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THE VOLUMETRIC DETERMINATION OF CHLORIDE WM.B. MELDRUM AND J. C. FORBES, HAVERPORD COLLEGE, HAVERFORD, PENNSYLVANIA In the teaching of quantitative analysis to students without experience in this branch of chemistry the determination of chlorine, both gravimetrically and volumetrically in the same sample, is a useful pedagogical device. The interest of the student is aroused by the check analyses obtained by such widely different methods and a t the same time he is introduced to the two general procedures which probably will weigh most heavily in the course. It is highly desirable, because of the student's inexperience, that the procedure be simple and straightforward and that he may obtain results that do check without too many repetitions. Repetition of analyses in the first few weeks of the course may be counted upon, as a rule, to kill interest pretty effectively. From this point of view the gravimetric determination of chlorine, using preferably the Gooch crucible, is as satisfactory as any useful method could be expected to be. The volumetric method has not met the specifications so well. The Volhard method, the one generally included in laboratory manuals, involves too many operations and is not readily understood by the beginner, and the results, a t least so far as the experience of the authors indicates, are poor. The Mohr method, though simple, yields results that are only approximately correct a t best. Nevertheless, the authors were convinced that the Mohr method was the only one usable in the early weeks of the course and so attempted to modify i t in such a way that the main errors might be eliminated to as large an extent as practicable. The results of this effort were so satisfactory that the publication of the modified procedure was considered worthwhile. The working out of the procedure given in this paper solved what has been a serious pedagogical problem in this laboratory. Three sources of error stand out most prominently: (1) Temperature effect on the solubility of silver chromate; (2) lack of sensitiveness of the end-point due to too great acidity or alkalinity of the solution; (3) adsorption of soluble chloride by the precipitated silver chloride. The necessity of titrating in the cold and of adjusting the pH of the solution have been emphasized in modern texts on quantitative analysis.' Even granting that these precautions are observed it has been recommended that Mohr's method he used only for determining small amounts of cblorine, as in water a n a l y s i ~ . ~Attempts to eliminate errors due to the thirdmentioned source do not seem to have been made and it seemed very 1 Fales, "Inorganic Quantitative Analysis," The Century Co., 1925, p. 175. Blasdale, "Principles of Quantitative Analysis," Van Nostrand, 1917, p. 2G0. Scott, "Standard Methods of Chemical Analysis," Van Nostrand, 1927, p. 126. Treadwell and Hall, "Analytical Chemistry," John Wiley & Sans. Inc., Vol. 11,p. 545.
probable that i t might account for the limitation of the method to small amounts of chlorine. The precipitated silver chloride comes down initially in a finely divided condition and does not coagulate until the end-point is very nearly reached--and does not do so completely then. Adsorption should then be expected. When provision was made to eliminate the error due to this cause the results were definitely better and the necessity to repeat the determination due to discordance of results has become rare. The procedure finally adopted was as follows: Dry the chloride material for one hour a t about 140°C. Cool in desiccator and weigh samples of about 0.4 gram into 200 cc. Erlenmeyer flasks and dissolve in about 75 cc. water. Add 1 drop methyl orange solution and dilute nitric acid until the solution turns pink. Now add 1% sodium bicarbonate solution until the color turns to yellow and then 1 cc. in excess. Add 5 drops 0.1 M potassium chromate solution (free from chloride) and titrate with standard silver nitrate solution, about 0.2 N, to a faint end-point. Boil for five minutes; cool in ice water to 20°C. or lower. Finally titrate with the standard silver nitrate to a faint end-point. The silver nitrate solution should be standardized against sodium or potassium chloride of known chloride content using the same procedure as above. I. STANDARDIZATION OP SILVER N ~ R A TSOLUTIONS E Experimenter Wt. NaCl Vol. ASNO,s o h Normality A
0.2312 g. 0.3104 0.2763
23.27 cc. 31.25 27.81
0.1699 0.1699 0.1699 Average 0.1699
B
0.3120 0.5537 0.3190
26.77 47.B 27.47
Average C
0.1946 0.1970 0.2151
21 2 2 21.48 23.39
0.1993 0.1993 0.1986 0.1991
0.1568 0.1568 0.1572 Average 0.1569
The first end-point reached is not permanent; if the flask be allowed to stand in the dark for half an hour the color due to the precipitated silver chromate fades out completely. The second end-point is permanent, indicating a real excess of silver nitrate. The amount to be added to attain the final end-point varies from about 0.5 to 2 drops, equivalent to about 0.03 to 0.12 cc. Other opinions to the contrary notwithstanding, the end-point is very sharp and when near the end-point drops should be
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divided. As little as 0.02 cc. makes a distinct difference. It should also be noted that there is no excuse for anyone, even the novice, to go over the end-point even to the extent of a single drop. The gradual change in the color of the precipitate in the region of the added silver nitrate is a sure indication of the proximity to the end-point. The silver nitrate should be added about 1cc. a t a time; a t first the color before shaking is a faint salmon pink because of the relatively small proportion of silver 11. ANALYSIS
OR
Experimenter
Wf. sample
A
0.3896g. 0.3929 0.3926
MATERIAL FOR SOLUBLE CRLORIDB Vol. AgNO.
21.51. 21.68 21.66
% Cl
33.26 33.25 33 24
-
Average 33.25
0.3702
C
0.3924 0.4011 0.3985
% CI gravimetric
33.22 33.31 33.26 33.26
33.29
33.36
Average 33.35
33.32
17.44
23.48 24.05 23.81
33.33 33.39 33.29
-
Average 33.34
chromate; as the titration proceeds and the excess of chloride becomes less the color deepens and as the end-point is approached becomes a distinct red. These results were obtained by three experimenters independently using different standard solutions, but the same material for analysis. They are not perfect but i t is highly probable that they would all be acceptable by an instructor in quantitative analysis. The instructor's analysis (gravimetrically) of the same material showed 33.35% C1.
