SEPHELOMETRIC TITRATIOSS. 111. THE EFFECT OF EXTRA COMPOUSDS* BY CLYDE R. JOHNSOS**
In the first paper of this series it was pointed out that atomic weight values derived from nephelometric titrations in which the equal-opalescence endpoint is used are not necessarily independent of the “extra” compounds inevitably present in the analytical solutions. Further, it was suggested that these values may also depend to some extent upon certain arbitrary features of the analytical procedures now used for determining the end-point in such titrations. The experiments described in the following report are designed to furnish information bearing upon both of the above points. They show in a general way the extent to which and the conditions under which the end-point and the stoichiometrical point coincide, in a number of typical titrations] involving a variety of extra compounds. More important, perhaps, the experiments outline and test a general method for placing the analytical procedures used in determining the end-point upon an experimental basis. That is, they provide a means of removing the arbitrary feature of these procedures when they are applied to the determination of any particular stoichiometrical ratio. The application of the method is quite simple. I t consists in predetermining the conditions under which the end-point of the titration will correspond with the stoichiometrical point with the necessary precision. The actual analyses are carried out under these predetermined standard conditions, with analytical systems of definite composition and volume. The details of the method used in handling the systems and conducting the nephelometric tests then lose their arbitrary character. The various time and temperature factors important in these tests are thus brought under control; any disturbing effects characteristic of the liquid phase in the systems under investigation are eliminated. It is evident that the method is applicable whether the tests are made by the equal-opalescence method of Mulder or by the standardsolution method outlined in the second paper of this series. The present research has been confined to experiments with systems containing silver chloride, since this compound has been employed very extensively in nephelometric atomic weight det,erminations. The systems used in such work consist of flocculent silver chloride in contact with one to five liters of its “saturated” solution in nitric acid. The solution generally contains an extra compound derived from the chloride undergoing analysis. Very frequently, although not necessarily, there are present various auxiliary compounds, of which ammonium nitrate is a common example. The methods by which these systems are synthesized in atomic weight determinations are described in detail in corresponding reports and will not be discussed here. * Contribution from the R i c k Chemical Laboratory, Princeton University. * * Kational Research Fellow in Chemistry.
2238
CLYDE R. JOHXSON
Exp erimenta1 In the present work systems of the above type were prepared by a reverse method of synthesis from pure flocculent silver chloride, nitric acid, and various pure compounds. These syntheses were carried out in parallel with a series of nephelometric tests, as described below. The technique characteristic of precise atomic weight determinations was observed throughout. All work with the silver chloride was done in the light of Series 0 and OA Rratten Safelights. Reagents. The water, nitric acid, hydrochloric acid, and the standard solutions used in this research were prepared according to the specifications outlined by Scott and Johnson.’ The silver nitrate was obtained from silver chloride residues from previous atomic weight investigations. This silver chloride was reduced to silver with purified sugar and sodium hydroxide. The silver was thoroughly washed, dried, and fused on a bed of pure lime in an atmosphere of methane. It was then etched and dissolved in nitric acid, and crystallized as the nitrate. This material was further purified by two precipitations from saturated solution with nitric acid, after each of which it was dried by centrifuging. Before each crystallization or precipitation the solution was filtered through a platinum hlunroe crucible. The other compounds used in the work were freed from chlorides (and silver) by repeated crystallization and centrifuging. The initial crystalline material was usually joo grams of the “reagent grade” compound; in a few cases C.P. material was used as a starting point. The compounds were subjected to four, five, or six recrystallizations, as necessary. After each of these the moist crystals were centrifuged in covered porcelain Buchner funnels, a t I joo--2000 r.p.m. for about fifteen minutes. Each solution was filtered a t least three times a t different stages in the purification, initially through a paper filter, and a t least twice through a hlunroe crucible. The crystals finally obtained were placed in porcelain dishes, and dried by vacuum desiccation over fused sodium hydroxide. By this treatment it was found possible to obtain 7 j to 150 grams of each compound, which met the requirements for purity described below. One exception must be noted. The initial thorium nitrate sample weighed less than z o o grams and could be recrystallized only four times. The system 0.10hf in T h ( S 0 3 ) 4prepared from the final material was found to contain excess chloride equivalent t o 0.00030 grams of silver per liter. It may be noted that the hygroscopic crystals of the cerium and thorium nitrates were distinctly more difficult to separate from their motherliquor in the centrifuge, than the crystals of any of the other compounds. Nephelometric Tests. The nephelometric tests made upon the systems finally obtained, and during their synthesis, were based upon the equalopalescence method of hlulder,* with innovations and precautions suggested by Richards and Wells,3 and Scott and Johnsor.: J. Phys. Chem., 33, 1978 (1929). Mulder: “Die Silherprohir-Methode ’ (18j9). Richards and Wells: Am. Chem. J., 31, 235 (1904). Wells: Am. Chem. J., 35, 99 (1906).
KEPHELOMETRIC TITRATIONS
2239
To prepare the systems for testing they were shaken vigorously 1000times, following each change in the composition of the solutions. After this treatment they were allowed to stand for several days, with the minimum of agitation required to keep the solutions uniformly mixed. For another period of days, and later, during the course of the tests, the systems were seldom disturbed. These specifications are of some importance, as preliminary and incidental data obtained in the present investigation show. Tests accompanied by casual “occasional shaking” of the silver chloride-nitric acid-extra compound systems almost invariably indicated that the liquid phase contained excess chloride equivalent to several tenths of a milligram of silver per liter. For each test two 20.0 cc. portions of clear supernatant liquid were withdrawn with a pipette and placed in a pair of clean, dry test tubes, which had been matched with respect to volume, clearness, and color. The suspensions to be compared were then formed in the usual manner by the addition of I .oo cc. portions of standard solutions of silver nitrate and sodium chloride containing the equivalent of 1.000gm. of silver per liter. These solutions were added drop by drop, with slow stirring, and each mixture was finally stirred with fifty strokes of a glass stirrer. The tubes were then covered; between thirty and ninety minutes later the suspensions were compared in a nephelometer. The comparison consisted in exposing 65 mm. of one cup and recording the mean of twenty settings of the jacket of the other. The tubes were reversed after ten readings to eliminate instrumental errors. The tests were made a t room temperature, which varied from 2 j” to 29°C. Comparative tests with Pyrex and soft glass nephelometer tubes showed the former t o be much more satisfactory. Furthermore, one may well suspect that the opaque white patches which soon form on bhe soft glass tubes are due to silver chloride, although they cannot be removed by the usual reagents. The above statement may thus be extended t o include the large flasks and bottles used in work of the present nature. Preparation of Analytical Systems. Thirteen 16 gram quantities of silver nitrate Tvere dissolved in one liter portions of water or nitric acid and mixed with one liter portions of hydrochloric acid containing either a 3% excess or deficiency of HC1, at the rate of 5 cc. per minute, drop by drop, with frequent shaking. In some cases both solutions were 0.3 11 in nitric acid; in others the solutions contained no additional acid. Each silver chloride precipitate, contained in a 4-liter glass-stoppered Pyrex bottle, was washed sixteen times with 2 jo cc. portions of pure water, over a period of one month, with frequent shaking. The final z j o cc. washings, after standing over the precipitates for another month, were removed, acidified, and tested in the nephelometer. After two or three additional washings the water was drained from the silver chloride samples and one liter of pure 0.30 M nitric acid was added to each bottle. The bottles were shaken; after standing for a time the supernatant liquids were again tested. The precipitates were washed twice more and then shaken vigorously with 1000cc. of water. Concentrated nitric acid was added to make the solutions 0 . 2 5 XI in HXO,. Kephelonietric tests were
2240
CLYDE R . JOHNSON
then made upon each system, with the precautions noted above, until three out of four consecutive tests, made over a period of a week, gave three results with an average deviation less than 3 5 . These last tests invariably indicated that the supernatant liquids contained equivalent amounts of silver and chloride. Blanks. All subsequent additions to the systems were covered by two series of “equal-opalescence” blanks. The first series of tests were used during the crystallization of the “extra” compounds to insure the removal of impurities which might form opalescent suspensions with the standard solutions. The tests differed from those of the second series, described below, mainly in that the factor of sensit’ivity was emphasized, at the expense of uniformity, by the use of larger volumes of the precipitating reagents and greater concentrations of the compounds in the blanks. I n general, the compounds were crystallized until any disturbing impurities originally present s e r e reduced to a concentration in which they produced no effect under conditions in which sodium chloride or silver nitrate impurities equivalent to 0.02-0.04 mg. of silver per 0.I mol of compound produced a noticeable difference of brightness in the nephelometer tubes. The second series of blanks, rather more uniform but less sensitive than the above tests, accompanied the addition of the compounds to the analytical systems. These blanks were intended to determine whether or not the compounds themselves produced any noticeable effect upon the precipitating reagents, under the same conditions used in the final tests, but with silver chloride absent.
Addition of Extra Compounds. In each case an amount of the crystalline compound sufficient to make the solution 0.0j o M in the compound, plus the amount needed for the blank, was weighed out on a watch-glass. This material was made up to 20, 40, or 60 cc. with 0.2 j &I nitric acid, and a 1.00 cc. to 5.0 cc. aliquot portion was withdrawn and diluted with 0.2j ?vl nitric acid until 0.050 M in the compound, for the blank. By virtue of a simple algebraic calculation, the blank test was always made upon two 20.0 cc. samples, withdrawn from a solution somewhat larger than 40 cc. in volume. These samples were tested by the usual equal-opalescence procedure. If the examination in the nephelometer showed no opalescence in either tube after two hours, the remainder of the concentrated solution was added to the main system. I t was necessary to add the sodium pyrophosphate, ammonium metavanadate, and boric acid as dry powders. The blanks for these compounds were prepared from representative samples of the powders. After the above additions, the supernatant liquids were again regularly saturated with silver chloride, and tested until four consecutive tests, separated by intervals of three days, gave three results with an average deviation less than 3%. I n an analogous manner, the solutions were made 0.10M in the compounds and tested again. In this case the blanks were 0.I O M in the compounds and 0.2 j M in nitric acid. X few exceptions should be noted. The average deviations of the tests made upon the thorium nitrate solutions were 8% and 1 2 % ~ respectively, a t the o.ojo &Iand 0.1o-M concentrations. System
2241
XEPHELOMETRIC TITRATIONS
KO.19 contained the equivalent of 0.025 mols of T\”,VO3 per liter and 2 0 grams of silver chloride. The solution was 0.58 hl in nitric acid. Experiments similar to those described above, in which the total amounts of silver and chloride in the systems were extremely small, were also tried. The initial systems mere made by diluting I .oo cc. to 4.00 cc. volumes of the standard sodium chloride solution with 1000 cc. of 0.25 nitric acid, and adding equal (small) volumes of the standard silver nitrate solution. These experiments offered a means of separating effects due to the presence of the silver chloride precipitates, and also served to check the accuracy of the standard solutions and the burette calibrations.
Discussion of Results
Preliminary Tests. The results of the tests made during the washing of the silver chloride samples are condensed below:
Formation and Washing of Silver Chloride Svstem Initial Conc.
Solution Added By Drops
S o . 16
I
0.3
Ag?;Os
1.21
2
0.0
4
0.0
6 7 8 9
0.3
IO
I1
0.3 0.3
I2
0.0
I3 16 I8 I9
0.0
SO.
“ 0 3
Adding 0.2 jM
HNOj I
AgSO3
0.0
AgKO,
0.3 0 3
*kg?;O,
0.0
0.0
0.3
AgSO3 Agxos HC1 AgS03
.30
1.14 1.18 1.29 0.96 I .OI 1.49 0.94 0.92 I.
HC1 Ag?;O, AgxOs
so
1.04 0.99
0.84
I
0.90
0.99
1.16 0.91 0.83
1.00
1.01 0.94
0.86 I .02
1.19
.69 1.04
1.00
.oo .oo 0.98 I I
0.90 0.94 0.91
I .02
1.06
1.10
1.00
0.78
0.99
1.08
0.67 I .oo
I .OI
1.15 I
.03
1.02
I
.03
I .OI
Kotes. Each addition of acid was immediately preceded by two washings with 250 cc. portions of water. Washing No. 16 stood for a month over the precipitates; N o . 1 7 only a few days.
K i t h the exception of the values in the last column each value in the table represents a single nephelometric analysis. The result of each analysis is expressed as the average ratio of the exposed length of the cup containing excess sodium chloride to that with excess silver nitrate, a t the condition of equal opalescence, as shown by the nephelometer. That is, a ratio greater than unity indicates that the original samples contained excess chloride; a ratio less than unity indicates that they contained excess silver. This same convention is observed in Table I. It is a drawback of the equal-opalescence method that the results are not in terms of absolute quantities of material.
2242
CLYDE R. JOHSSON
There are available no trustworthy data for effecting this conversion. h s a rough approximation, holding only in certain limited cases, it may be taken that 0.01 ratio unit corresponds to about 0.01 mg. of silver or its chloride equivalent, per liter of solution. The data tabulated above furnish a little general information concerning the precipitation and washing of silver chloride which may be passed over without comment. They also reveal a characteristic of the analytical systems which is worthy of more detailed examination. Most of the supernatant liquids, standing after fifteen washings with non-acidified water, contained an excess of chloride over silver. On the other hand, the precipitates generally held excess silver, which was removed,-at least partially removed,-only by a large increase in the concentration of electrolyte in the liquid phase. I t is difficult to believe that the excess of chloride in the sixteenth washing was due to the “soaking out of occluded hydrogen chloride.” This hypothesis does not explain the reasonable assumption that fifteen z j o cc. .ivsshings with water, covering an entire month, are more than enough to remove any “occluded” material which the sixteenth washing might remove, even in another month. Nor does it explain the excess of silver left in (or on) the precipitates. It is easier to accept an explanation based on the view that colloidal silver chloride, on coagulation, tends to carry down more silver than chlorine atoms. However, without reference to any particular mechanism, it may be concluded that in working with these analytical systems one may consistently obtain tests for excess chloride in the solutions, even though the precipitates hold an excess of silver. This source of constant error, operating in any nephelometric atomic weight titration, would tend to make the calculated atomic w i g h t too low. The departure from accuracy would also be enhanced by the constant errors in the antecedent atomic weights used in the calculation, e.g., the atomic weights of silver and chlorine. The accepted values of these constants depend to a considerable extent upon determinations involving silver chloride precipitated from dilute solution. The Blanks. The extent to which the absence of disturbing impurities was guaranteed by the first series of blanks has already been noted. In the second series of blanks, with the exception of those covering the thorium nitrate, all of the paired tubes appeared equally “black” when v i e w d in the nephelometer I t follows that the corresponding compounds, listed in Table I , had no effect upon the equal-opalescence end-point due to any reactions with the standard solutions, at the concentrations used in this work. The tests also show the effectiveness of crystallization with centrifuging in removing chloride (and silver) impurities from these compounds.
Color a n d Stabilz‘ty of the A’uspensions. At the risk of making a few inaccurate statements in attempting t o generalize, some observations conccrning the color and stability of the equal-opalescence suspensions may be recorded. All of these apply to systems practically at the end-point. When the tests were made with the necessary precautions, the two suspensions were
NEPHELOMETRIC TITRATIONS
2243
usually quite alike, sometimes even identical, in color and appearance. No marked and regular differences in color between the two suspensions due to the action of any of the extra compounds were observed, although there were indications of small differences of this nature. The colors of the suspensions derived from the separate systems varied from blue-gray and yellow-gray to yellow-white with increasing solubility of the silver chloride in the presence of different extra compounds. In other words the color varied with the amount of material in suspension, also, of course, with its state of division. It would appear that effects due to the latter factor were largely eliminated by the uniform conditions of precipitation. In general, the above statements apply to tests made upon solutions regularly saturated with silver chloride a t room temperature. On the other hand there was a persistent tendency for the suspensions in the paired tubes to differ markedly in color and appearance. This tendency was aggravated by shaking or cooling the analytical systems or by deliberately reducing the silver chloride concentration in the nephelometer tubes by dilution. Changes usually appeared in both tubes simultaneously, and accompanied changes in the relative brightness of the suspensions in a characteristic manner, almost invariably taking the same course. The suspensions formed by addition of excess silver nitrate inclined toward bright blue and blue-gray colors, differing nevertheless from the normal colors. Those formed by the addition of excess sodium chloride became dull gray, with yellow, red, or brown tints. I n comparing the suspensions with respect to their opalescence or brightness color differences were found to increase the uncertainty of the nephelometric observations, and were avoided in the final tests, as indicated. As a rule the slispensions were quite stable. Although they often changed rapidly in relative opalescence immediately after precipitation, they did not subsequently alter for long periods. After standing for several hours they settled noticeably. The suspensions, once fully formed, were in no case observed to be very greatly affected by the presence of the extra compound. Eflect of Eztra Compounds. The results of the tests designed to examine the possible effect of extra compounds on the equal-opalescence end-point are summarized in Table I. Each nephelometric ratio in the table is the average of the values obtained from a t least three separate analyses. The method of obtaining these results has been described in detail; it seems to warrant the conclusion that the ratios show the correct experimental endpoints of the titrations represented in the table, with an accuracy corresponding to a few hundredths of a milligram of silver or its chloride equivalent, per liter of analytical solution. Reservations concerning the solid phase in the analytical systems have been noted, The deviations from the theoretical (equal-opalescence) value of I .oo are in most cases quite small. Those observed for System No. 1 8 are mainly due to a chloride impurity in the thorium nitrate, as the blanks testify. Some of the others may be ascribed, not unreasonably, to the action of the extra compounds. Even for precise titrations these deviations are equivalent only to minor errors, which may be eliminated in any case by the use of the correct
CLYDE R. JOHNSON
2244
experimental end-point. In brief, when the tests are made with the necessary precautions, the accuracy claimed for the nephelometric method of analysis by investigators who have used it in atomic weight work may be attained, in so far as any effects due to the presence of extra compounds come into consideration. The writer takes this occasion to thank Professor George A. Hulett for his interest and co-operation in this work. TABLE
1
The Effect of Extra Compounds w t . of System
so. I 2
3
solid ARC1 Grams
13 '3 0.001
4
I3
5 6 7 8 9
0.004
I3 I3 13 I3
Observed Sephelometric Ratios in .4eCl Solutions 0.25 hl in H S O - : Compound compo& Cornpoind Absent o.ojo XI o 10 hf I .03 0.99 0.99 I .oo 0.98 I .oo
-
-
1.16 I .oo 0.86 0.89
0.97
1.00
.oo
0.98 I .02
1.02
0.98
.oo 0.98 0.96
0.98 0.93
I .oj
I .OI
1.0;
1.06 1.08
I
I .02
I1
13 73 I3
I .02
IS
16 I7 I8 I9
Sone Sone I3 0 001
13 20
1.00
1.00
I3
I3 I4
I .02
0.98 0.97
IO
I2
I .OI
I I
.03 .oo
1.00 I .oo 0.99 1.00 I .OI 1.01
I
I
.07
1.18
0.8; 0.86 I . I4 (1.03)
1.0;
I ,
14
1.04 1.24
-
Xote. Silver rhloride solution 14 was saturated at room temperature; solution 15 contained I .33 mg. of AgCl per liter.
Summary A method for testing arbitrary features of the procedures used in precise nephelometric titrations has been outlined. The effect on the equal-opalescence end-point of thirteen compounds of various types has been studied. Princeton, Seu, Jersey
