CHAPTER X APPLICATION OF SOLUBILITY PRODUCT PRINCIPLES GRAVIMETRIC AND VOLUMETRIC DETERMINATION OF CHLORINE. INDIRECT DETERMINATIONS 169. Gravimetric Determination of Chlorine. General Remarks. This determination is based upon the precipitation of chloride ion as silver chloride by means of a solution of silver nitrate.1 The reaction is, if we take the case of sodium chloride as an illustration of a soluble chloride, NaCl + AgNO3 → AgCl ↓ + NaNO3 or writing the reaction ionically: As silver ion also forms insoluble salts with bromide, iodide thiocyanate, cyanide and phosphate ions, these ions must be absent. In the case of the cyanide and phosphate ions, however, their concentration can be sufficiently diminished so that they will not interfere if the acidity of the solution is sufficiently increased (see § 159). This remark also applies to the presence of hydroxyl ion. It is the practice, therefore, when precipitating silver chloride to render the solution slightly acid with nitric acid, i.e., its content of nitric acid should be almost 0.002 molar. 170.-Silver chloride is one of the most insoluble precipitates, its solubility in pure water at 25° being about 1.8 mg. per liter. In washing this precipitate, however, the wash water should be slightly acidulated with nitric acid (0.01 M HNO3 is a suitable washing solution) because if the precipitate is washed with pure water it tends to pass into the colloidal condition and 1 The chlorine cannot be determined in this way in certain salts in which the chlorine exists more or less in the form of a complex ion in association with the metallic radicle of the salt. Examples of such salts are mercuric chloride, stannous chloride, antimony chloride, platinic chloride, and chromic chloride. so go through the filter. When the washing is complete, i.e., when a fresh portion of the filtrate shows no test for silver ion, the nitric acid should be removed by washing the precipitate twice with distilled water. The use of a Gooch crucible is much to be preferred over filter paper in the filtration of silver chloride because in the subsequent and necessary ignition of the paper, the resulting carbon reduces some of the silver chloride to metallic silver, and this of course must be converted back to the chloride, a procedure which is both time-consuming and bothersome. Silver chloride exhibits the phenomenon of becoming lightstruck. This phenomenon is evidenced by the precipitate's acquiring a purplish blue cast that varies in depth of color according to the intensity and duration of exposure. As this light action results in the formation of a sub-halide of silver with attendant loss of chlorine, silver chloride should always be protected from the influence of daylight as much as possible and under no conditions ever be exposed to direct sunlight. Silver chloride is not very stable toward the action of heat; it may be dried safely at 110°-115°; at 451° it fuses, and if heated much above this temperature it begins to decompose. 171. Summary of Conditions. The general conditions of the determination are as follows. The amount of sample to be weighed should not contain more chlorine than corresponds to 0.500 g. of silver chloride. The volume of the solution from which precipitation is effected should be about 150-200 c.c., and should just be rendered acid by an excess of about 5 c.c. 0.1 molar HNO3. The precipitation should be conducted at room temperatu e (18°-25°) by adding the precipitant dropwise and only a slight excess of precipitant should be added beyond the calculated amount. After the precipitant is known to be in excess, the solution should be heated to about 90°-95° to coagulate the precipitate and thus facilitate subsequent filtration. Throughout the whole procedure the silver chloride must be protected from daylight as much as possible for the reason that it gets light-struck as mentioned above. In filtering the precipitate, a Gooch crucible is much to be preferred, although filter paper may be used. In washing the precipitate free of excess silver nitrate solution, 0.01 molar nitric acid should be used as the washing medium and when a fresh portion of the filtrate shows no test for silver ion, the nitric acid should be removed by washing twice with pure water. The subsequent treatment now depends on whether a Gooch crucible or a filter paper was used for filtration: if a Gooch crucible, the precipitate may be dried at 115° and weighed; if a filter paper, the procedure mentioned in § 173 must be followed. Precision. If all due attention and care are paid to details, the precision which is attainable in the gravimetric determination of chlorides as silver chloride is 1 part per 1000. In the hands of beginning students it is seldom that such a precision is obtained, their results ranging from 3 to 30 parts per 1000 low. 172. Exercise No. 7. Gravimetric Standardization of Approx. 0.1 Molar HCl by Precipitation as AgCl. Use of Gooch Crucible.Make up 1 liter of approx. 0.1 M HCl.2 Prepare two porcelain Gooch filters as directed in § 46. Select two 300 c.c. beakers, and measure accurately into each by means of a standard pipette or burette 25.00 c.c. of the acid. Add about 150 c.c. of water and 5 c.c. of 0.1 molar HNO3. Protect the solution from strong daylight and add dropwise from a pipette and with constant stirring 0.1 molar AgNO, until in slight excess (this excess should not exceed 5 c.c.). After this addition but not before, heat the solution to about 90°-95° and stir until the precipitate coagulates, so that it will settle and leave a clear supernatant solution. Allow the precipitate to settle, and add to the clear solution one or two drops of the silver nitrate solution to make sure that an excess of precipitant is present. If there is no further precipitation, cover the beaker with a watch-glass, and set it aside for an hour or so to digest. After digestion, wash the precipitate of AgCl several times by decantation, and then transfer it to the Gooch crucible using a "policeman" (§ 20) if necessary, and continue washing with water which is slightly acidulated with nitric acid (0.01 M HNO3 is a suitable washing solution) until the filtrate shows no test for silver. Now wash the precipitate twice with pure water to remove the nitric acid. Dry the crucible to constant weight at 115° in an air bath or electric oven. Duplicate determinations should agree within 3 parts per 1000. * 2 If a standard approx. 0.1 molar HCl has already been prepared as directed in § 136, Exercise No. 3, it can be used here very advantageously. From the weight of AgCl found, calculate the value of the hydrochloric acid solution. 173. Exercise No. 8. Gravimetric Standardization of Approx. 0.1 Molar HC1 by Precipitation as AgCl. Use of Filter Paper.Proceed exactly as described in the preceding exercise up to the point where digestion has been effected; from there on proceed as follows. Fit a 12 cm. ashless filter paper exactly to its funnel and then moisten it thoroughly with water and tamp down the seams as directed in § 44, so that no precipitate can lodge back of the folds. The next operation is the filtration. Wash the precipitate of AgCl several times by decantation and then transfer it to the filter paper, using a "policeman" (§ 20) if necessary, and continue washing with water slightly acidulated with nitric acid (0.01 M HNO3 is a suitable washing solution) until the filtrate shows no test for silver. Now wash the precipitate twice with pure water to remove the nitric acid. The funnel is now covered by a filter paper which is made secure by crumpling its edges over the rim of the funnel so that they will engage the outer conical wall of the latter. The funnel is next marked for identification, placed in a drying oven at 100°-110°, and left there until the precipitate and the filter paper are completely dry. The precipitate is next detached from the filter paper as much as possible. To accomplish this select a large watch-glass about 6 inches in diameter and open the filter paper just above it in a place that is free from draughts. By very gently rubbing the sides of the filter paper together the greater portion of the precipitate becomes detached from the paper and drops upon the watch-glass. Care must be taken not to rub too hard as such rubbing will cause the paper itself to become abraded and fall into the main portion of the precipitate. When as much of the precipitate has been detached as possible, the large watch-glass is covered with a smaller one for the time being, and attention directed to the filter paper with its small amount of remaining precipitate. The paper is now folded carefully and placed inside of a weighed porcelain crucible; the crucible is placed on a triangle and ignited gently until the paper is charred, after which the 3 * If a watch-glass is not available, a similar sized piece of black glazed paper will answer the purpose. ignition is conducted at a higher temperature until the carbon is all oxidized. The crucible is then allowed to cool, and the ignited residue, consisting of metallic silver, is moistened with 2-3 drops 6 M HNO3 and 1 drop 12 M HCl, which may be advantageously added by dropping from a stirring rod. After this the crucible is very cautiously heated (preferably in an air bath) to avoid spattering, until the acids have been expelled. The main portion of the precipitate is now transferred to the crucible by means of a camel's-hair brush, and similarly moistened with 2-3 drops 6 M HNO3 and 1 drop 12 M HCl. The acids are likewise expelled by very cautious heating, and when the precipitate is white the temperature is slowly raised until the silver chloride just begins to fuse at the edges (about 450°). The crucible is then cooled in the desiccator and weighed. The ignition is repeated (without the addition of the acids however) and a duplicate weighing obtained. If the duplicate weighing does not agree with the first within 0.0003 g., a third heating, cooling, and weighing are made, and so on, until this concordance is obtained in two consecutive weighings. From the weight of AgCl found calculate the value of the hydrochloric acid solution. a 174. Exercise No. 9. Gravimetric Determination of Chlorine in Sodium Chloride. If the proposition is to determine the percentage of chlorine in a sample of sodium chloride "as is," any ordinary grade of sodium chloride, such as table salt, will suffice. If, however, the proposition is to see how nearly we can verify the theoretical percentage of chlorine in the chemical compound, sodium chloride, then the purity of our salt must be established beforehand. The most certain way of establishing this point is for the analyst to prepare the pure salt himself matter which is not very difficult. To prepare the pure salt, make up a saturated solution of a fairly good grade of sodium chloride (table salt will answer if we have nothing better at hand); then pass hydrochloric acid gas into this solution until a sufficient amount of sodium chloride precipitates; when this has happened the supernatant liquid is poured off and the salt transferred to a Gooch funnel which has an asbestos filter bed at its bottom; the salt is then washed several times with water. A portion of the salt, 5-10 g., is now transferred to a large |