Constant-boiling hydrochloric acid

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CONSTANT-BOILING HYDROCHLORIC ACID JOHN L. SHELDON Bat* Creek College, Battle Creek, Michigan

A short survey of the important work which has been done on the subject of constant-boiling hydrochloric acid as a n analytical standard i s presented. Instructions for preparing and using the mixture are ginen.

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LTHOUGH constant-boilmg hydrochloricacidhas been recommended as a standard for acidimetryalkalimetry since 1909, it has not had the extensive use which its many desirable qualities deserve. The existence of this constant-boilmg mixture has been known since the time of Dalton. Bineau (I), in 1843, and Roscoe and Dittmar (2,3,4), in 1859-60, published papers concerning it. The composition of the mixture varies with the pressure a t which i t is distilled. These last two investigators determined the composition of the constant-boiling mixture distilled a t pressures from 65 to 2510 mm. Hulett and Bonner (5) were the first. to suggest it as a standard. In their paper, published in 1909, they described the method of preparing and using the standard. They stated that the composition of the distillate collected according to their instructions should not vary more than one part in ten thousand from the values given in their table. The subsequent work of Morey (6),Hendrixson (7),Foulk and Hollingsworth (8),Bonner and Branting (9), and MacInnes and Dole (11) has shown the mixture to be a very accurately reproducible standard. Shaw (10) investigated the keeping characteristics of the mixture and found that several samples kept tightly stoppered in pyrex flasks for periods of time up to 39 months displayed no variation from their initially determined concentrations greater than one part in a thousand. The work of Bonner and Titus (12) and Bonner and Wallace (13) in determining the composition and boiling point of constant-boiling hydrochloric acid over a pressure range of 50 to 1220 mm. is probably the most accurate of any of the work cited. They also investigated the effect of the rate of distillation upon the composition of the distillate, a t rates varying from 1.7 to 4.9 cc. per minute, and were unable to corroborate the previous conclusion of Foulk and Hollmgsworth that the rate of distillation affects the composition. The following practical instructions for the preparation of constant-boiling hydrochloric acid for use as a standard have been compiled from the preceding references. Concentrated hydrochloric acid is first diluted to a density of approximately 1.10. This is done by diluting 100 cc. of 35537% HCl (density 1.178-1.185) to approximately 190 cc. The distilling flask should

rest on an asbestos-centered wire gauze. A screen of heavy asbestos paper with a hole in the center just a little smaller than the asbestos center of the gauze is used to protect the flask. The flask should be further protected by a cylinder of asbestos paper which rests on the horizontal asbestos screen and which is as high as the flask. This protection of the flask is very important, because it prevents super-heating a t the end of the distillation. Distillation should be a t the rate of 3 to 4 cc. per minute and once started it should not be interrupted. The condenser should be fitted with an adapter. The first 75'% of the distillate is set aside for use as high grade 6N reagent acid and the next 15% is distilled through the adapter directly into a pyrex flask and saved. If a large amount of acid is distilled, distillation may be continued until only about 50 cc. remains in the flask. The barometer should be read a t the beginning and a t the end of the distillation and the average of the two readings is recorded. The distillate should be tightly stoppered with a new rubber stopper and should be kept preferably in a closed cupboard. When needed it may be used in two ways: (1) small weighed samples may be used for standardizing alkali solutions, or (2) i t may be used directlpfor the preparation of standard solutions of acid tiy diluting weighed amounts to a definite volume. The weighing can be done in small flasks, the weight being accurately adjusted by means of a capillary pipet. Smce the mixture is not very volatile and does not change in strength upon being exposed it is not necessary th take unusual precautions in the weighing. The following table, from Foulk and Hollingsworth (8), may be used to determine the amount of the constant-boiling distillate to be weighed: P~essw, mm. 770 700 750 740 730

Air Weighf of Connnnfdoiling Didillae Thof Confoi*1 1 Mol Vacvvm Weight of % HCl-Vncuum RCI (Gmnir) Wcighf Bois 20.197 180.407 20.221 180.193 179.979 20.245 20.289 179.700 179.555 20.293

Although i t is not usually feasible for all the members of a quantitative laboratory section to prepare this standard, it is desirable to have a few, a t least one, do so. Students who have previously had experience in the organic laboratory should be able to make the set-up and carry out the distillation in a minimum of time. In schools where a supply of standard acid is kept for student use in courses other than quantitative analysis, a considerable quantity of the mixture can be

made up and used when it is desired to prepare standard solutions of HC1 of any exact normality. Students of physical chemistry who are determining the temperature.composition curves for the liquid and vapor of the binary pair hydrochloric acid-water may advantageously prepare a sample of this standard as a supplementary project. LITERATURE CITED

(1)

BINEAU, Ann. chim. phys., [31,7, 257 (1843), ascited in (9).

(2) ROSCOE AND DITTMAR. Ann.. 112, 343 (1859). as cited in (9) and (13). (3) Roscoe AND D m ~ m I. , Gm.Soc., 12, 128 (1860), as cited in (9) and (13). (4) Roscoe. Ann., 116,213 (1860). as cited in (9) and (13). (5) H a E T r AND BONNER.3. Am. Chem. Soc.. 31,390 (1909). (6) MOREY, ibid.. 34, 1027 (1912). (7) HENDRIXSON, ibid.,37,2352 (1915). AND HOLLINGSWORTH, ibid.,45, 1220 (1923). (8) FOULK (9) BONNER AND BRANTING, ibid.. 48,3093 (1926). Ind. Eng. Chem., 18, 1065 (1926). (10) SHAW, (11) MACINNE~ AND DOLE, I.Am. Chnn.Soc.. 51,1125 (1929). AND TITUS, ibid., 52,633 (1930). 12) BONNER 13) BONNER AND WALLACE. ibid., 52, 1747 (1930).

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