 3.9 containing the same relative amount of indicator. At the end of the titration read the burette. Note the temperature of the room and, if it differs from the reference temperature (20" C.), correct the volume of standard acid used accordingly with the aid of Table IV. From the corrected burette reading and the weight of pure sodium carbonate taken, calculate the strength of the acid in grams per ml. and thence express the result as a factor of N , 0-5 N or 0.1 N as appropriate. STANDARDISATION OF 0.01 N HYDROCHLORIC ACID- The standardisation of 0.01 N hydrochloric acid is only carried out as a check on the calculated factor.Measure accurately from a 5- or 10-ml. burette 4.50ml. of standard 0.1 N alkali into a 100-ml. hard-glass flask. Add 4 drops of phenolphthalein solution and titrate rapidly with the 0.01 N acid from a 50-ml. burette until the pink colour just disappears. Calculate the factor of the acid by use of the phenolphthalein factor of the standard 0.1 N alkali. The factor so obtained should agree with the calculated factor. STANDARD ALKALIS PREPARATION OF STANDARD ALKALI- N sodium hydroxide-Standard solutions of sodium hydroxide are best prepared from a solution having a density of 1.525 g. per ml. at 20" C. and containing 50 per cent.of sodium hydroxide. The solubility of sodium carbonate in this solution is almost negligible. Solutions of this strength can be filtered through ordinary stout filter-paper and yield a clear filtrate. If the 50 per cent. solution is not available, dissolve 106 parts by weight of commercially * For purposes where the highest accuracy is required, it is necessary to use for both the determination and the standardisation water that has previously been brought to the mid-point of the pH range of the indicator employed.692 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS [Vol. 75 pure powdered sodium hydroxide in 100 parts by weight of water in an enamelled-iron, nickel, silver or stainless steel pan. Allow a stock solution prepared in this way to stand for a few days, and filter it through paper into hard-glass bottles.About 53 ml. of this solution are required for a litre of N sodium hydroxide. Use boiled-out water for the dilution to normal strength, and set the resulting solution aside for several days to allow traces of iron and manganese to settle as a precipitate of hydroxides to the bottom of the bottle. Siphon off the clear solution into storage bottles fitted with rubber bungs of good quality. 0.1 N sodium hydroxide-Prepare 0.1 N sodium hydroxide by diluting the N solution with boiled-out water. 0.1 N and 0.01 N barium hydroxide-For most purposes 0.1 N and 0-01 N barium hydroxide are preferable to the corresponding solutions of sodium hydroxide. Prepare standard solutions of barium hydroxide from a stock solution saturated with pure crystalline barium hydroxide.Store the stock solution undisturbed for several days and siphon off the clear liquid as required. The saturated solution has an approximate strength of 0.4 N . Prepare the standard solutions with boiled-out water in an atmosphere as free as possible from carbon dioxide, preferably in the open air, and allow them to stand several days to deposit any traces of barium carbonate. Fit soda-lime guard tubes to all containers from which standard solutions of alkalis are to be siphoned. STANDARDISATION OF ALKALIS- N and 0.5 N sodium hydroxide-Standardise N and 0.5 N sodium hydroxide by titrating with an accurately measured volume, about 9Oml., of the standard acid of corresponding normality, using a 105-ml.bulb burette, graduated from 80 to 105 ml. in 0.05 ml., and bromo- phenol blue as indicator. A standard buffer solution of pH 3.9 containing the same relative amount of indicator may be used to facilitate the determination of the end-point. When the phenolphthalein factor of the standard alkali is required it is necessary to remove carbon dioxide from the standard acid. For this take an accurately measured volume of the standard acid, dilute it to 200 ml. with water, boil for 5 minutes to expel carbon dioxide, cool the solution out of contact with atmos- pheric carbon dioxide and titrate with the alkali, with phenolphthalein as indicator. 0.1 N sodium hydroxide-Standardise 0.1 N sodiuni hydroxide by titration of accurately measured volumes of about 90 ml. of standard 0.L N acid, using as indicators bromophenol blue, methyl red or phenolphthalein.It must be borne in mind that carbon dioxide begins appreciably to affect indicators that give an end-point above pH 4.0 and consequently carbon dioxide must be removed from the standard acid where an accurate methyl red or phenolphthalein factor is required. 0.1 N and 0.01 N barium hydroxide-Standardise 0.1 N and 0.01 N barium hydroxide by titrating accurately measured volumes, about 90 ml., of the corresponding standard hydro- chloric acid solutions which have been freed from carbon dioxide by boiling after dilution with water. Use phenolphthalein as indicator. Re-standardise 0.1 N and 0.01 N alkalis at comparatively frequent intervals if reliance is to be placed on results obtained with them.The acid and alkali must be at the same temperature. When more than one indicator is used, factors are given for each indicator. CORRECTIONS OF VOLUME OF STANDARD SOLUTIONS OF ACIDS AND ALKALIS FOR A REFERENCE TEMPERATURE OF 20" c.- Where the measurement of volumetric solutions of acids of alkalis at temperatures other than the normal reference temperature (20' C.) is unavoidable, adjust the observed volume at temperature to C . by means of the corrections given in Table IV. OXIDIMETRY The relationship between the solutions used in oxidimetry is shown in the scheme in Fig. 1 (p. 577). GENERAL REMARKS- Several methods are available for the standardisation of sodium thiosulphate solutions, but a direct titration with standard iodine (method 1) is most frequently used.When the SODIUM THIOSULPHATENov., 19501 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS 693 solution is used for the determination of copper, standardisation with pure copper (method 2) is to be preferred. Other methods that may be employed in special circumstances involve (3) a direct reference to secondary standard potassium dichromate by titration, the dichromate being used for the liberation of iodine from potassium iodide, (4) titration of the iodine liberated from an acid solution of potassium permanganate to which potassium iodide has been added, TABLE IV CORRECTIONS IN ML. TO BE APPLIED TO REDUCE 1000 ML. MEASURED AT to C. TO VOLUME AT 20” c. (SCHLOESSER1* AND SCHOORL15) Correction in ml. A I Temperature, t “C. 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 7 0.1 N 1.1 1.0 0.9 0.8 0.6 0-5 0.3 0.2 Hydrochloric acid 0-5 N 1.4 1-2 1-1 0.9 0.8 0.6 0.4 0.2 A 7 N 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 Sodium hydroxide r 0.1 N 0.5 N 1.3 1.7 1.1 1.5 1.0 1.3 0.9 1.1 0.7 0.9 0.6 0.7 0.4 0-5 0.2 0.2 1 Nitric acid, sulphuric acid and sodium hydroxide N 2.0 1.8 1.6 1.3 1.1 0.8 0.6 0.3 0.0 0-0 0.0 0.0 0.0 0.0 - 0.2 - 0.4 - 0.6 - 0.8 - 1.0 - 1.3 - 1.5 - 1.8 - 0.2 - 0.4 - 0.7 - 0.9 - 1-1 - 1-4 - 1.7 - 2.0 - 0.2 - 0.5 - 0.7 - 1.0 - 1-2 - 1.4 - 1.7 - 2.0 - 0.2 - 0.4 - 0.8 - 0-9 - 1.1 - 1.4 - 1.7 - 2.0 - 0.2 - 0.5 - 0.8 - 1.0 - 1.3 - 1.5 - 1.8 - 2.1 - 0.3 - 0.6 - 0.9 - 1.2 - 1-6 - 1.8 - 2.1 - 2.4 -2.1 - 2.3 - 2.3 - 2.3 - 2.4 - 2.8 the potassium permanganate solution having been referred to secondary standard arsenious oxide, or (5) titration of a solution of iodine which has been referred to secondary standard arsenious oxide.The stoicheiometric relations involved in the above reactions may be expressed as follows- (1) 2Na,S203 + I, = 2NaI + N+S406 (3) K2Cr,0, + 6KI + 14HC1= 31, + SKU+ 2CrC1, + 7H,O (4) 2KMn0, + lOKI + 16HC1= 51, + 12KCl+ 2MnC1, + 8H20 (5) 21, + As,03 + 2H20 = As205 + 4HI. When a solution of thiosulphate is allowed to stand and the titre is examined at intervals, it is sometimes observed that there is an initial rise in the value, followed by a gradual fall with the simultaneous deposition of sulphur. This increase in titre is attributed to the con- version of polythionate, present as an impurity, into thiosulphate- (2) 2Cu(NO3)2 + 4KI = 1 2 + 4KNO3 + CuJ, 4s40,” + 60H’ = 5S2o3’‘ + 2s306” + 3H20.The subsequent decrease in the titre is ascribed to such influences as (a) light and (b) micro-organisms, the influence of which may be reduced to some extent by the use of salt of high purity, by storing the solution in dark-coloured glass bottles and by the preparation of the solution with water free from carbon dioxide. The effect of micro-organisms, particularly sulphur bacteria, can be further reduced by taking care to boil thoroughly in water any rubber bungs and rubber tubing that are to be used with solutions of sodium thiosulphate. Sodium thiosulphate solution does, however, deteriorate slowly, and it is advisable to re-standardise at weekly intervals any solutions that are intermittently exposed to the air.If the solution is protected from carbon dioxide, less frequent checking is necessary.594 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS [Vol. 75 PREPARATION OF 0.1 N SODIUM THIOSULPHATE--. Weigh 25 g. of crystallised sodium thiosulphate for each litre of 0.1 N solution required, Dissolve the salt in water free from carbon dioxide and dilute to the required volume. Allow the solution to stand in the dark for at least a fortnight and, at the end of this period, siphon off the supernatant liquid without disturbing the deposit of sulphur. Determine the strength of the solution by titration with 0.1 N iodine and, if necessary, add sufficient water to give a solution which is as nearly as possible 0.1 N . Shake the solution well and proceed with the standardisation as follows.STANDARDISATION OF 0-1 N SODIUM THIOSULPHATE- Place approximately 5 g. of pure potassium iodide, free from iodate, and 0.5 ml. of water in a small weighing bottle. Insert the stopper in the weighing bottle, place the whole in a balance case for half an hour and weigh. Remove the stopper from the bottle and with the least possible delay add 1.1 to 1.2 g. of working standard iodine. Replace the stopper and weigh the whole again. Rotate the bottle gently to ensure that all the iodine is dissolved. Place in a 500-ml. conical flask 100 ml. of boiled-out water and add 85 ml. of the 0.1 N sodium thiosulphate from a 105-ml. bulb burette. Hold the flask in an inclined position, loosen the stopper of the bottle containing the iodine and allow the bottle and stopper to drop into the liquid.Promote solution of the iodine by swirling the liquid gently and then add more of the thiosulphate solution until the liquid in the flask is pale yellow. Add 2 to 3 ml. of starch solution and continue the titration slowly, shaking well after each addition of reagent, until the blue colour has just disappeared. 0.01 N SODIUM THIOSULPHATE- Prepare the solution by accurate dilution of 0.1 N sodium thiosulphate with water free from carbon dioxide, and calculate the factor. Introduce 1 g. of potassium iodide into a 200-ml. conical flask. Add from a 5- or 10-ml. burette 4.50 ml. of standard 0.1 N iodine accurately measured; titrate the solution immediately with 0-01 N sodium thiosulphate, delivered from a 50-ml. burette, until the colour of the iodine has nearly disappeared.Add 2ml. of starch solution and continue the titration, shaking the liquid vigorously after each addition, until the blue colour is just discharged. The factor so obtained should agree with the calculated factor. Check this factor by titration, as follows. IODINE GENERAL REMARKS- Standard solutions of iodine deteriorate slowly owing to volatilisation of iodine, and in order to reduce this effect to a minimum it is necessary to have a concentration of at least 2 per cent. of potassium iodide in the diluted solution. Solutions of iodine are affected (a) by light, so they should therefore be stored. in dark-coloured glass bottles, and (b) by organic matter, hence contact with cork or rubber in any form should be avoided. Iodine solution is standardised with standard arsenious oxide.It is necessary in the titration of arsenious acid with iodine to keep the solution approximately neutral to phenol- phthalein, and this is accomplished by the addition of sodium bicarbonate. The stoicheiometric relation may be expressed as follows- As,O, + 21, + 8NaHC0, = 2Na,HAsO, + 4NaI + 3H,O + KO,. PREPARATION OF 0.1 N IODINE- Weigh roughly 12.7 g. of re-sublimed iodine, mix it with 26 g. of potassium iodide that is free from iodate, add 30ml. of water and alllow the whole to stand until solution of the iodine is complete. Finally dilute to 1 litre with water and mix well. STANDARDISATION OF 0.1 N IODINE- Weigh accurately 0.42 to 0.45 g. of secondary working standard arsenious oxide, previously dried to constant weight at 105" C. ; transfer it to a, 500-ml.conical flask, add 20 ml. of N sodium hydroxide and warm the flask to aid solution. Cool to room temperature, add 15 ml. of N hydrochloric acid to neutralise the excess of alkali, dilute to 200ml. and add 20ml. of a saturated solution of sodium bicarbonate. Titrate the solution with iodine solution, whichNov., 19501 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS 595 should be added from a 105-ml. bulb burette; towards the end of the titration add 2 to 3 ml. of starch solution as indicator, and continue to add the iodine until a pale purple-pink colour is obtained throughout the solution. 0.01 N IODINE- Prepare the solution by accurate dilution of 0.1 N iodine and calculate the factor. This factor may be checked by means of 0.1 N sodium thiosulphate as follows.Place 1 g. of potassium iodide that is free from iodate, 50 ml. of water and 2 ml. of starch solution in a 250-ml. conical flask, and add 4-50 ml. of standard 0.1 N sodium thiosulphate accurately measured from a 5- or 10-ml. burette. Titrate the solution with 0.01 N iodine, added from a 50-ml. burette, until the liquid becomes pale blue. The factor found in this way should agree with the calculated factor. SODIUM ARSENITE PREPARATION OF 0.1 N SODIUM ARSENITE- Weigh 4.948 g. of sublimed arsenious oxide, transfer it to a 500-ml. beaker, add 200 ml. of N sodium hydroxide and warm until the solid is completely dissolved. Then add 100 ml. of N hydrochloric acid to neutralise the excess of alkali, cool the solution to room temperature, transfer it to a 1-litre graduated flask, dilute to the mark and shake well.STANDARDISATION OF 0-1 N SODIUM ARSENITE- Measure 85ml. of the solution from a bulb burette into a conical flask containing 2g. of iodate-free potassium iodide, 100 ml. of water and 20 ml. of a saturated solution of sodium bicarbonate. Add 2 to 3 ml. of starch solution as indicator and titrate with standard 0.1 N iodine until a pale purple-pink colour is obtained throughout the solution. POTASSIUM PERMANGANATE GENERAL REMAKKS- solution as indicated by the following equations- Potassium permanganate may serve as an oxidising agent either in acid or alkaline (Acid medium) 2KMn0, = K20 + 2Mn0 + 50 (Alkaline medium) 2KMn0, = K20 + 2Mn0, + 3 0 Oxidation in acid medium in which the permanganate is reduced to the manganous state is nearly always employed in analysis and is, therefore, used throughout the methods of standardisation.Potassium permanganate solutions are conveniently standardised by means of secondary standard arsenious oxide according to the following equation- 4KMn0, + 5AS2O3 + 6H2S0, = 2K.30, + 4MnS0, + 6As20, + 6H20. An alternative method that is given is based on the liberation of the equivalent of iodine in an acid solution of potassium iodide, followed by titration with standard sodium thiosulphate solution. According to MacBridels and Popoff and Whitman,17 the order of mixing the reagents is a factor which should be carefully observed, since it has been shown that oxygen is lost when permanganate is allowed to accumulate in an acid solution.For this reason, the conditions recommended in such titrations are as follows. The potassium permanganate solution is added to an acidified solution of potassium iodide, the liberated iodine being subsequently titrated with thiosulphate solution. Potassium permanganate solutions are very susceptible to the influence of impurities such as manganese dioxide, which results in progressive decomposition of the solution, and organic matter. In view of this, the following precautions should be taken: (a) the solution should be filtered through a plug of glass wool or purified asbestos or a No. 4 sintered glass filter, (b) dark-coloured (amber) glass bottles should be used for storage and (c) the solution should be allowed to mature for two or three weeks before standardisation.PREPARATION OF 0.2 N POTASSIUM PERMANGANATE- Weigh 6.4 g. of recrystallised potassium permanganate for each litre of 0.2 N solution required, transfer the salt to a large porcelain mortar, add sufficient water to cover it and596 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS [Vol. 75 triturate the mixture with a pestle. Half fill the mortar with water, stir well, allow the undissolved permanganate t o settle and filter the solution through a plug of glass wool or a No. 4 sintered glass filter into the stock bottle. Continue to triturate the residue in the mortar, diluting with water and decanting the supernatant liquid as before until solution is complete. Dilute the bulk solution to the correct volume, shake it well and allow it to stand in the dark for a fortnight.At the end of this period, siphon off the solution without disturbing the sediment formed by the slight decomposition of the permanganate. Determine the titre as described below and, if necessary, add sufficient water to give a solution that is as nearly as possible one-fifth normal. Shake the solution well and proceed with the standardisation. STANDARDISATION OF 0-2 N POTASSIUM PERMANGANATE WITH SECONDARY WORKING STANDARD ARSENIOUS 0 x 1 ~ ~ ~ ~ - Accurately weigh 0.46 to 0.48 g. of arsenious oxide that has recently been dried at 105" C. and transfer it to a 600-ml. beaker. Add 20ml. of a cold 20 per cent. solution of sodium hydroxide that is free from oxidising or reducing substances. When all the arsenious oxide is dissolved, add 2OOml.of water, 20ml. of 10M hydrochloric acid, and 1 drop of 0.0025M potassium iodate or potassium iodide solution. Titrate with the permanganate solution until a faint pink colour persists for 30 seconds. Add the last 1.0 to 1.5 ml. dropwise, allowing each drop to become decolorised before the next is introduced. Determine the volume of permanganate required to duplicate the faint pink colour of the end-point. This is done by adding permanganate to a solution containing the same amounts of alkali, acid and catalyst as were used in the test. The correction should amount to less than 0.03 ml. Allow the whole to stand for 8 to 10 minutes with occasional stirring. Calculate the factor from the relation- 1.000 g. of As,O, = 101.10 ml. of 0.2 N KMnO,. PREPARATION OF 0.1 N POTASSIUM PERMANGANATE- required and proceed as above under the heading of 0.2 N potassium permanganate.Weigh 3.2 g. of recrystallised potassium permanganate for each litre of 0-1 N solution STANDARDISATION OF 0.1 N POTASSIUM PERMANGANATE- (a) With secondary working stafzdard arsenious oxidel*-Accurately weigh 0-23 to 0-24 g. of arsenious oxide that has recently been dried at 105" C. and transfer it to a 400-ml. beaker. Add 10 ml. of a cold 20 per cent. solution of sodium hydroxide that is free from oxidising or reducing substances. Allow the whole to stand for 8 to 10 minutes, with occasional stirring. When all the arsenious oxide is dissolved, add 100 ml. of water, 10 ml. of 10 N hydrochloric acid and 1 drop of 0.0025 M potassium iodate or potassium iodide solution.Titrate with the permanganate solution until a faint pink colour persists for 30 seconds. Add the last 1.0 to 1.5 ml. dropwise, allowing each drop to become decolorised before the next is introduced. Determine the volume of permanganate required to duplicate the faint pink colour of the end-point. This is done by adding permanganate to a solution containing the same amounts of alkali, acid and catalyst as were used in the test. The correction should amount to less than 0.03 ml. Calculate the factor from the relation- 1.000 g. of As,O, = 202.20 ml. of 0.1 N KMnO,. (b) With 0.1 N sodium thiosuZj5hate-Place 100ml. of boiled-out water in a 600-ml. glass-stoppered flask and add 15 ml. of 10 per cent. hydrochloric acid and 3 g.of potassium iodide. Run into the flask an accurately measured volume of about 85 ml. of potassium permanganate solution from a 105-ml. bulb burette, and then titrate the liberated iodine with standard 0.1 N sodium thiosulphate (recently standardised with working standard iodine) until a pale yellow colour is obtained. Add 2 to 3 ml. of starch solution and continue the titration, shaking vigorously after each addition, until the blue colour has just disappeared.Nov., 19501 THE STANDARDISATION OF VOLUMETBIC SOLUTIONS 597 0.1 N CERIC SULPHATE GENERAL REMARKS- following equation- Ceric s ~ l p h a t e l ~ ? ~ ~ behaves as an oxidising agent in acid solution as indicated by the 2Ce(S04), + H20 = Ce2(S04), + H2S04 + 0. They are quite stable, they have a high oxidation potential, and many of the reactions in which they are employed take place instantaneously and in the cold.For these reasons standard ceric sulphate can be employed with advantage in place of standard potassium permanganate in most, if not all, determinations where the latter is customarily used. The colour change corresponding to the reduction of the ceric to the cerous state is not very marked, but this difficulty is readily overcome by the use of an appropriate oxidation - reduction indicator ; o-phenanthroline ferrous sulphate ("Ferroin") is generally the most valuable indicator for the purpose. N-phenyl anthranilic acid is also useful when ceric sulphate solutions are titrated with ferrous salts. Solutions of other ceric salts, in' particular ceric ammonium nitrate, are useful for certain determinations.Their preparation is described in the literature,19 p2* to which reference should be made; they are standardised against arsenious oxide in the presence of iodine monochloride or in the presence of osmic acid as described below. The stoicheiometric relationship involved in the standardisation of ceric sulphate by means of arsenious oxide is expressed as follows- PREPARATION OF SOLUTION- Add 600 ml. of concentrated sulphuric acid slowly to 600 ml. of water contained in a 4-litre beaker and stirred with a large mechanical stirrer; immerse a thermometer in the solution. When the temperature of the dilute acid falls to 100" C., add 420 g. of technical quality ceric oxide in quantities of a few grams at a time, and a t such a rate that the temperature remains between 95" and 105" C.; the reaction is exothermic.When all the ceric oxide has been added, allow the temperature to fall to about 90" C., add about 2 litres of water and stir the solution for a further 15 to 20 minutes; finally divide it into two equal portions, dilute each to about 4 litres, stir each for about 15 minutes and allow the insoluble matter to settle. Place a Whatman No. 541 filter-paper in the funnel, fit the funnel to a large filter flask and add sufficient good quality diatomaceous earth in the form of a slurry to produce a thin layer on the surface of the paper and act as a "filter-aid." Suck the excess water from the funnel and wash the layer with water until the filtrate is free from fine particles.Discard the washings and rinse the flask thoroughly. Filter the ceric sulphate solution through the prepared paper, transfer the insoluble matter to the filter and wash it several times with water. Finally transfer the filtrate to a graduated 10-litre aspirator and mix the solution well. STANDARDISATION- Weigh out 200 to 240 mg. with an accuracy of 0.1 mg. and transfer it to a 250-ml. beaker. Add approximately 1 g. of A.R. grade sodium hydroxide and 10 ml. of water. Warm the mixture slightly and stir it until the arsenious oxide has dissolved completely; then add 30 ml. of water and 10ml. of 9 M sulphuric acid, stir the liquid and add a further 50ml. of water. Add 3 drops of a 0.01 M solution of osmium tetroxide in 0.1 N sulphuric acid and 1 drop of Ferroin indicator, and add the ceric sulphate from a 50-ml.bulb burette until the colour changes from red to colourless. Calculate the normality of the ceric sulphate solution and, if necessary, adjust it by dilution with 2 N sulphuric acid; re-standardise it as above. GENERAL REMARKS- The reaction, which takes place readily at ordinary temperature in presence of either sulphuric or hydrochloric acid, is expressed as follows- 6FeS0, + K2Cr20, + 7H2S0, = 3Fe2(S0,), + Cr,(SO,), + K,S04 + 7H20. Solutions of ceric sulphate possess certain characteristics. As203 + 4Ce(S04), + 2H20 = As20, + 2Ce2(S0,), + 2H2S04. Prepare a 12.5-cm. Buchner funnel as follows, Dry a sufficient amount of arsenious oxide at 105" C. and cool it in a desiccator. POTASSIUM DICHROMATE The chief use of 0.1 N potassium dichromate is in the determination of ferrous iron.598 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS [Vol.75 The salt is easily obtained in a state of high purity by recrystallisation from water, and material of A.R. quality that has been ground to powder and dried to constant weight at 150" C. is usually suitable for making the standard solution; the criterion of suitability is the comparison with the secondary working standard potassium dichromate by the methods given on p. 586 for the standardisation of the latter. The solution, which is quite stable, is best made from an accurately weighed amount of the purified salt diluted to an exact volume, because the calculated concentration is more reliable than the figure obtained by standardisation. Should the concentration be in doubt, however, it may be verified by titration of a solution of ferrous sulphate the strength of which has recently been determined by reference to working standard potassium dichromate.Diphenylamine is a satisfactory internal indicator for use with potassium dichromate and has almost completely superseded potassiurn ferricyanide, which can only be used externally. It should be noted that diphenylamine may be used when the iron has been reduced to the ferrous state by means of stannous chloride and the excess of the latter removed by addition of mercuric chloride. Knop21 has shown that if phosphoric acid is added the titration is reversible. PREPARATION OF 0-1 N POTASSIUM DICHROMATE-- Weigh accurately 4.9035 g. of pure potassium dichromate, previously dried to constant weight at 150" C., for each litre of solution required, dissolve the salt in water, dilute the solution to the correct volume and mix it well.VERIFICATION OF CONCENTRATION OF 0.1 N POTASSIUM DICHROMATE- The method which follows is to be used where it is necessary to verify the factor of 0.1 N potassium dichromate. Measure accurately about 85 ml. of 0.1 N ferrous sulphate solution into a conical flask. Add 50 ml. of sulphuric - phosphoric acid reagent and 0.2 ml. of diphenylamine indicator. Dilute the solution to about 200ml., and titrate it with the potassium dichromate until the green colour deepens to an intense blue-violet that remains permanent after shaking. Determine the titre of the 0.1 N ferrous sulphate by carrying out a similar titration with a solution prepared by dilution of an accurately weighed amount of the secondary working standard potassium dichromate.From the results thus obtained calculate the factor of the potassium dichromate solution under test. GENERAL REMARKS- Solutions of ferrous sulphate gradually deteriorate owing to the absorption of atmospheric oxygen; because of this they require to be standardised immediately before use. The relatively more stable double salt, ferrous ammonium sulphate, is used in the preparation of the solution, and its hydrolysis is prevented by using dilute sulphuric acid as solvent. PREPARATION OF 0.1 N FERROUS AMMONIUM SULPHATE- Weigh 39-2g. of ferrous ammonium sulphate hexahydrate and dissolve it in boiled-out water to which 100 mi.of 50 per cent. sulphuric acid have been added. Transfer the solution to a 1-litre graduated flask, dilute to the mark with boiled-out water and mix well. STANDARDISATION OF 0.1 N FERROUS AMMONIUM SULPHATE- Decinormal ferrous ammonium sulphate is standardised by means of 0.1 N potassium dichromate as follows. Measure accurately about 85 ml. of the 0.1 N ferrous ammonium sulphate solution into a conical flask. Add 50ml. of sulphuric-phosphoric acid reagent and 0.2 ml. of diphenyl- mine indicator. Dilute to about 200ml. with boiled-out water, and titrate it with 0.1 N potassium dichromate until the green colour changes to an intense blue-violet, which remains permanent after shaking. TITANOUS CHLORIDE AND TITANOUS SULPHATE PREPARATION OF 0.2 N SOLUTIONS- (a) Titanous chZoride-Add 200 ml.of commercial titanous chloride solution (15 to 20 per cent. of TiClJ to 200ml. of concentrated hydrochloric acid, boil for 1 to 2 minutes in a flask, cool and dilute to loo0 ml. with boiled-out water. FERROUS AMMONIUM SULPHATENov., 19501 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS 599 Place the solution in the bottle, A, of the apparatus shown in Fig. 5. Siphon the solution over into the burette, B, by way of its two-way stopcock. Pour dilute sulphuric acid into the hydrogen generator, C, the bulb of which has been charged with granulated zinc. Turn the burette stopcock so as to run out the solution already in the burette and leave the stopcock B Fig. 5. Titanous sulphate titration apparatus open until the air above the solution and in the burette has been replaced by hydrogen from the generator.The apparatus is then allowed to stand for one day with occasional shaking before using the solution. (b) Titanous suZ@ate-Add 800 ml. of commercial titanous sulphate solution (15 per cent. of Ti,(SO,),) to 1000 ml. of dilute sulphuric acid consisting of 1 volume of 95 per cent. sulphuric acid to 4 volumes of water, and boil the mixture for 1 to 2 minutes. Cool to room temperature in a stream of hydrogen or carbon dioxide and dilute to 2000 ml. with boiled-out water. Store the solution under hydrogen as for the chloride.600 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS [Vol. 75 STANDARDISATION OF 0.2 N SOLUTIONS- The standardisation is carried out with 0.1 N potassium dichromate, using as intermediary 0.2 N ferric ammonium sulphate.As the titre of the latter remains constant, the concentra- tion of the titanous solution is conveniently expressed as the volume in ml. equivalent to 1 ml. of 0-2 N ferric alum solution of known factor. Measure into a 300-ml. conical flask 25 ml. of boiled-out water, 50.0 ml. of 0.1 N potassium dichromate and 25 ml. of 40 per cent. sulphuric acid. Fit the flask with a rubber bung bored with two holes, through one of which passes a leading tube reaching about 2 inches into the flask, and pass a rapid stream of carbon dioxide into the flask for several minutes in order to displace the air. Place the jet of the burette containing the titanous solution in the second hole of the rubber bung and, whilst continuing to pass carbon dioxide, run in 40.0 ml.of the titanous solution. Mix the liquids by swirling the flask gently, and titrate the excess of titanous salt with 0.2 N ferric ammonium sulphate added from a 25-ml. burette, the jet of which is placed inside the second hole in the rubber bung. Towards the end of the titration, i.e., when the dark colour due to the titanous salt has nearly disappeared, add 10 ml. of 10 per cent. ammonium or potassium thiocyanate and continue the addition of the iron solution until a pink colour is obtained that is permanent for 1 minute. Maintain a slow stream of carbon dioxide throughout the titration. In the same way titrate 40.0 ml. of the titanous solution with the 0.2 N ferric ammonium sulphate, the conditions being exactly the same, except for (a) the omission of the 0.1 N potassium dichromate and (b) the use of a 50-ml.burette for the iron solution. From the data thus obtained calculate the normality factor of the 0.2 N ferric ammonium sulphate and the volume of it that is equivalent to 1 ml. of the 0-2 N titanous solution. PREPARATION OF 0.025 N SOLUTION OF TITANOUS CHLORIDE- Add 25 ml. of commercial titanous chloride solution (15 to 20 per cent. of TiCl,) to 50 ml. of concentrated hydrochloric acid, boil for 1 to 2 minutes in a flask, cool and dilute to 1000 ml. with boiled-out water. STANDARDISATION- Carry out the standardisation exactly as described for 0-2 N titanous solutions, sub- stituting 0.025 N titanous solution for the 0.2 N , 0.0125 N potassium dichromate for the 0.1 N and 0.025 N ferric ammonium sulphate for the 0.2 N.FERRIC AMMONIUM SULPHATE PREPARATION OF 0.2 N , 0.1 N AND 0.025 N FERRIC AMMONIUM SULPHATE- Dissolve the quantity of ferric ammonium sulphate (iron alum) indicated below in 500 ml. of boiled-out water that has been acidified with 10 ml. of 50 per cent. sulphuric acid and dilute to 1000 ml. with boiled-out water. Store the solution under hydrogen as described above. For 0.2 N use 96-44 g. of ferric alum For 0.1 N )) 48.22 g. 33 For 0.025 N 79 12.05 g. 37 STANDARDISATION OF 0.2 N , 0.1 N AND 0.025 N FERRIC AMMONIUM SULPHATE- Carry out the standardisation of 0.2 N solutions by the method given above for 0.2 N Standardise the 0-1 N and 0.025 N solutions in the same way, with titanous solutions. modifications appropriate to the concentrations of the solutions.GENERAL REMARKS- Potassium bromate is used as an oxidising agent and, in the presence of potassium bromide, as a brominating agent. In the former. use the reducing agent to be titrated is acidified with hydrochloric acid and titrated hot with the bromate solution; any excess of bromate then reacts with the bromide produced in the reaction, equation (l), with liberation of bromine, equation (2), the free bromine being usually detected by the decolorisation of methyl orange, or potentiometrically- POTASSIUM BROMATE (1) KBrO, + 6HC1+ 3SbC1, = 3SbC1, + KBr + 3H20 (2) KBrO, + 5KBr + 6HC1= 6KC1+ 3H20 + 3Br,.Nov., 19501 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS 601 When potassium bromate is used as a brominating agent, an excess of potassium bromide is added to the reaction mixture before adding the bromate solution.Equation (2) shows the reaction. It is usual to add an excess of the bromate solution and to determine the excess by addition of potassium iodide and titration with a standard solution of sodium thiosulphate. PREPARATION OF 0.2 N AND 0.1 N POTASSIUM BROMATE- Weigh 5.56 or 2-78 g., as appropriate, of potassium bromate for each litre of 0-2 N or 0.1 N solution required, dissolve the salt in cold water, transfer the solution to a litre flask, dilute to the mark and mix thoroughly. Standardise the resulting solution preferably by method (a), or, if it is to be used for the determination of arsenic or antimony, by method (b). STANDARDISATION OF 0.2 N AND 0.1 N POTASSIUM BROMATE- (a) With 0.2 N and 0.1 N sodium thiosulphate res$ectiveZy-Measure accurately about 40 ml.of the bromate solution into a 500-ml. glass-stoppered flask or bottle containing 150 ml. of water. Cool the flask and its contents under the tap, then add 4 g. of potassium bromide, 4 g. of potassium iodide and 5 ml. of concentrated hydrochloric acid; stopper the flask, well mix the solution and allow it to stand for 15 minutes. At the end of this period titrate the liberated iodine with 0.2 N or 0.1 N sodium thiosulphate, according to the normality of the potassium bromate solution, at a rate of 10ml. per minute until the end-point is almost reached; then add the sodium thiosulphate solution drop by drop. When the colour of the iodine is faint, add 2 or 3 ml. of starch solution and complete the titration, adding the thio- sulphate solution drop by drop, with shaking, until the blue colour just disappears.Calculate the normality factor of the bromate solution from the following equation- KBrO, + 6KI + 6HC1= KBr + 6KC1+ 3H,O + 31, (b) With standard arsenious oxide-Weigh accurately 0.40 g., or 0.20 g. for a 0.1 N solution, of secondary working standard arsenious oxide, previously dried to constant weight at 105" C., transfer to a 500-ml. conical flask and dissolve in 20 ml. of 1 M sodium hydroxide solution, warming to aid solution. When solution is complete add 25 ml. of water and 25 ml. of concentrated hydrochloric acid, density 1.16 at 20" C. Heat the solution almost to boiling and titrate while hot with the bromate solution, running the bromate in slowly with vigorous stirring until 36ml.have been added. Then add 0-2ml. of methyl orange solution and continue the titration until the colour of the indicator disappears. Carry out a second titration in which the indicator is not added until the volume of bromate added is within 0.2 ml. of the volume previously used. Complete the titration after adding the methyl orange and use the value so obtained. Calculate the normality factor of the bromate solution from the following equation- ZKBrO, + 3As,03 = 2KBr + 3As20,. PREPARATION OF 0.2 N AND 0.1 N BROMA4TE - BROMIDE- Weigh 5.56g. of potassium bromate and 30g. of potassium bromide, or 2.78g. of potassium bromate and 15 g. of potassium bromide, for each required litre of the 0.2 N or 0.1 N solution respectively. Dissolve the salts in cold water, transfer the solution to a 1-litre flask, dilute to the mark and mix thoroughly.A standard solution of bromate - bromide that is 0.2 N or 0.1 N in respect of bromine may also be prepared and used for bromination. STANDARDISATION OF 0.2 N AND 0.1 N BROMATE - BROMIDE SOLUTION- Measure accurately about 40 ml. of the bromate - bromide solution into a 500-ml. glass- stoppered flask or bottle containing 150 ml. of water. Cool the flask and its contents under the tap, add 4 g. of potassium iodide and 5 ml. of concentrated hydrochloric acid, stopper the flask, mix the solution well and allow it to stand for 15 minutes. At the end of this period titrate the liberated iodine exactly as described above for the standardisation of 0-2 N and 0.1 N potassium bromate with standard arsenious oxide.Calculate the normality factor of the bromate - bromide solution on the basis of the following equations- KBrO, + 5KBr + 6HCl= 6KC1+ 3H20 -+ 3Br, 3Br2 + 6K1= 6KBr + 31, 31, + 6Na$,03 = 6NaI + 3Na,S,O,.602 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS [Vol. 76 PRECIPITATION REACTIONS The relationship between the solutions used in precipitation reactions and the working standards is outlined in Fig. 1 (p. 577). SILVER NITRATE GENERAL REMARKS- periods provided they are stored in well-stoppered dark-coloured bottles. Standard solutions prepared from silver nitrate of good quality remain stable for long PREPARATION OF 0.1 N SILVER NITRATE- the salt in water in a graduated measuring flask, dilute to the mark and mix well. Weigh 17.0 g.of recrystallised silver nitrate for each litre of solution required; dissolve STANDARDISATION OF 0.1 N SILVER NITRATE- Transfer about 0.25 g. of working standard sodium chloride to a platinum crucible and dry it in an electrically heated air-bath at 270" C. until it is constant in weight. Use the weighed quantity of dried salt thus obtained for the standardisation by one of the following methods. (a) Mohr's method-Transfer the sodium chloride to a 400-ml. conical flask, washing the last traces of salt into the flask with water. On the assumption that the silver nitrate is exactly 0.1 N , calculate the volume required to precipitate all the chloride and add 0.2 ml. less than this. Shake the mixture vigorously and filter it through an 11-cm.Whatman No. 1 filter-paper into another 400-1111. flask and wash the paper once with water. Add a few drops of 10 per cent. potassium chromate to the filtrate and continue the titration until a faint reddish-brown colour persists throughout the solution. (b) Drechsel's modification of Volhard's method-Transfer the sodium chloride to a 360-ml. glass-stoppered flask, washing it in completely with a total of 130 ml. of water. Add 5 ml. of 1.5 M nitric acid. Calculate the volume of silver nitrate solution, assuming it to be exactly one-tenth normal, necessary to precipitate the sodium chloride, and add this amount, plus 0.2 ml. in excess, slowly and with vigorous shaking until the precipitate is coagulated. Filter the solution through a washed filter-paper and wash the precipitate by decantation several times, each time shaking the precipitate thoroughly with the washing water.Finally transfer the precipitate to the filter and complete the washing. To the combined filtrate and washings add 1 ml. of ferric ammonium alum solution for every 50 ml. of liquid present, and titrate the excess of silver nitrate with 0.01 N ammonium thiocyanate until a pale reddish- brown colour persists. Note-The 0.01 N ammonium thiocyanate for this titration should be standardised in terms of the silver nitrate solution with which it is to be used. (c) Electrometric method22-Transfer the weighed quantity of sodium chloride to a porcelain basin, and wash it in with 50 ml. of water. Add 5 ml. of 1.5 M nitric acid and place a silver wire electrode in the solution and connect it in series with a high-resistance galvano- meter, a tapping key and a reference electrode.The latter consists of a silver wire coated electrolytically with silver chloride and immersed in 0.1 N nitric acid containing 0-05ml. of 0.1 N silver nitrate per 100 ml. in a small wide-mouthed bottle. Make electrolytic contact between the titrating vessel and the reference electrode by means of an H-type salt-bridge, the limbs of which are filled with saturated potassium sulphate or ammonium nitrate solution. Titrate the prepared solution with the silver nitrate solution, and after each addition close the circuit momentarily by depressing the tapping key. Mechanical stirring of the liquid is advisable. The end-point is reached when the galvanometer needle remains stationary or the direction of its swing is reversed.Check the e.m.f. of the reference electrode at frequent intervals by means of a potentiometer. PREPARATION OF 0.01 N SILVER NITRATE- Prepare the solution by accurate dilution of 0.1 N silver nitrate and calculate the factor. The calculated factor may be checked by one of the three methods used for the standardisation of 0.1 N silver nitrate as follows.Nov., 19501 THE STANDARDISATION OF VOLUMETRIC SOLUTIONS 603 STANDARDISATION OF 0.01 N SILVER NITRATE- Transfer about 0.25g. of working standard sodium chloride to a platinum crucible and dry it in an electrically heated air-bath at 270" C . until it is constant in weight. Transfer the sodium chloride to a 500-ml.graduated flask, washing it in completely with water; dilute the solution to the mark and mix well. Measure 50.0 ml. of this solution and titrate it with the 0.01 N silver nitrate by methods (a), (b) or (c) as given under 0.1 N silver nitrate. The factor so obtained should agree with the calculated factor. AMMONIUM THIOCYANATE GENERAL REMARKS- Thiocyanate solutions are standardised by direct titration with standard silver nitrate; sufficient nitric acid is added to discharge the brown colour of the iron alum indicator. When thiocyanate is added to a solution of silver nitrate, white insoluble silver thiocyanate is pre- cipitated in accordance with the equation- AgNO, + NH,CNS = NH,NO, + AgCNS When the silver has been completely precipitated, the excess of thiocyanate is indicated by the appearance of a red-brown colour due to the formation of ferric thiocyanate.Ammonium thiocyanate is hygroscopic, so that the salt cannot be weighed out and dissolved to give a one-tenth normal solution with sufficient accuracy. It is, therefore, necessary to prepare a solution slightly stronger than decinormal in order to obtain an exact solution by final adjustment when the strength has been determined by titration with silver nitrate as described below. Thiocyanate solutions are not very stable. PREPARATION OF 0.1 N AMMONIUM THIOCYANATE- in water, transfer the solution to a graduated flask, dilute it to the mark and shake well. Weigh 8 g. of ammonium thiocyanate for each litre of solution required, dissolve the salt STANDARDISATION OF 0.1 N AMMONIUM THIOCYANATE- Measure accurately 40.0 ml. of the ammonium thiocyanate solution into a 450-ml. flask provided with a ground-glass stopper. Add 115 ml. of water and 5 ml. of 1-5 M nitric acid. Run into the flask, slowly and with vigorous shaking, a very slight excess of standard 0.1 N silver nitrate and shake the flask vigorously until coagulation of the precipitate is complete. Excess of silver nitrate is indicated by the fact that as long as ammonium thiocyanate is present the mixture remains milky, but as soon as the silver nitrate is in excess the precipitate coagulates and settles at once. Filter the solution through a washed filter-paper and wash the precipitate several times by decantation, each time shaking the precipitate thoroughly with the washing water. Finally transfer the precipitate to the filter and complete the washing. To the combined filtrate and washings add 1 ml. of ferric ammonium alum for every 50 ml. of liquid, and titrate the excess of silver with 0.01 N ammonium thiocyanate until a pale reddish-brown colour persists. Calculate the factor by means of the following expression- C B -- lo Factor = - A A = ml. of 0.1 N NH,CNS taken B = ml. of 0.1 N AgNO, added C = ml. of 0.01 N NH,CNS required. where PREPARATION OF 0.01 N AMMONIUM THIOCYANATE- the factor, This factor may be checked as follows. Prepare this solution by accurate dilution of 0.1 N ammonium thiocyanate and calculate STANDARDISATION OF 0.01 N AMMONIUM THIOCYANATE- Accurately measure 4.50 ml. of 0.1 N silver nitrate from a 5- or 10-ml. burette into a 250-ml. stoppered flask, add 50ml. of water, 5 ml. of 1.5 M nitric acid, and 2 ml. of ferric604 MANLEY: THE ANALYSIS OF PEI‘ROL - KEROSINE MIXTURES, ammonium alum solution and then titrate with the 0.01 N ammonium thiocyanate, shaking vigorously after each addition until a pale reddish-brown colour persists. The factor thus obtained should agree with the calculated factor. [Vol. 75 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. REFERENCES Hinks, E., Analyst, 1930, 55, 238. Rosin, J., “Reagent Chemicals and Standards,” Chapman & Hall, Ltd., London, 1937. Richards, T. W., and Wells, R. C., “A Revision of the Atomic Weights of Sodium and Chlorine,’ Washburn, E. W., J . Amer. Chem. Soc., 1908, 30, 32. Menzies, A. W. C., and McCarthy, F. N., Ibid., 1916, 37, 2021. Chapin, R. M., Ind. Eng. Chem., 1918, 10, 522. -, J . Amer. Chem. Soc., 1919, 41, 351. Gooch, F. A., “Methods in Chemical Analysis,’’ J. Wiley & Sons, New York, 1912. Collins, W. D., Farr, H. V., Rosin, J., Spencer, G. C., and Wichers, E., Ind. Eng. Chem., 1927, Briihns, G., J . pr. Chem., 1916, 93, 73 and 312; Chem. Abs., 1916, 10, 2674 and 2857. Kolthoff, I. M., and Furman, N. H., “Volumetric Analysis,” 2nd Ed., Vol. 11, Interscience Pub- Publication No, 28, The Carnegie Institute of Washington, 1905. 19, 1369. lishers. Ltd.. London. 1947. Collins, W. D.; Farr, H. V., Rosin, J., Spencer, G. C., and Wichers, E., Ind. Eng. Chem., 1925, 17. 759. British Standard Specification No. 846, “Burettes and Bulb Burettes,” British Standards Schloesser, W., Chem. Ztg., 1905, 29, 510. Schoorl, N., Chem. Weekbl., 1926, 23, 581. McBride, R. S., J . Amer. Chem. SOC., 1912, 34, 393. Popoff, S., and Whitman, J. L., Ibid., 1925, 47, 2259. Fowler, R. M., and Bright, H. A., J . Res. Nat. Bur. Stand., 1935, 15, 493. Smith, G. F., “Cerate Oxidimetry,” G. Frederick Smith Chemical Co., Columbus, Ohio, 1942. Smith, G. F., and Richter, F. P., “Phenanthroline and Substituted Phenanthroline Indicators,” Knop, J., J . Amer. Chem. SOC., 1924, 46, 263. Callan, T., and Horrobin, S., J . SOC. Chem. Ind., 1928, 47, 3 2 9 ~ . Institution, London, 1939. G. Frederick Smith Chemical Co., Columbus, Ohio, 1944. COMMUNICATED BY C. R. N. STROUTS, SECRETARY THE ANALYTICAL CHEMISTS’ COMMITTEE O F IMPERIAL CHEMICAL INDUSTRIES LIMITED NOBEL DIVISION STEVENSTON, AYRSHIRE March, 1960