116
ANALYTICAL CHEMISTRY
RJ( C H ~ C H ~ O H ) C H ~ C OT+ OR&HCH~COO-
+ CH~CHO
Tlie water-soluble secondary amine chosen for use with sodium nitroprusside, in detecting the formation of acetaldehyde, was diethanolamine, since its stability, water-solubility, and low volatility made it appear eminently suitable for the purpose. Table I lists the reactions to this test of various amines and nitrogen-containing surface-active agents. PROCEDURE
Two hundred milligrams (or 4 drops) of a riitrogeii-coritairiirlg compound, 0.2 to 0.3 gram of sodium chloroacetate (Dow Chemical Co., technical grade, was used), and 1 to 1.5 ml. of tetraethyleneglycol dimethyl ether (Ansul Chemical Co. ) are placed in a 5-inch test tube and agitated vigorously for a few seconds. The test tube is clamped a t an angle of no more than 30’ from the horizontal (to eliminate spatt’eringduring the pyrolysis) and a glass delivery tube with a GOO-angle bend is att,ached by means of a one-hole rubber stopper. The end of the delivery tube passes beneath the surface of the “detecting solution” contained in a +inch test tube support’edby a wire gauze placed across an iron ring. I n order to facilitate observation of color changes in the detecting solution, a piece of whit’e paper is placed over thr wire gauze supporting the test tube containing the solution. The detecting solution consists of 1 ml. of water to which havr been added 2 drops of sodium nitroprusside solution (20 grams of‘ Xa2Fe(CN)&O.2H20 dissolved in 50 ml. of water and diluted with 450 ml. of methanol) and 1 drop of diethanolamine. The contents of the 5-inch test tube are now heated with a m a l l flame a t such a rate that bubbles of gas pass through the detecting solution a t a rate not exceeding one per second. The contents should boil vigorously and the solvent should reflux from the upper portion of the test tube, but distillation of any appreciable amount of solvent must be avoided. The heating should be continued for no longer than 5 minutes. The appearance of a definite blue color in the detecting solution during the pyrolysis period constitutes a positive result. As indicated by Table I, most P-hydroxyethylamines give a royal blue color (usually after heating the quaternized amine for about 3 minutes). In those cases where a light blue color is obtained, the detecting solution should be allowed to stand for up to 10 minutes and then re-examined. If the blue color has deepened to a brilliant royal or “copper sulfate” blue, the test is considered positive. If the light blue color persists or fades, the tePt is (sonsidered negative, since traces of blue may he d u e to
P-hydroxyethylaniine impurities present in commercial materials of related structure. DISCUSSION OF RESULTS
All P-hydroxyethylamines tested give positive results with this test. In addition, amines containing one or tm-o polyethoxyethm o l groups attached to the amino nitrogen give positive results, alt~hough,. as the size of the polyethoxyethanol group increases, with consequent decrease in the nitrogen content of the molecule, the results become less definite (Priminox 43 us. 32; Ethomeen T/15 us. 18/60!. I n di(8-hydroxyethyl)aniline, the presence of the aromatic nucleus, with its electron-attracting capacit,y, apparently inhibits the decomposition to acetaldehyde, and only a light blue color (which deepens on standing, however) is obtained. This tendency is intensified in t,he amides where the strongly electron-attracting C=O group inhibits the reaction to such an extent that ethoxylated amides give negative results (Ethomids C/15 and R0/25). lsopropanolamines (mono-, di-, and tri-) all give negative results, since they cannot decompose to acetaldehyde. This makes the test valuable in distinguishing between emulsifying soaps made with triethanolamine or diethanolamine and those made with di- or triisopropanolamine. The positive results obtained wit,h diethanolamine hydrochloride, and triethanolamine phosphate indicate that in analyzing amine-containing compositions it should not be necessary to isolate the amine per se, but that a salt of the amine, often more conveniently obtained, may be tested instead. The triethanolamine salt of an anionic surfactant (Duponol W.4T) can be deiected with this method. ACKNOWLEDGMENT
The author \vishes to acknowledge gratefully the valuable suggest,ions made by David Davidson of this department during thr course of this investigation. LITERATURE CITED
(1) Hofinann, A4. W., Ber., 14, 494, 659, 710 (1881). (2) Rosen, AI. J., ANAL. CHEM.,26, 1 1 1 (1954). (3) Simon, L. J., Compt. rend., 125, 536 (1897). KFEEIYEDfor review May 7, 1954.
Accepted July 16, 1954
Colorimetric Determination of Niobium in the Presence of Tantalum MOHAMMED NAB1 BUKHSH’ and DAVID N. HUME Department
of Chemistry
a n d Laboratory of Nuclear Science, Massachusetts institute of Technology, C a m b r i d g e 39, Mass.
The major sources of error in the thiocyanate method for the colorimetric determination of niobium are loss of niobium due to hydrolysis of tantalum present and incomplete extraction of the niobium thiocyanate complex with ether. These effects have been minimized by adding tartaric acid to the reagents, changing the order of additions, and replenishing the thiocyanate and acid between extractions.
T
HE greatest drawback to the colorimetric determination of
niobium with thiocyanate has probably been the interfering effect of tantalum a t high ratios of tantalum to niobium. The authors have observed, in using a recently published procedure (a), that although satisfactory results are obtained a t a 10 to 1 ratio of tantalum to niobium a t low levels of niobium concentration, poor results are obtained a t the same ratio with larger con1 Present address, Central Testing and Standards Laboratories, Karachi, Pakistan.
centrations. They therefore extended their studies on the thiocyanate method with particular attention to the tantalum interference problem. The two main sources of error have been found to be: incomplete extraction of the niobium from the aqueous phase, and loss of niobium due to hydrolysis of tantalum present, the latter effect being the more important. It has been shown that addition of tartaric acid to the reagents and a change in the order of addition eliminate the erratic interferences of tantalum. EXPERIMENTAL
A majority of the reagents were prepared as in previous work ( 4 ) . Niobium and tantalum stock solutions were made up from spectrographically analyzed high purity oxide as before. The oxides were fused in silica crucibles with potassium pyrosulfate, with special care to obtain clear melts, and the cooled masses were taken up in 10% tartaric acid. Close attention to detail was found necessary in high tantalum mixtures if clear solutions were to result. The pure niobium stock solutions were found to be stable, but tantalum stock solutions showed a tendency to hvdrolyze on standing. The spectrographically pure ouidr
117
V O L U M E 2 7 , N O . 1, J A N U A R Y 1 9 5 5 samples were sometimes found to assay as low as 70% niobium pentoxide owing t o the presence of moisture and volatile salts. The stock solutions were standardized gravimetrically by classical procedures. Radioactive niobium-95 was prepared from a zirconium-95niobium-95 mixture derived from uranium fission and obtained from the United States Atomic Energy Commission. Pure radioactive niobium tracer was prepared by carriage on manganese dioxide according to the method of Siegel, Bigler, and Hume ( 5 ) . Gamma counting was done on liquid samples mounted in imall glass cups and covered with lacquer films according to the technique of Freedman and Hume ( 8 ) . A conventional scaling circuit and thin window, bell-shaped Geiger Muller tube was used for counting. Beta rays were removed by 435 mg. per sq. rm. of aluminum absorber. The techniques of manipulation, extraction, and measurement with a Beckman DU spectrophotometer were essentially those of the previous puhliration. ETHER EXTRACTION
In previous work, the presence of much tartrate appeared to affect the absorbance index of niobium in the thiocyanate complex. Since high tartrate concentrations offered the most promising path to avoidance of tantalum hydrolysis, the first efforts were directed to the elimination of direct interference by tartrate. I t was observed that even traces of oxalate have a bleaching effect on the niobium thiocyanate color; however, careful analysis of the reagent grade tartaric acid used failed to show the presence of oxalate. Chemical analysis of the ether extracts revealed, however, that the initial portion of ether used to extract niobium also removed about 70% of the total thiocyanate and about 4% of the chloride. Since a high concentration of thiocyanate in the aqueous layer is necessary for efficient extraction, especially in the presence of tartaric acid, subsequent extractions were not removing much additional niobium. The effect of the volume of ether and removal of thiocyanic acid in a single eytraction step is shown bv the data in Table I.
Table I. Absorbance of 19.47 of Niobium Extracted with Various Volumes of Ether and Diluted to 25 M1. for Measurement Extraction Volume, Ml. 2.0 5.0 10.0 15.0 25.0
Absorbance 0,302 0.302 0.292 0 282 0 208
The extensive removal of thiocyanic acid by a larger volume of ether results in a significant decrease in the efficiency of extraction When the effect of dilution of the already-extracted color with more ether was determined, it was found that a 5-ml. extract n-hich had been obtained in the usual way was significantly less intense in color when diluted to 25 ml. with pure ether than when diluted with ether which had been saturated with thiocyanic acid When two extractions were made with 5-ml. portions of ether, with the addition of sufficient potassium thiocyanate and hydrochloric acid to bring back the original concentration, the addition of 15 ml. of fresh ether to the combined ether extract had no bleaching effect. ICividently, sufficient thiocyanic acid is extracted in two operations to prevent decomposition of the vomplex. In case of doubt, however, ether previously equilibrated with a potassium thiocyanate-hydrochloric acid mixture ('ail he used profitably. I t was concluded that extractions subsequent to the first should be made only if sufficient acid and thioryanatt. i q added to restore the optimum concentrations for niohiuni extrartion, and that the final dilution of the ether extract should have a high enough concentration of thiocyanic acid to give thr. maximum color intensity of the niobium thiocyanate complex Although a high thiocyanate concentration in the aqueous phaw favors the formation of the complex and its extraction into ethw , the thinwanate concentration must not be raised too high 01' t h v
blank correction becomes large. \Vhen 5 ml. of 20% (grams per 100 ml. of solution) pot,assiuni thiocyanate are used as in the standard procedure, the blank, against ether, amounts to only 0.01 or 0.02 absorbance unit. If 40% thiocyanate reagent is used, the blank increases fourfold; and with 80% thiocyanate, the blank increases 50- to 100-fold owing to the rapid formation of colored thiocyanate decomposition products. The color intensity of the niobium in the aqueous phase and, under conditions of incomplete extraction, in the ether phase, is promoted by a high Concentration of hydrochloric acid. I t has heen suggested that both the hydrogen and chloride ions are involved, inasmuch as color intensity is increased by the presence of magnesium chloride (3). In order to verify the importance of the chloride ion, the effects of hydrogen and chloride ions were studied separately. On the assumption that the effect of hydrochloric acid is due to hydrogen ion alone, it was reasoned that other acids would be equally effective; and if the effect of magnesium chloride were due to magnesium ion, other magnesium salts would behave Rimilarly. When perchloric acid was tried in place of hydrochloric acid, and sodium salts substituted for potassium, it was found that the rolor development was again increased by increasing the wid concentration in the same range. In another esperimerit, dolutions of equivalent strength, of magnesium chloride and magnesium perchlorate, were added separately to solutions containing niobium, hydrochloric acid ( l M rather than 43f, to allow the effect to be more readily observable), tartaric acid, and potassium thiocyanate. When the solutions were made 0.4M in magnesium chloride or magnesium perchlorate, the absorbances of the extracted samples were identical and some 35% greater than if the magnesium salts had not been used. It was therefore concluded that the effect of hydrogen ions and magnesium salts was due to t'heir reaction with tartrate, thereby freeing the niobium for extract'ion as the thiocyanate complex, and that the chloride ion as such did not enhance the color. Too high a concentration of hydrochloric acid is undesirxblc>, :is it is then extracted into the ether. High concentrations of hromide bleach the thiocyanate color. The efficiency of the ether extraction step was next determiiicd using radioactive niobium-95 as a tracer. Amounts of niobium of the order of 25 y were t,aken for extraction after the addition of sufficient niobium tracer to give a gamma counting rate of wound 2000 counts per minute. The residual activity in the aqueous phase after two ether extractions was found to be very low, of the order of 20 to 30 count,s per minute. This corrcsponds to a minimum extraction efficiency of 98 to 99%, if all the residual activity is actually niobium, and not traces of zirconium impurity. The estimate by Alimarin ( 1 ) of the efficiency as less t,han 50% for a two-st,ep extract>ionis evidently not valid under the conditions of this procedure. For all intents and purpo.ws, :L two-step extraction is quantitative. EFFECT OF TANT.4LUM
The interfering action of tantalum was studied in some detail. If niobium and tantalum solutions were extracted separately and the extracts combined, no interference due t,o tantalum was observable. I t was found by the use of radioactive niobium, however, that, although niobium could be extracted quantitatively \vhen alone, the presence of equal or greater amounts of tantalum somet,imes result,ed in t,he st,ubborn retention of variable but rignificant amounts of niobium, even on repeated extraction. The tendency of niobium to remain in the aqueous phase increiisrtl with the proportion of tantalum and with the age of the niobiumtantalum solution. XI1 evidence pointed to the co-separation of niobium with colloidal tantalum oxide as the source of the difiaulty. Factors which tend to diminish the hydrolysis of fantalum likewise diminish the interference. The best rcsults \ Y P W obtained in the following manner. Thc hydrochloric acid and dnnnous chloride reagenb iv('re
118
ANALYTICAL CHEMISTRY
made up to be 1M in tartaric acid. The order of addition of reagents was changed so that thiocyanate was the first reagent to be added to the sample. Although the change in order of reagents made no difference when tantalum was absent, a great improvement was observed when tantalum was present. Under these conditions, the niobium was thus forced into the soluble and extractable thiocyanate complex before appreciable hydrolysis of tantalum could take place. For samples containing high percentages of tantalum, it was found necessary to carry through the analysis promptly after dissolution of the melt in tartaric acid: the longer the period of standing, the greater the chance for error, even though the solution appeared to be perfectly clear. With these precautions, it was found that the presence of ten to twenty times as much tantalum as niobium had no effect on the niobium results, the average values of two series of determinations agreeing within 3%, an accuracy and precision comparable with the original method. Reliable standard samples containing high ratios of tantalum to niobium were not available, but the results of a few determinations run on oxide mixtures suggest that the method is applicable without modification a t tantalum to niobium ratios as high as 100 to 1. RECOMMENDED PROCEDURE
The sample is prepared for analysis by fusion with potassium pyrosulfate and dissolution of the melt in 10% tartaric acid. A volume of 1 to 2 ml. of the solution (containing 1 to 50 y of niobium) is measured into a GO-ml. separatory funnel followed immediately by 5 ml. of 20% potassium thiocyanate, 2 ml. of 15% stannous chloride containing 1M tartaric acid, and 5 ml. of 9M
hydrochloric acid, also containing 1M tartaric acid. The funnel is shaken thoroughly after t,he addition of each reagent. When all the reagents have been added, the mixture is allowed to stand for 5 minutes, and 5 ml. of ether are added. After 10 seconds of vigorous shaking, the mixture is allowed to stand for 5 minutes and the lower (water) portion is run off into a second separatory funnel. One milliliter of 9M hydrochloric acid and 0.i ml. of 50% potassium thiocyanate, freshly prepared, are added to bring back the original concentration of reagents, and a second extraction is made with 5 ml. of ether. The combined ether extracts are diluted to 25 ml. with additional ether. After standing for 30 minutes to allow water droplets to settle out, the absorbance of the ether extract is read a t 385 mH against a blank treated in exactly the same way. ACKNOWLEDGMENT
The authors are indebted to the Massachusetts Institute of Technology Laboratory for Nuclear Science and the r n i t e d States Atomic Energy Commission for partial support of this work. LITERATURE CITED
(1) hlimarin, 1. P., and Podvalnaja. R. L., Zhur. Anal. Khim., 1, 30 (1946). (2) Freedman, A. J., and Hume, D. S . ,Science, 112, 461 (1950). (3) Freund, H., and Levitt, A. E., . ~ N A L .CHEM.,23, 1813 (1951). (4) Lauw-Zecha, A. R. H., Lord. S. S., and Hurne, D. N., Ibid., 24, 1169 (1952). (5) Siegel, J. bl., Bigler, W. P., and Hume, D. X., in “Radiochemical Studies, the FissionProducts,”ed. byC.D.Coryelland?;.Sugarman, Rook 3, p. 1532, New York, McGraw-Hill Book Co.. 1951. RECEIVED for review August 5 , 1953. Accepted August 30. 1954.
Polarographic Determination of Cadmium and Zinc in Zinc Sulfide-Cadmium Sulfide Phosphors SAMUEL B. DEAL Tube Division, Radio Corp. o f America, Lancaster, P a .
In a method for the quantitative determination of cadmium and zinc, in which lead is used as an internal standard, the diffusion currents of cadmium and zinc ions are measured in relation to the diffusion current of lead ion. The concentrations of cadmium and zinc are determined by comparison of the results obtained with a calibration curve. The materials used include a maximum suppressor containing methyl red and bromocresol green, a base solution of potassium chloride, and a modified electrolysis cell. The method is simple and rapid in application, and the necessity for constant temperature control is eliminated.
AN
IMPROVED method was needed for the quantitative determination of cadmium and zinc in zinc sulfide-cadmium sulfide phosphors used in the manufacture of cathode ray tubes because the established chemical methods for the separation and determination of cadmium and zinc are difficult and time-consuming and not highly accurate. Polarographic analysis of cadmium and zinc appeared to afford the best method for this determination because it eliminates the need for separating the cadmium from the zinc, which is the most difficult part of a chemical analysis because of the similarity in their chemical properties. Experimental evidence presented by Lingane ( 3 )indicated that the half-wave potential of cadmium ion in a 0,LV solution of potassium chloride referred to the saturated calomel electrode a t 25’ C. is -0.60 volt, and the half-wave value for zinc under
the same conditions is - 1.00 volt. Because a difference of only 0.1 volt between the half-wave potentials of two different ions is necessary for distinction of the polarographic “waves” ol)tsined on a current-voltage curve, determination of the t n o ions in question is feasible. Compensation for temperature variations was accomplished by the use of an internal standard as first suggested by Forche ( d ) , who worked with cadmium and lead. In 1948, Loofbourow and Frediani (4)discussed the internal standard method and provided experimental data for the system, lead, cadmium, zinc, in 0 . 1 s potassium chloride. Lead was selected as an internal standard because the half-wave potential of lead rrferred to the saturated calomel electrode is -0.40 volt. The solubility of lead chloride is sufficiently high for complete solution in the concentrations ordinarily employed in polarographic analysis. Only four voltage settings and subsequent current readings were used in routine work in the manner descrihed by Copeland and Griffith ( 1 ) . EYPERIMENTAL
Apparatus. The Fisher Elecdropode and a modified electrolysis cell requiring 5 to 10 ml. of test solution a ere used to obtain the current-voltage curves. A saturated calomel electrode w a p used as a reference elertrode Reagents. The following reagents were used. Potassium Chloride Solution. A 0.2N solution containing 5 ml. of a solution of methyl red and bromocresol green per liter. Methyl Red-Bromocresol Green Solution. Three parts of a
