ANALYTICAL CHEMISTRY
1342
tially h e d increases in amount, it soon becomes less effective in promoting the deposition of silver. The foregoing method for estimating silver has been successtully applied in practical problems. Because the silver in the sample is separated from the other constituents when it is fixed as the bromide, the method has a decided advantage over the a-benzoin oxime test for copper, in which repressing the interCerence of other ions reduces the sensitivity of the test. The precision with which silver can be determined by the transmittance measurements described above is difficult to estimate, but it probably approaches * 10% under the best conditions.
2nd Annual Summer SwmposZum
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LITERATURE CITED
(1) Bull, H. B., Hahn, J. W., 71, 550 (1949).
and Baptist, V. H., J. Am. Chem. Sac..
(2) Feigl, F., “Qualitative Analysis by Spot Tests,” p. 45, New
York, Elsevier Publishing Co., 1946. (3) Ibid., p. 65, (4) Michaelson, J. L., 445 (1936).
and Liebhafsky, H. A., Gen. Elec. Reo., 39
(5) Jlllller, R. H., and Clegg, D. L., “dutomatic Paper Chromatography,” Yew York .4cademy of Sciences Conference March 26, 1949. RECEIVEDingust 4 , 1949.
Organic Reagent8
Organic Reagents in Inorganic Analysis Sources of Error and Interferences PHILIP W. WEST, Louisiana State University, Baton Rouge, La. Such problems as reagent isomerism and tautomerism, isomerism of reaction products, and variations in coordination numbers may be mentioned as peculiar but important aspects in the use of organic reagents for inorganic analysis. A general survey of these and other problems connected with the use of organic compounds for qualitative and quantitative work is presented.
T
HE general applicability of organic reagents for inorganic
analysis seems to compare most favorably with that of inorganic reagents from almost every standpoint. On the basis of reliability, which is the sole consideration covered in the present discussion, they can be assumed to possess a number of distinct advantages. This discussion is therefore introduced not as a deterrent to the use of such reagents but rather in the hope that by directing attention to the modes of interfering reactions and the sources of general errors, more rapid development and utilization of organic reagents for analytical purposes can be promoted. Brief mention is justified of some of the factors contributing favorably to the use of organic reagents when considered on the basis of reliability. A very significant characteristic of many such reagents is a high degree of selectivity, or even specificity, when applied under appropriate conditions. Specificity is one of the most important attributes in analytical chemistry, and although progress of real significance along such lines is already marked (1-3’), continued development can be anticipated through the invaluable investigations of such men as Feigl, Baudisch, Yoe, Kolthoff, and other brilliant pioneers in this phase of scientific development. Factors other than specificity add to the reliability of organic reagents. Inorganic-organic compounds encountered in gravimetric analyses usually have such low solubilities that solubility effects are of minor significance. Such substances, particularly of the chelate type, are usually nonhygroscopic in character, which is a most important attribute for gravimetry. Of prime importance, hou ever, are the very favorable equivalent weights of practically all inorganic-organic precipitates, such that very small chemical factors occur in gravimetry; in titrimetric procedures involving indirect methods such as the bromate technique (iodometric), equivalent weights of metallic ions may be as little as a tenth or even l/**thof the ionic weight. PURITY OF REAGENTS
One would be naive indeed to assume that analytical chemists fail to appreciate the importance of purity of reagents. Organic
reagents are of somewhat special character as compared to inorganic compounds. Someone once said that organic chemistr) is the chemistry of reactions and side reactions. Organic reagents are sometimes contaminated with impurities-these ma! be derived from side reactions in the process of manufacture or they may result from general instability of the reagents or decomposition of their solutions. An interesting but extreme example of the importance of impurities in organic reagents has been noted by West and Hamilton (9), who have obtained evidence that resorcinol, which is often used as a reagent for the colorimetric detection and determination of zinc, is not itself active in this instance. Instead, an impurity present only as minute traces in resorcinol serves to produce the test color. Falea ( 1 )has called attention to an important instance where impurities present in a reagent may lead to unexpected error. He has noted that 1-nitroso-2-naphthol may contain an impurity which reacts with nickel to give a precipitate. This impurity was found in a number of commercial samples of the reagent as well as in material freshly prepared in the laboratory. The indication in this case is that some side reaction occurs in the normal preparative procedure to produce the active impurity. That the 1nitroso-2-naphthol is itself stable is evidenced by the observation of Fales that solutions of the reagent that have stood for a f e a days can be used with confidence, because the deleterious impurity settles out as a brown precipitate. ERRORS IN QUALITATIVE ANALYSIS
Errors resulting from the use of organic reagents for qualitative inorganic analysis usually take the form of simultaneous competitive reactions. In fact, a very important form of interference, the so-called “negative” or “masking” interference, is due very often to concurrent reactions tThich involve the reagent and reduce its effective concentration so low that the characterizing test reaction is inhibited. The more generally understood type of interference is the one involving the formation of reaction products similar to those of the true test reactions. Such false tests are most often due to the presence of ions which are verj similar to the ion sought, so that analogous reactions occur which may be indistinguishable from the test reaction itself. Detailed
V O L U M E 21, NO. 11, N O V E M B E R 1 9 4 9 consideration of the topic of interferences encountered in qualitittive analysis has been included in a paper by West (8). ERRORS IN QUANTITATIVE ANALYSIS
A wide variety of sources of error is possible in the application of organic reagents to quantitative work. Fortunately, such possibilities seldom materialize, and by careful attention to the details of procedure most potential hazards can be avoided. Gravimetry. Probabl~ the most general source of error in gravimetric work rmults from the low solubility of many organic precipitants in water. Excess reagents, when added to aqueous solutions, may result in the precipitation of the reagent and consequent contamination of the precipitate. Such contamination obviously introduces serious error v here the reaction product IS to he dried and weighed as such. True, it is often expedient to convert such precipitates to the form of the metal oxides before 11 eighing and thus obviate errors due to contamination by the reagent, but such procedures invalidate one of the significant advantages of organic precipitants-the possibility of very favorable equivalent weights. I t seems preferable, therefore, to recognize the source of such error and then circumvent it by carefully avoiding harmful excesses of reagent; most precipitates involved are so insoluble that the addition of excess reagent for mass action effects is unnecessary. Isomerism is somewhat overlooked in analytical chemistry but cannot be avoided as an important source of error. Isomwic forms may be encountered in the reagent itself and in the reaction product; in either case wide variation in analytical chemical behavior may result. The well known case of the reactivities of the dioximes may be cited in this regard, inasmuch as such wide differences exist between the a-, p-, and 7-dioximes in connection with their reactions with metallic ions. The configuration of these isomers is depicted as:
'
H0
O 'H a
ITd
\OH
P
\ \
OH OH 7
The LY form is highly selective in its reactions, the p isoinri gives no reaction with metals, and the y form reacts with man3 metals but is of no value because of its almost total lack of selectivity. A second type of isomerism involves the reaction product itself and was considered as early as 1905 by Tschugaeff ( 7 ) . Where unsymmetrical molecules are employed, cis-trans isomers may be formed which vary considerably in their physical characteristics. Sugden has shoivn ( 6 ) that methylbenzylglyoxime can form such isomeric compounds in its reactions with nickrl, as shown by formulas I and 11:
1343 He was able to produce these products, identical in molecular weight, but differing in both solubility and melting point. Obviously, such situations are of importance to the analyst, although equilibrium conditions ordinarily exist so that under controlled conditions of analytical experiment, products of reproducible properties are obtained. In the case cited above, for example, the a form was found to melt at 168" C. while the p form melted a t 75-77" C. The p isomer was thermolabile and waq rapidly transformed a t 120" C. to produce an equilibrium mixture melting at 152' C. I t can be assumed that the solubility of the mixture would vary at different temperatures and in different solvents and that change of solvent would affect the equilibrium position; such a precipitate would require careful study before reliable analytical procedures could be devised. Coprecipitation and postprecipitation phenomena may also be sources of error in the gravimetric application of organic reagents, but little positive evidence is available to show the extent A survey of accepted procedures employing organic precipitants fails to disclose references to such difficulties and it is interesting to speculate concerning the effect of coordination on the tendency toward coprecipitation. I n the case of chelate formation where coordination spheres of the central atom are filled, little coprecipitation should occur through adsorption. I n such cases, precipitation itself should be a more complex process than where normal salt formation is involved; as a conaequence, there should be a reduced tendency for occlusion and mixed crystal formation because not only the ionic radii of the coprecipitating ions would need to be of the same order of magnitude a3 the ion being determined but, in addition, the coprecipitating ions should have corresponding tendencies toward coordination. Because of the relatively large size of organic precipitants, few inorganic ions can replace them in a crystal lattice nithout undue strainanother condition minimizing the likelihood of occlusion. Low results sometimes are caused by competitive reactions Rhich reduce the effective concentration of the reagent. For example, in the determination of nickel by precipitation with dimethylglyoxime in the presence of zinc, cobalt, and copper, extra amounts of reagent are required because of the formation of soluble complexes of these metals. The volatility, together with the decomposition, of precipitates constitutes an important source of error. Especially in the case of rhelate compounds, volatility of precipitates is appreciable and low temperature drying nil1 usually be required for accurate work. In cases where precipitates must be ignited, great care should be taken and i t is \vel1 to add oxalic acid or ammonium nitrate to the precipitate prior to the ignition as a means of obviating excessive volatilization. ' Titrimetry. Few errors in titrimetric analysis involving organic reagents can be attributed solely to the use of organic compounds. Purity of the reagents can be a factor, especially where oxidimetric methods are used, and, similarly, stability of the reagents and of the reaction products should be considered. Where the bromate procedure is employed, volatilization of bromine or of the iodine liberated can constitute a serious source of error. One of the most serious criticisms of many titrimetric procedures is the utilization of empirical reactions in indirect technique However, such methods are giving good results and although some of the reactions employed may be complicated, they are often stoichiometric in nature. Colorimetry. Three main types of errors in colorimetric work are generally peculiar to organic reagents: isomerism of reagents and reaction products, tautomeric shifts, and competitive reactions. In the case of isomerism the most important types would seem to involve reaction products. Such possibilities can be shown by reference to the investigation of Ray and Bose ( 4 ) of the cis-trans isomerism of ferrous quinaldate. See structural formulas I11 and IV, next page. Two forms, differing in color, were demonstrated. I n the case
ANALYTICAL CHEMISTRY
1344 0
0
0
I11
0
phenomenon to a variation in coordination number of the iroD from 4 to 6. This theory is consistent with the findings of Reihlen (6), that a red iron-pyrocatechol complex exists of the type
IV
cited, a red compound representing one of the isomers proved to be unstable and was transformed into a stable violet form. Although this illustration is mainly of academic interest, potential dangers of this type of phenomenon are being encountered in practice. Tautomeric equilibria are of real importance in colorimetric work: the well known example of diphenylthiocarbazone serves to illustrate this. Diphenylthiocarbazone exists in either the keto or enol forms, depending on the pH of the solution.
I
-
-\
L
-I
while a blue salt having a formula
could be isolated. By careful attention to the details of pro cedure such variations can be eliminated. The possibility of interferences arising from such shifts should be kept in mind however, because many colorimetric tests depend on the rhemiral hehavior of coordinated compounds SUMMAR1
A survey of the sourceb of error and interferences occurring
It is well known that the form of the reagent affects its reactions and thus becomes a factor in quantitative analysis. In most cases involving tautomerism, careful control of the hydrogen ion concentration serves to establish reproducible conditions. The significance of competitive reactions in colorimetry is of apecial interest because so many of the color-forming reagents lend themselves readily to complexation. Where soluble but colorless complexes are formed by diverse ions, low results may be caused because of the lowered concentration of the reagent. The practical importance of this phenomenon is attested to by the fact that even the reliable 1,l’-bipyridine method for the determination of iron suffers from interferences when large amounts of zinc are present: the deleterious effect of zinc is undoubtedly due to the utilization of the reagent in the formation of ammine-type complexes. A fourth type of interference may be best described as a coordination number phenomenon. In certain cases the coordinating capacity of metals may vary, depending on the solvent, temperature, pH, and concentration of complexing agent. Where there is a variation in the amount of addenda coordinated, a variation in color can be anticipated. Yoe and Jones (IO) in their studies of sodium 1,2-dihydroxybenzene3,5-disulfonate found that three complexes were formed with ferric iron, depending on the pH. At low pH values a blue complex was found to exist. As the pH was raised a sudden change to a violet colored compound occurred a t p H 5.7, which was then gradually converted to a red form a t pH 9.0 to 10.0. They ascribed thip
ii
the use of organic reagents indicates many possible hazards Actual experience, on the other hand, shows that organic reagents compare very favorably Rith inorganic reagents. There is considerable evidence, in fact, to show that organic compounds can be used with greater confidence, and with the development of more compounds that are specific in their reactions it seems highly probable that the analytical chemist will depend more and more on organic reagents. Users of organic reagents should keep in mind the specia properties of organic compounds and of organic-inorganic com pounds, as well as such factors as solubility of reagents, stability and purity of organic compounds. It is extremely importani also that the analyst be familiar with the chemistry of coordinated compounds, for much of the work involving organic reagents actually entails the use and behavior of complex ions and mole rules LITERATURE CITED
h’ales, H. A., and Kenny, F., “Inorganic Quantitative Analyuis.’ p. 349,New York, Appleton-Century, 1939. Feigl, F., ANAL.CHEM.,21, 1298 (1949). Feigl, F., “Chemistry of Specific, Selective, and Sensitive Hractions,” New York, Academic Press, 1949. Ray, P. R., and Bose, M. K., 2.anorg. Chem., 95,400 (1933) Reihlen, H., Ibid., 123, 173 (1922). Sugden, S., J . C h a . SOC.,1932, 246. Tschugaeff, L.,2.anorg. Chem., 44, 146 (1905); 46, 144 (1905) West, P.W., J . Chem. Education, 18, 528-32 (1941). West, P.W., and Hamilton, W. C . , unpublished studies. Yoe, J. S., and Jones, A. L., IND. ENG.CHEM.,ASAL. ED.,16 111-15 (1944). R ~ C B I V E July D 14, 1949.
