NEPHELOMETRIC T I T R A T I O S S . I1 The Standard-Solution End-Point*
BY CLYDE R. JOHXSON**
A previous paper1 from this laboratory has called attention to a possibk source of error in the equal-opalescence end-point used in nephelometric atomic weight titrations. It is the purpose in the present article to suggest an alternative end-point for such titrations, free from this source of error, and possessing certain other advantageous features. To distinguish the proposed end-point from the other, it may be termed the standard solution end-point. The proposed standard solution method differs from the equal-opalescence method only in the determination of the end-point of the rea’ction under investigation. That is, one precipitates an acid solution containing the chloride (or bromide) ions from a weighed quantity of a pure compound in the usual manner, using wit’hin a few tenths of a milligram of the theoretical amount of pure silver, weighed, and dissolved in nitric acid. At this stage, without necessarily making further additions to the system, one determines the endpoint by measuring the absolute amounts of silver and halide ions in the supernatant liquid. This is done by comparing test portions of the supernatant’ liquid with standard solutions having practically the same composition as the supernatant liquid itself, with the aid of the nephelometer. In each individual titration, two sets of standard solutions are required, for the determination of the silver and the halide, respectively, in the test portions of the analytical solution. The details of the preparation of these standards must, of course, vary with the particular “atomic weight” ratio under investigation. The general procedure in the use of the proposed endpoint may be inferred from the tests described below. It may be noted that all of the water and nitric acid used in these experiments was purified by the methods usually employed in atomic weight determinations. Experimental A number of 18 gm. portions of pure silver nitrate were dissolved in 1100 cc. volumes of mater in glass-stoppered 3-liter Pyrex Erlenmeyer flasks and precipitated a t the rate of about 4 cc. a minute wit,h equal volumes of hydrochloric acid containing a slight excess of HC1. The precipitates, after stand* Contribution from the Chemistry Department of The Rice Institute. * * National Research Fellow in Chemistry. Johnson: J. Phys. Chem., 35, j40 (1931).
NEPHELOMETRIC TITRATIONS
83 1
ing, were washed about 2 0 times with 300 to joo cc. portions of water, with intermittent soaking and shaking during a period of a week. After this time, varying calculated amounts of nitric acid were added to the flasks, and the volume was in each case made up to I joo cc. with water. The solutions were then saturated with silver chloride by intermittent shaking over a period of Peveral days. The silver and chloride content of each solution a t approximately 0°C'. was determined as follows. The flask containing the solution was partially immersed in an ice-salt bath until the contents had completely frozen. I t was then removed, carefully washed, and allowed to stand in air until about three-fourths of the contents had melted, whereupon it was placed in a bath of cracked ice and allowed to stand, usually for a t least eight hours, with occasional gentle but thorough shaking. About two hours after the final shaking, 300 cc. of the clear supernatant liquid were withdrawn with a pipette and placed in a Pyrex bottle, which had been cleaned, dried, and rinsed with a small amount of the liquid. I n other cases the solution was not frozen, but was withdrawn after cooling in an ice bath for several hours, with frequent shaking. Some preliminary experiments showed the inadvisability of removing the flask from the ice bath until after the sample had been withdrawn, even for the purpose of shaking the solution. The sample was allowed to stand overnight before making the nephelometric tests, as it was also found important that it should be a t exactly the same temperature as the standard solutions used in its analysis. The acid concentration of each sample was next determined by titration of z j cc. to I O O cc. portions with 0 . j o g 11 sodium hydroxide, using methyl orange as an indicator. The chloride and silver content of each of the samples was determined by nephelometric tests, which consisted in comparing 22.00 cc. portions of the samples with standard solutions of equal volume. These standards \yere prepared from pure water, j.16 %I nitric acid, and solutions made by diluting measured volumes of primary standard solutions of silver nitrate or sodium chloride containing 1.000 mg. of silver or its equivalent per cc. I n all of the volumetric measurements calibrated apparatus as used, of dimensions intended to give the measurements, without exception, a precision of I part in j o o or better. I n analyzing for chloride, the standard and test solution were precipitated with two equal 1.00 cc. portions of the primary standard solution of silver nitrate; in analyzing for silver, the standard and test solution were precipitated with two equal I .oocc. portions of the primary sodium chloride standard. I n the precipitation of the solutions, and in reading the nephelometer, the precautions given by Richards and Kellsl, and Wells,' were observed. The procedure employed was adapted from that described by Scott and J o h n ~ o n , ~ I
Richards and Wells: Am. Chtm. J., 31, 235 ( 1 9 ~ 4 ) J. ; Am. Chem. POC..27,484 (1905). Wells: Am. Chem. J., 35, 99 (1906). Scott and Johnson: .J. Am. Chem. Soc., 52, 2644 (1930).
83 2
CLYDE R. JOHKSON
with slight modifications to make it suitable for the present experiments. The silver chloride used in this work, except the minute amounts employed in nephelometric tests, was illuminated only by red light. The typical data of a single analysis is given below in condensed form:
Analysis of Sample from Flask No. 1 Sample: 400 cc. of solution; frozen; melted; cooled for 9 hours; withdrawn a t 1.0' C.; warmed to 30' C . Found to be 0.485 M in nitric acid a t 30' C. Test Solution: z 2 . 0 0 cc. of above sample. Standard Solution: Made from 2.07 cc. of j . 1 6 > nitric I acid and 19.93cc. of sodium chloride solution containing the equivalent of 0.001124 gm. of AgCl per liter. It thus contained sodium chloride equivalent to O . O O I O Z gm. of AgCl per liter, in 0.485 JI nitric acid, a t 30' C. One hour after precipitation of the two solutions with excess silver nitrate, the mean of 2 0 settings gave a value of 1 . 0 1 for the nephelometric ratio, with an average deviation of o . o z j unit. The standard tube had the weaker opalescence. Therefore the test solution contained 0.00103 gm. of BgCl per liter. The results of other similar analyses are given in Table I. Each result recorded in the table corresponds to a set of 2 0 nephelometric readings, made with an independently-prepared standard and a fresh portion of the sample. Analysis of Silver Chloride Solutions T.4BLE
Molarity of sample in HNOl
Time of Temp at cooling removal Ratio Hours Corr "C
Part 0.000
0.111
0.241
0.485
1
Xephelometer Ratios and Grams of A4gClper liter Found as Ratlo Found as NaCl AgSOa I
IO
o 5
I
40
o 00106
I 00
o 00104
IO
0
4
I
34
0 00101
I 0:
0 OOIII
6
0.4
1.68
o.00172
1.68
11
0.4
1.10
o.ooog3
I
3
1.4
1.55
o.oo19;
1.31
0.00238
3
1.4
1.14
0,00217
6
0.5 0.5
1.54
0.00186
1.16
0.00148
1.08
0.00166
6
3j
o.o01;3 (0.00138)
NEPHELOMETRIC TITRATIONS
833
TABLE I (Continued) Molarity of sample in H S O l
Time of Ternu. at cooling rembval Ratio Hours Corr “C.
Part 0.000
0.111
9
0.4
9
0.4
8 8 8
1.35
1.33 Average
6 8
1.53
.63
0.00064 0,00068 0.00066
O.oOOj8
1.00
0.0009I
1.18
(0.000 j j)
1.01
I .OI
0.6
1.04
o .00092 0.00094 o .ooo92
.06
0.0009 2 0.00086 0.00089
I
0.6
1.13
0.00100
1.16
0.00103
0.6
1.10
o.ooog8
1.15
O.OOI02
0.00099
0.00
103
I
0.5 1.0
1.08
o .00109
.45 1.14
(0.00100)
6 9 9
1.0
1.01
0.00103
I .02
0.oOIoq
I .02
0.00100
I
I
.o
Average 0.988
I
O.OOOj8
0.6
Average 0.48j
2
0.ooojj
0.6
Average 0.241
Nephelometer Ratios and Grams of AeCl Der liter: Rkio Found as Found as AgNOa XaC1
8
8 8
0.4 0.4
0.4
o ,00106
(0
00089)
O.OOIO2
1.08
0 .OOIOO
I .OI
0.00107
1.03
o .00104
I
.04
0.00104
1.04
0.00103
I .02
0.00110
.-iverage
0.00102
o .00107
Part I of the table includes analyses of solutions cool 1 without freezing; Part 2 includes analyses of solutions frozen and melted before cooling. The nephelometric measurements were made at temperatures varying from 30° to 33O C. For purposes of comparison, both of the constituents determined have been calculated as AgC1. The nephelometric ratios included in the table show only the relative compositions of the standard and test solutions, as the ratios have been calculated so that they are uniformly greater than unity. IGo analyses have been omitted; the bracketed values I am inclined to reject. While the tests described above were not intended primarily as solubility measurements, it is interesting to note that the data in Part z of Table I form a consistent set of determinations of the solubility of silver chloride in varying
83 4
CLYDE R. JOHNSOX
concentrations of nitric acid. Giving each independent analysis equal weight, and reducing all of the values t o 0.5’ C., the data combine as follows: Gm. AgCl
Temp.
Molarity of Iiitric .kcid
0.5
0.000
0.OOOjZ
0.j
0.111
o.00090
0 .j
0,241
0.00100
0.5
0.48;
0.00102
0.5
0.988
0
per liter
.ooro~
Discussion of Results It seems reasonable t o conclude that the data in Table I show that the standard solution end-point is suitable for use in nephelometric atomic weight titrations. In solutions at the stoichiometrical point (saturated silver chloride solutions) the method indicates that the silver and chloride ions are equal, with the desired precision. That is, the average silver and chloride, calculated as .IgC1, agree Jyithin a few hundredths of il milligram, when the tests are made under the proper conditions (Table I , Part 2 ) . These conditions, as defined by the present cxperiments, may be summarized: ( I ) The cooling of the solutions should be preceded by freezing and melting. This treatment hastens the attainment of equilibrium, decreases the amount of dissolved material t o be determined, and reduces the mwsurenicnts t o a common basis. Thc latter feature niay prove of value in detecting any abnormality in the end-point of the titration. ( 2 ) I n general, the measurements become somewhat more dependable as the nephelometric ratio approaches I .oo. It follow that successively-prepared standards should approximate more and more closely t o the test solution in composition. I n any case, in the determination of each constituent the analyses should be made a t least in triplicate, with fresh, independently-prepared standards. ( 3 ) I n every analysis the standard and test solutions should be at exactly the and 1.0 11 are same temperature. (4) Acid concentrations betrveen 0.3 suitable for use in the analytical solutions. ( j ) The method used in preparing the standard solutions evidently involves no serious error. Severtheless, the possibility of introducing improvements and refinements in the method is apparent. The use of solutions of silver chloride as primary standards has been considered. I t is recognized that in some atomic weight determinations the prcparation of standard solutions containing suitable amounts of the “extra” ions present in the analytical solutions may involve difficulties. However, in all but exceptional cases it should easily be possible to prepare standards of the necessary purity with an accuracy at least ten times greater than that required. I n spite of the great sensitivity of the nephelometer, nephelometric observations are accurate only to about I part in j o .
NEPHELOMETRIC TITRATIONS
83 5
General Discussion I t is not the intention in this paper to imply that the possibility of an error in the equal-opalescence procedure, in certain types of titrations, affects in any way its past or present general usefulness, or that the proposed standard solution procedure is anything more than a possible alternative or supplementary one. Both methods have their characteristic advantages. For the present, attention is confined to two advantages peculiar t o the standard solution method: ( I ) I t offers the opportunity of comparing, in the nephelometric tests, systems which are practically identical in composition. The importance of this feature has often been stressed by Richards.' The same opportunity does not appear in the equal-opalescence method, in which one compares systems precipitated with e q w i i v l e n t rather than with equal amounts of the precipitating reagents. ( 2 j In the standard solution method one measures the absolute amounts of silver and halide ions in the analytical solutions; in the equal-opalescence method only the iclntiw amounts are found. The use of the standard solution procedure gives valuable data regarding the solubility of silver chloride in the analytical solutions, and information concerning the equilibrium between silver and halide ions in the neighborhood of the end-point. This feature is of value if the nephelometric analyses are to be followed by incidental gravimetric analyses, as it permits an exact correction for chloride lost i n the nrplielometric tests. Reports of such atomic weight determinations, in which it has been customary to employ the equal-opalescence method, give one the impression that this rather important correction has frequently been neglected, although corrections involving smaller quantities of material have been taken into consideration. In conclu.qion, it may be said that while the measurements recorded in this paper show the essential validity of the proposed end-point, the usefulness of such an end-point can only be established by actual trial under the conditions arising in atomic weight work. Such a trial is desirable, if only for the reason that it is good policy in atomic weight investigations to gather evidence from many sources. summary
An end-point for use in nephelometric atomic weight titrations has been proposed. Experiments show that under a specified set of conditions the endpoint corresponds to the stoichiometrical point with the desired accuracy. Honstori, Texas. l
Richards: -4111. Chem. J., 35, j~I (1906). See also, Richards and Wells: Am. Chem. J.,
31, 242 (1904).
