NEPHELORIETRIC‘ TITRATIONS. I. THE EQUa4L-OPALESCESCE ESD-POIST* BY CLYDE R. JOHNSOX**
One of the outstanding procedures now in use in the determination of exact atomic weights is the so-called “nephelometric method” of titration, The general method consists in estimating the stoichiometrical ratio between a pure compound furnishing chloride or bromide ions, and pure silver, with the use of the nephelometer. This method was first used by Richards and Wells’ in 1 9 0 j to determine the SaCl:+ig ratio, from which the atomic weight of sodium may be calculated, by taking suitable values of the antecedent atomic weights of chlorine and silver. The method has subsequently been applied to the analysis of other chlorides and bromides, to determine the atomic weights of many elements. In these analyses, the procedure described by Richards and Wells has been used without essential modification, in determining the end-point of the nephelometric titrations (the equal-opalescence end-point). At the present time the accepted values of a large number of atomic weights rest almost entirely upon ratios determined by the nephelometric method. This is in part due to the convenience and simplicity of the method; it has been preferred by investigators who have used modern methods to prepare pure compounds for atomic weight analysis. Furthermore, the “probable error” of atomic weights calculated from ratios determined nephelometrically is rather generally lower than the “probable error” of corresponding values calculated from ratios found by other chemical methods. In short, there has developed a tendency toward giving “nephelometric” values preference over the values obtained by other methods, which are regarded mainly for the confirmatory evidence which they furnish. Curiously enough, in view of the wide application of the nephelometric method in atomic weight work, and the general reliance placed on “nephelometric” results, there have been few attempts t o demonstrate the unqualified applicability of the method in the case of particular analyses. Richards and Wells, in a series of tests which showed the applicability of the method to the XaC1:Xg titration, established the essential soundness of the equalopalescence end-point. Kevertheless, there are certain features of this endpoint which make it seem desirable to extend the experiments of Richards and Wells, more particularly to titrations in which multivalent ions are present in the analytical solutions. I n the titrations under consideration, a n acid solution containing the chloride or bromide ions from a weighed quantity of a pure compound is ?Contribution from the Chemistry Department of The Rice Institute. **?iational Research Fellow in Chemistry. Richards and Wel!s: J. Am. Chem. SOC., 27, 502 (190j).
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precipitated with almost the t’heoretical amount of pure silver, weighed, and dissolved in nitric acid. The titration consists in adjusting to equality the silver and halide ions in the resulting supernatant’ solution, by the use of standard silver and halide solutions. I n the equal-opalescence method, this adjustment is based on nephelometric tests which, presumably, show the relntice amounts of silver and halide ions in the solution. The end-point of each titration, and hence the calculated atomic weight, depends upon the comparison of two colloidal suspensions of silver chloride. These suspensions are formed in equal samples of the supernatant analytical solution, in two matched test-tubes, under conditions as nearly identical as possible. Silver nitrate is added in excess to one tube. An equivalent excess of a suitable alkali halide is added to the other tube, so that, according to the “solubility product” principle, one suspension represents practically all of the silver ions, the other practically all of the chloride ions present in the supernatant analytical liquid. At the stoichiometrical point, the numbers of silver and halide ions in the supernatant liquid are, supposedly, equal. If, from a solution at the stoichiometrical point, the above procedure results in the production of two suspensions possessing equal light-reflecting power during the time required for the nephelometric observations, the desired correspondence between the end-point and the stoichiometrical point is attained. In this case the opalescences in the two tubes are equal, and the two parts of the divided field seen through the nephelometer eyepiece are the same, when the ratio of the exposed lengths of the tubes is 1.00. I n every equal-opalescence titration, in spite of the attempt to compare the silver chloride suspensions under exactly similar conditions, there must be a t least one very marked difference in the two sols. One, stabilized by the adsorption of excess C1- ions, is negative; the other, stabilized by the adsorption of excess Ag+ ions, is positive’. Experiments may be cited to show that under certain special conditions this dissimilarity introduces no error into the nephelometric readings. For example, Richards and Kells’ demonstrated that the “extra” ions present in the analytical solutions had no effect upon the end-point of the SaC1:Ag titration. Scott and Johnson3 tested five saturated solutions of silver chloride containing varying amounts of nitric acid and found no deviation from equality of silver and chloride ions which would affect even the most accurate atomic weight analyses. For the present purpose, it seems desirable to emphasize the fact that the ions involved in the above-mentioned tests were all univalent. In other atomic weight determinetions, “extra” ions of the most widely varied character, di-, tri-, and tetravalent, derived from the compounds undergoing analysis, have been present in the test solutions examined in the nephelometer. The effect of the adsorption of these ions upon the quantity, state of division, structure, color, and stability of the sols (Le., upon their lightreflecting power) cannot certainly be stated. However, it is known that the lLottermoser: J. prakt. Chem., (2) 72, 39 (190j); 73, 374 (1906). Richards and Wells: .J. Am. Chem. SOC.,27, 503 (1905). Scott and Johnson: *J. Phys. Chem., 33, 1981 (1929).
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coagulating power of some ions is hundreds of times greater than that of other ions. Furthermore, any given ion is more likely to affect a sol of opposite charge than one having the same charge’. Only rarely, then, would one expect the action of any particular “extra” ion to be equal in the two nephelometer tubes,-in most cases its effect upon the properties of the two oppositely charged colloidal suspensions should be different, The good general agreement between atomic weights determined nephelometrically and corresponding values determined by other methods insures that the effect under consideration must be small. Severtheless, one would be unjustified in assuming that the effect is negligible. The nephelometer, as Richards and Kells2 have often pointed out, is an extremely sensitive instrument. It seems very likely that the unbalanced action of certain ions (mainly multivalent ions) may cause differences in the light-reflecting power of the sols large enough to produce serious constant errors in the nephelometric observations. There is nothing in the procedures described in reports of atomic weight titrations to assure one that the above-mentioned source of error has been avoided. The conditions (temperature, time after precipitation, etc.) under which the suspensions of silver chloride are compared in nephelometric titra tions are really quite arbitrary, although they depend more or less upon the “solubility product” idea. In most cases it has been customary to compare the suspensions at the equilibrium condition which most facilitates the nephelometric observations. Since the equilibrium is influenced by factors which the “solubility product” rule does not take into account, the point of equal opalescence in any given titration may or may not correspond to the stoichiome h a 1 point. Under the circumstances, one may reasonably question the unqualified general applicability of the equal-opalescence endpoint to extremely accurate titrations. Only applications based on experiments such as those described by Richards and \Tells would appear to be entirely trustworthy; the procedure should be tested for each analysis. Some preliminary tests, made in this laboratory, indicate that a small effect exists, and emphasize the need for further experiments, which are to be undertaken at Princeton University, under a grant from the National Research Council. summary .1 possible source of error in the equal-opalescence end-point used in nephelometric atomic weight titrations has been pointed out. The nature of the possible error suggests the advisability of testing some of the more general applications of the original procedure described by Richards and Wells. The writer wishes to acknowledge his indebtedness to Dr. H. B. Weiser and to Dr. A. F. Scott for suggestions concerning this article. Houston, Texas.
’ Schulze. J. prakt. Chem., (2) 25, 431 (1882); 27, 32n (1883). 2Richards and Wells: Am. Chem. J., 31, 239, 241, 242 (1904); J. Am. Chem. Soc., 27. 486 (1905).
