139
L U M E 28, N O . 1, J A N U A R Y 1 9 5 6 Hendenon, L. M., and Kracek, F. C., J . Am. Chem. SOC.49, 738 (1927). Hermann, J. rl., “Separation of Americium from Lanthanum by
Fractional Oxalate Precipitation from Homogeneous Solution,” U. S. rltomic Energy Commission, AECD-3637 (July
less error in these analyses. This minimizes the error in X factors for small degrees of precipitation, but leaves considerable possibility of error in extensive precipitations, where the compositions of t h e total precipitate and original are similar.
22, 1954).
Kolthoff, I. hl., and Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,” 3rd ed., p. 123, llacmillan, New York, 1952.
Table I.
Salutsky, M. L., Stites, J. G., and Martin, A . W., ANALCHEM. 25, 1677 (1953).
Shaver, K., Division of Physical and Inorganic Chemistry, 125th Meeting, AM.CHEM.SOC., Kansas City, Mo., 1954. Wahl, A. C., and Bonner, N. -4.,”Radioactivity Applied to Chemistry,” p. 106, Wiley, New York, 1951. Weaver, B., ANAL.CHEM.26, 474 (1954). Ibid., p. 476. Ibid., p. 479. Weaver, B., “Fractional Separation of Rare Earths by Oxalate Precipitation from Homogeneous Solution,” U. S. Atomic Energy Commission, ORNL-1629 (Dee. 4, 1953). Nonsanto Chemical Co. Dayton, Ohio Syracuse University Syracuse, N. Y.
MURRELL L. SALUTSKY LOUISGORDOS
SIR: I n an article published in ANALYTIC.4L CHEMISTRY[26, 474 (1954)l I pointed out t h a t most workers engaged in difficult separations, such as between the rare earths, have failed t o publish a n y standards b y which the efficiency of their separations can be measured I n m y own separations, and I assumed in those of most other M orkers, there was a schematic arrangement of pairs of fractions. T h e two fractions of a pair !yere usually of nearly equal quantity. .is I was interested in the difference between the members of a pair, I expressed t h e degree of separation of two elements as the ratio of the relative abundances of these elements in the two fractions and called this the separation factor. I applied this rrlationship t o the measurement of the efficiency of the few other vparations published with adequate data, and in tTvo concuriently published articles applied it t o m) own work on two methods of separating rare earth. Subsequently L Gordon and 11 L. Salutsky have pointed out t h a t this homogeneous distribution factor, D , applies strictly only t o separations in vhich the two fractions are in complete equilibrium throughout t h e process of separation, as in liquidliquid extractions. In pi ecipitations or crystallization8, espec~:dly those made from homogeneous solutions, the solution is in equilibrium only with the surfaces of t h e solids. T h e separation efficiency here can be iheasui ed more accurately by the logarithmic distribution coefficient,
Fractionation of Samarium-Neodymium Mixtures (Yariation in degree of urecipitntion)
Ratios of SmnOa t o iFd20P Degree of P p t n , Filtrate 7% Precipitate (calcd.) 1 59 0 988 2 8 1 49 0.973 6 6 1 68 0 962 7 0 1.44 0.931 I6 2 1 RR 0.904 16 8 1 35 0.848 35 7 1 48 0,800 36 2 1 33 0.792 44 8 1.35 0.782 45 1 1 . 3 2 52.4 0.737 54 2 1.30 0.735 1.33 0 700 55 7 1.24 74 3 0.530 1.22 75 1 0.537 1 13 83 5 0.528 1.03 89 2 0.770 a
St-Paration F a c t o r ~~D h 1.61 1 . 53 1,73 1.55 1.84 1.59 1,85 1 08 1.73 1 i9 1.77 1.90 2.34
1.55 1,32 1.73 1.51 1.75 1.22 1.65 1.48 1.51 1.51 1.50 1.55 1.50 1.55 1.42
2.27 2 14 134
1.12
Originally 1: 1.
T h e d a t a in Table I show t h e X factor t o be considerably more nearly constant than t h e D factor. However, its use does leave one rvithout a simple relationship betveen the products of a separation. I do not know why the X values calculated solely on the basis of analyses of precipitates are more nearly constant than if calculated from filtrates, b u t this is a fact in this instance. As D values involve both fractions, they are not so greatly affect,ed by analytical inaccuracies in only one fraction. I can understand t h e critics’ puzzlement regarding the difference in ORSL-1629 and the published article. The embarrassing fact ip that, while the article was not submitted until after ORXL1629 had been written, the copy submitted was an earlier version written some time before the additional work given in the larger table had been done. There was no selection of items. The enlarged table had been given at Chicago, and I intended to publish it. While all analyses of rare earths are subject t o some error, the most obviously inaccurate analyses were obtained in the case of 16.8% precipitation, the case which Gordon and Salutsky continually point out a s typical example. I should prefer to omit it from the table. BOYDWEAVER Union Carbide Xuclear C o . Oak Ridge, Tenn.
Both factors have appeared in previous publications. The applicability of each expression can be tested b y calculating it for different degrees of precipitation. I n m y article on separation of rare earths by oxalate precipitation I applied the D factor t o a few varying degrees of precipitation of samarium-neodymium mixtures consisting initially of equal amounts of the oxides of these elements. Subsequently the data n e r e extended hv more euperiments. I have now calcuh t e d both D and X factors for the full set of experiments. Littention must be called t o the fact t h a t when the extent of precipitation iq small, the composition of t h e final fraction is only slightly different from t h a t of the initial material. A slight error in malysis, easily made in the case of rare earths, can make a great difference in t h e logarithmic ratio. I n the present case the compositions of the final fractions have been calculated from those of the original mixtureP and the p i wipitates, as there appeared t o be
“Standard Addition” Method of Polarographic Analysis SIR: One of the most valuable technique3 of quantitative polarographic analysis is the standard addition method devised by Ilohn ( 1 ) and discussed by KolthofT and Iingane ( d ) , Taylor ( 4 ) ,and Meites (3). Startingwith a known volume of the sample, the diffusion current, il, of t h e desired wave is measured; then a known volume of a known solution of the sulxtance being determined is added, and the diffusion current, il, is measured again. The calculation of the concentration of the substance in question in the original solution follom from an equation of the form C, = ilcCs/ [i2(1-
+ v) - i I V ]
(1)
ANALYTICAL CHEMISTRY
140 where C, and CS are the concentrations of the sample and the standard, respectively, and V and z are the corresponding volumes. Reiterated in the literature on this simple technique are statements t o the effect t h a t for maximum precision the amount of standard solution added should be sufficient just about t o double the wave height. As this would be a nuisance in practical work, and the truth of this assertion seems t o have been generally accepted, the present demonstration of its falsity would appear to be of some interest. It is convenient t o assume t h a t the error in C, arises from errors in il and ipalone-in other rvords, that v, V , and CS are measured with considerably better accuracy than the two diffusion currents. It is also convenient t o assume that, as is ordinarily true in practice, si is much larger than v, although the validity of the conclusion is in no way affected if this is not the case. This permits rewriting Equation 1 in t h e form
C, = h: ill(& - il) A simple differentiation then gives t h e relative error of the result:
T h e first term on the right of Equation 2 is simply the relative error of il. For a given relative error of measurement of il or iz,
however, the second and third terms have their smallest values, not at i p = 2il, but a t i p > > il. I n other words, maximum precision is attained when the diffusion current of the unknovm is negligibly small compared t o that obtained after adding the standard. If it is assumed t h a t a 1% error may be made in each diffusion current measurement, the relative error of the answer may reach 4% if i z = 2il. If i p = 1Oi1, however, the corresponding relative error of the answer cannot exceed 2.2%, which is almost a twofold gain in accuracv. It is evident from this simple analysis that the amount of standard solution added should actually be sufficient t o increase the observed diffusion current by a factor of 10 or so, except in the event that the diffusion current is a linear function of concentration over too narrow a range t o permit so large an addition. LITERATURE CITED
(1) Hohn, H., “Chemische Analysen mit dem Polarographen,” p. 51, Springer-Verlag, Berlin, 1937. (2) Kolthoff, I. II., and Lingane, J. J., “Polarography,” p. 251, Interscience, Xew York, 1941. (3) Meites, L., “Polarographic Techniques.” p. 178, Interscience, New York, 1955. (4) Taylor, J. K., ANAL. CHEM.19, 368 (1947). LOUISMEITES Polytechnic Institute of Brooklyn Brooklyn 1, K. Y.
MEETING REPORT
Society for Analytical Chemistry MEETING of t’he Scottish Section was held Sept.
30 a t
A Glasgow, at which t h e following papers were presented and discussed. Lead in Biological Materials. S. L. TOMPSETT, Northern General Hospital, Edinburgh. The clinical applications were described under the headings: the level of blood lead as an aid in the diagnosis and treatment of lead poisoning in the human, the excretion of lead, and the distribution of lead in the tissues of the “normal” subject and in cases of lead poisoning. The determination of lead in urine, feces, blood, soft tissues, and bone was then dealt with under the headings: destruction of organic matter, separation of lead with ether as a diethyldithiocarbamate complex, and colorimetric determination with dithisone by the reyersion technique. Determination of Lead by Square-wave Polarography. D. J. FERA.E.R.E., Harwell, Berks. A brief survey of the principles of square-wave polarography illustrates how, with this technique, reducible metal ions-e.g., lead-may be determined at concentrations down t o 2 X lo-’ (0.05 y of lead per ml.). The differential nature of the technique eliminates much of the chemical treatment normally needed before polarographic determinations. These advantages were illustrated in the determination of lead in cocoa (0.7 p,p,m,), in analytical reagents (0.7 p.p.m.), in aluminum, zinc, and tin-base alloys (0.003 t o 0.1%), and in steels (0.2%). RETT,
At a meeting on Oct. 5 in London, the following papers were presented. Colorimetric Determination of Phosphorus in Steel and CopperBase Alloys. W. T. ELWELL ASD H. N. WILSON, Physical Chemistry Group, I.C.I., Ltd., Billingham. A colorimetric procedure was recommended for the determination of phosphorus in all classes of steel; it has also been successfully applied to two standard bronze samples. The procedure permits a considerable saving in time, particularly in the examination of highly alloyed steels, when the determination can be completed in under 1.5 hours, compared with about 8 hours required by the existing gravimetric method.
The determination is based on the formation of phosphovanadomolybdic acid, which is soluble in amyl alcohol. The yellow color, which is proportional to the amount of phosphorus present, does not fade and can be measured in any convenient way. Determination of Small Amounts of Carbon in Steel by LowPressure Analysis. R. M. COOKA N D G. E. SPEIGHT, hlond Nickel Co., Ltd., Birmingham, and Richard Thomas & Baldwin, Bucks. 9 simplified low-pressure method was described for the determination of carbon in steel. The use of a normal combustion furnace in conjunction with the low-pressure analytical apparatus permits samples to be analyzed with a precision of =tO.OOOl% of carbon up to 0.036% of carbon, the deviation being slightly greater with higher carbon contents. The method lends itself readily to the analysis of carbon contents on a semimicro basis when only small weights of sample are available. TTith medium and high carbon steels, analyses may be carried out on samples weighing as little as 0.05 gram with a degree of accuracy at least equal to that given by the normal gravimetric combustion method operated under the best conditions, 2.729-gram samples being used. Determination of Small Amounts of Sulfate by Reduction of Hydrogen Sulfide, and Titration with Mercuric or Cadmium Salts using Dithizone a s Indicator. E. E. ARCHER. Distillers Co., Ltd., Great Burgh, Epsom, Surrey. Sulfate is reduced to hydrogen sulfide with an acid reduction mixture. The hydrogen sulfide is absorbed in an alkaline solution containing dithiaone, and titrated with a solution of mercuric acetate or cadmium sulfate. At the end point there is a sharp color change from yellow to red, owing t o the formation of dithiaonates.
.kt a meeting of the Midlands Section on Oct. 12, a paper on “The -4nalyst’s Dilemma: Color or Stability” by R. J. P. Williams, Merton College, Oxford, was presented. Intense color as the property of a chemical compound, such as a complex ion, is frequently accompanied by instability of the compound. Inspection of the reasons for instability and of the sources of color shows that the two properties are often related. The analyst can use only unstable colored compounds in spot tests. Quantitative reagents demand a stabilization of color. The differences between stable and unstable colored compounds can be explained and the knowledge used t o design reagents. Most of the examples discussed were complex ions.
