NEPHELOMETRIC TITRATIOKS. IT. T H E EFFECT OF SHAKIXG ASD COOLISG T H E ANALYTICAL SYSTEMS” BY CLYDE R. JOHPisOS**
In the work described in the preceding paper‘ of this series, concerning the action of extra compounds in nephelometric atomic weight analyses, irregularities due to shaking and cooling the analytical systems were avoided. However, since the practices of “cooling to reduce the solubility of the silver chloride” and “occasional shaking” are a t present employed in such analyses, it seemed advisable to continue the experiments, to determine the effect of shaking and cooling the analytical Systems, in the presence of various extra compounds. The work was facilitated by the fact that the carefully prepared systems used in the earlier experiments were available.
Experimental
Equal Opalescence Tests. The analyses of the supernatant liquids were continued in the same manner described in the earlier paper. Systems KO. 3, 4, 5 , 7, 9 , 14, 16, 1 7 , and 18 were diluted with 0 . 2 5 31 nitric acid until o . o j o RI in the extra compounds. The extra compound concentration in the others remained a t 0.10M. Each system was shaken 1000times, and samples were withdrawn and tested in the nephelometer after 8 and 24 hours. The bottles were then three-quarters immersed in shaved ice and samples were withdrawn for testing after intervals of 4, 8 and 24 hours. The samples were allowed to come to room temperature before t h e precipitating reagents mere added. The bottles were next placed in ice-salt mixtures, until the contents had completely frozen. After melting, the supernatant liquids were subjected t o another series of analyses, in which the time and temperature factors were allowed to vary rather widely. The systems were then brought to room temperature for the final tests. The results of these analyses are given in Table I. A nephelometric ratio greater than unity indicates that the samples contained excess chloride, one less than unity indicates that they contained excess silver. Standard Solution Tests. To obtain additional information regarding the cause and magnitude of the effects observed in the equal-opalescence tests, the following experiment was tried. Three saturated solutions of pure silver chloride in different concentrations of nitric acid, over I j gram quantities of flocculent silver chloride, were prepared and analyzed according to the method described by Johnson.2 These analyses showed that the solutions contained silver and chloride in equivalent amounts, within 0.03 mg. of silver. The
* Contribution from the Frick Chemical Laboratory, ** National Research Fellow in Chemistry J. Phys. Chem., 35, 2237 (1931). *Johnson: J. Phys. Chem., 35, 830 (1931).
Princeton University.
CLYDE R . J O H S S O N
2 j82
TABLE I Effect of Shaking and Cooling the Analytical Systems So.
Kephelometric Ratios; .4&1 Solutions 0.25 XI in HSOz: After After After Final Shaking in; Cooling, for: Freezing: At Room 8 hrs. 24 Hrs. 4 Hrs. 8 Hrs. 24 Hrs. Max-Min. Temp.
Extra Compound
1.10
1.0;
1.10
1.29
0.9;
1.00
1.10
1.18
1.23
0.86 0.89 0.78
0.99 1.28
1.10
o.dz
1.22
1.12
1.08 0.96 1.46 0.82
1.17 1.2j
0.93
1.13
1.04
1.33 1.33
1.41
1.02
1.22
1.4j 1.42
1.03 1.01
1.25
1.04 1.19
1.46
I . ~ I
1.51 1.70 1.28
0.92 1.16-1.08 0 . 9 6 0.98 I . 01-1.00 I .oo I . 33-1.00 I . 28-1.02 0.99 I . j2-I . 0 2 0.91 I . 2 j-I. I 3 0.95 I . 32-1.09 0.83 0.98 2.19-0.90 1.16-1.1; 0.94 0.91 1.02-0.95 I .oo I . 48-1.01 0.91 I . 65-1.36
2.40-0.98
0.94
1.11
1.42
1.63
1.02
1.11
1.76
2.12
1.00
1.08
1.18
1.03
1.00
1.10
1.20
1.00
0.88
1.00
1.46 1.12
1.j8 1.63
I .04 1.47 I , 21-1. I I 1.09 2.22-1.16 I . 13 I .38-I. 26 1.00 0.79 1.00 0.93 1 . 2 2 - 1 . 0 0 2.49-1.01 0 . 9 7 1.50
1.07
1.j3
1.82
1.46 0.94
I . j1-1.01
I
.03
systems were then shaken occasionally over a period of several days, allowed to stand, and again analyzed by the same method, this time at room temperature. Equal-opalescence tests made upon the same solutions gave ratios from 1.08 t o 1.18. The results of the other analyses are summarized below: Grams AgC1 per Liter:Found as Found as SaCl AgXOa
Tzmp;
S O .
c.
3Iolarity of H S O ? .
Time of Standing. Hours.
20
29
0.2j
24
0.00342
50
0.00304
24
0.0037j
49 I20
0.00336 0.00310
IO 10
0.00331
0.002
24
0.00340
26
0.00352
0.00296 0.00306
System
21
228
22b
29
29 30
0.50
I
.oo
I .oo
0.00288
0,0027 5
0.00~76 0,00252
53
NEPHELOMETRIC TITRATIONS
2583
Discussion of Results In the preceding paper of this series are recorded observations which show that most of the supernatant liquids in the systems under investigation contained practically equivalent amounts of silver and chloride a t the beginning of the experiments described in this paper. The present results taken as a whole, indicate that there is a marked tendency for the shaking and cooling treatments to leave in the supernatant liquids an excess of chloride, equivalent to several tenths of a milligram of silver per lit'er. Real or virtual, this effect represents a source of constant error which, if permitted to operat'e in any nephelometric atomic weight titration, would tend to yield a low value for the calculated atomic weight. An additional error in the same direction might enter into corrections applied for the removal of nephelometric test portions. It may be assumed that cooling1 and shaking initiated in the systems the same series of changes, whenever these were not modified or inhibited by the presence of extra compounds. That is, colloidal silver chloride, in excess of the normal solubility a t the final temperature, was first formed, and next slowly coagulated in the supernatant liquids. The data in the above tables offer a somewhat fragmentary record of the accompanying changes in the silver and chloride concentrations, complicated by the action of a number of disturbing factors, operating both in the main systems and in the nephelometer tubes. The fluctuations in the observed nephelometric ratios for any given system may be ascribed to some combination of the following factors: ( I ) adsorption of ions by silver chloride during peptization and coagulat,ion, (2) adsorption on the precipit'ate, (3) differences in the coagulating action of the two precipitating reagents, and (4) the peptizing and coagulating actions of the extra compound. Certain trends in the tabulated data suggest that coagulating silver chloride may have a tendency to carry with it more silver than chlorine atoms. This point is worthy of some attention, as the observed effects accompanied the peptization (or condensation) and subsequent coagulation of only a few milligrams of silver chloride, in the presence of thoroughly washed and aged precipitates. Adsorption effects would be much greater in actual titrations of the type under consideration, as the analytical procedures include the precipitation of 5 to z j grams (or more) of silver chloride in every analysis. These precipitates are not washed and are not usually aged. If silver chloride did carry down excess silver during coagulation, in any nephelometric atomic weight determination, it would produce constant errors tending to make the calculated atomic weight too low. I t is evidently quite important in precise work in which silver chloride precipitated from dilute nitric acid is used, to know the relative amounts of silver and chloride atoms in the material, and the nature of the other inclusions. A result obtained in experiments now under way in this laboratory may have some bearing on the question. A quantitative study of the solution and 1
J. Phys. Chem;,
35, 832 (1931).
2584
CLYDE R. JOHNSOX
reprrcipitation of silver chloride samples has been undertaken, with the idea of obtaining information regarding their relative silver and chloride content, and the possibility of removing impurities by reprecipitation. Large samples of silver chloride can readily be dissolved by distilling ammonia into the analytical systems after replacing the supernatant liquids with pure water. The silver chloride is easily reprecipitated by boiling away the ammonia or by distilling it off under reduced pressure. The technique characteristic of atomic weight investigations may be maintained throughout. However, even the most carefully prepared and thoroughly washed samples, weighing about I z grams each, deposit as much as two or three milligrams of silver during solu. tion in aqua ammonia. This may be due to some reducing action occurring on the walls of the Pyrex flnsks. I t may indicate that part of t’he silver in the samples is not present as silver chloride, for this compound is quite resistant to reduction under conditions in which other silver compounds, such as the nitrate, are quickly reduced. These experiments are to be continued, in connection with experiments concerning the “loss on fusion” correction appied in precise ivork with silver chloride. Thanks are due to Professor George A. Hulett, who has kindly placed his laboratory at my disposal for this work.
Summary Data concerning the effect of shaking and cooling on a number of typical systems used in nephelometric atomic weight determinations have been recorded. Princeton, iYew Jersey.
