Reaction between Zinc and Salicylaldoxime S. H. SIMONSEN and PHILIP CHRISTOPHER Department of Chemistry, The University of Texas, Austin 72, Jex.
S o . 2956) in 5 ml. of ethyl alcohol and then diluting to 100 ml n i t h marm water. The solution nas stored in an amber bottle in a refiigerator to avoid possible decomposition. A standard zinc solution was prepared from reagent grade zinc nitrate hexahSdrate to contain 2 to 3 mg of zinc per ml I t n-as standardized gravimetrically by the zinc ammonium phosphate method and found to contain 2.38 mg. of zinc per ml. All other reagents used weie of reagent grade. X Becbman Model G p H meter with a glass electrode assembly. standardized frequently against a buffer solution of pH 7.00, was used for all p H measurements. h Haves \-ray diffraction unit and a Spectron spectrometer with filtered copper radiation were used to record the diffraction niayima of precipitates prepared under different conditions. Procedures and Results. COVPARISOS O F L-XDIGESTED GD DIGESTED PRECIPITATES. Several sets of precipitates were prepared by mixing 60 ml. of 1 % salicylaldo\ime solution with 50 ml. of standard zinc solution and adjusting the pH to 8 50 by the s l o ~addition of dilute ammonia solution from a buret. hfter thorough stirring, one set of precipitates was filtered and washed with water immediately after precipitation, whereas another set v a s heated for 10 minutes at 90" to 100" C. before filtration. A 1% ferric chloride solution was used t o test for completeness of washing. Both sets of precipitates m-ere dried to constant weight at 80' C. and then analyzed for zinc by ignition of the precipitate to zinc ovide and for nitrogen bv the micro-Dumas method. The ignition of the precipitate was carefully carried out by slow heating in a porcelain crucible to avoid loss of precipitate (the compound burns very easily with a vigorous evolution of heavy smoke); after the initial heating the residue was ignited t o constant weight a t 900" C. in an electric muffle furnace. The results of the analyses are given in Table I.
The reaction between zinc(I1) and salic! laldoxime jields an insoluble product, the composition of which \aries with the conditions of precipitation. This investigation was undertahen to study the reaction more completely. I t was found that the composition of the precipitate \aries with the pH of the solution from which it was precipitated. A t pH 6.8, the precipitate had the composition of zinc disalicylaldoximate; at pH 8.5, the precipitate had the composition of zinc monosalicylaldoximate; and at the intermediate pII, the precipitate was a mixture of the t w o . Digestion at 90" to 100" C. converted the precipitate into the monosalicylaldoximate, regardless of the pH of the solution from which i t was precipitated. When precipitated from a solution of pII 7.5 at a temperature of 10" C., pure monosalicylaldoximate was obtained; whereas when precipitated from a solution of the same pH at room temperature, a mixture was obtained. The presence of neutral salts in the solution affected not onl? the completeness of precipitation, as previously reported, but also the composition of the precipitate. If the reaction between zinc(I1) and salicylaldoxime is to be used as the basis for the gravimetricdetermination of zinc, the conditions of precipitation must be rigorously controlled.
T
HE formation of an insoluble complex of zinc with salicylaldoxime was first reported by Ephraim ( 2 ) . Later, Pearson ( 4 ) made a more thorough investigation of the properties of zinc salicylaldoximate and concluded that it was not suitable for the gravimetric determination of zinc because the pH range for complete precipitation \vas too limited, the precipitate was appreciably soluble in various neutral salt solutions, and the precipitate slowly decomposed above 80" C. Precipitated from solutions of p H 7 to 8, the compound corresponded to the formula Zn(C;HsOzS)2,which contains 19.36y0 zinc. Attempts to determine zinc gravimetrically in this form gave extremely varia1)Ie results, depending upon such factors as the time of standing before filtration and p H of solution from which precipitate was formed. Flagg and Furman ( 3 ) studied the reaction more completely and found t,hat if the freshly precipitated zinc salicylaldosimate was allowed t o digest for 10 minutes a t YO" to 100" C. a significant transformation took place. The original voluminous precipitate became dense and curdy, and could, after filtering and washing, be dried for several hours a t 110' C. n-ithout loss of weight. This product contained 32.50y0 zinc, corresponding to the formula ZnC:H60J. The first turbidity appeared in the range pH 6.2 t o 6.8 and the precipitate redissolved at, pH 8.8 to 9.4. Precipitation was complete b e h e e n p H i and 8. Good analytical results were obtained on synthetic mixtures and Kational Bureau of Standards brass samples. Biefeld and Ligett ( 1 ) also investigated the feasibility of the reaction for gravimetric determination of zinc and their results confirmed the work of Flagg and Furman. The present investigation was undertaken in an attempt t o learn more about the conditions under nhich the two forms, zinc mono- and disalicylaldosimate, are formed.
The theoretical composition of the zinc monodcvlaldoximate is 32.61% zinc and 6.95% nitrogen. The x-ray diffraction recording. of both sets of precipitates were identical eycept that the one of the undigested precipitate gave broadened maxima and many of the weak line. were absent. Thiq indicated a very small particle qize.
Table I.
Composition of Undigested and Digested Precipitates Prepared at pII 8.50
L-ndinested Zn, R % 32 74 6.89 32.43 .. .iv. 3 2 . 39
Digested Zn, 70 s,% 32 12 6.91 32.79 .. 32,46 -
Table 11.
Effect of pH on Composition of Precipitate
L-ndigested Final Zn, PH 7c 7 20 28 23 7 50 29 30 8 00 31 20 8 :o 32 48
Digested Final PH 7 00 7.23 8.00 8.50
Zn.
70
32.61 32 43 32 69 32 61
EFFECT O F pH O N COMPOSITIOS O F PRECIPITATE. Precipitations weie carried out by the procedure described except that the p H of the final qolution was varied. The initial p H of all solutions \\as hetween 4.7 and 5.0 and the p H was adjusted by the slow addition with thorough stirring of dilute ammonia solution from a buret. Completeness of precipitation was not conqidered. Tmo sets of precipitates were prepared, one as filtered immediately after formation and the other was digested for 10 minutes at 90" to 100" C. The precipitates were dried t o constant neight a t 80" C. and analyzed for zinc by ignition t o zinc oxide. The results are given in Table 11. It was observed that the higher the final p H of the solution,
EXPERIMENTAL
Reagents and Apparatus. h 1yc solution of salicylaldoxime was prepared by dissolving 1 gram of salicylaldoxime (Eastman,
681
682
ANALYTICAL CHEMISTRY
the whiter the color of the precipitate. At lower p H values, the precipitate was yellowiPh. In order to avoid a localized region of high hydroxyl ion concentration, sodium acetate was used to adjust the p H of the solution containing the zinc and salicylaldoxime. Standard solutions of zinc and salicylaldoxime were mixed thoroughly and 50 ml. of 2.5M sodium acetate was added slowly from a buret. The first turbidity occurred in all cases a t a p H of about 5.8. The final pH was from 6.85 t o 6.90. One set of precipitates was filtered and washed immediately after filtration; another set was digested for 10 minutes a t 90" t o 100" C. before filtration. All precipitates were dried to constant weight a t 80" C. and analyzed for zinc. The results obtained are given in Table 111. X-ray diffraction recordings were made of both sets of precipitates. The principal lines are given in Table IV.
Table 111. Composition of Precipitates Obtained by Adjusting pH with Sodium Acetate Zn, % Undigested
Digested
19.31 19.81 19.56
32.81 32.39 32.60
AI..
Table IV.
Zinc Disalioylaldoximate, Undigested a Intensity5 13.2 VS 10.37 W M 6.47
VS
13.5 10.64 6.40 5.83 5.01 4.75 4.46 4.07 3.93 3.80 3.48 3.41 3.15 3.02 2.65 2.45 2.17 2.01 1.88 a
hl
S
.\I hi hl W
w
4.81 4.65 4.30 4.11 4.03 3.82 3.35 2.86 2.21 2.16
vw W W S
S
W W M
M
hl S S W W M W W
vw M vw W
VS, very strong;
Effect of pH on Completeness of Precipitation Zn Recovered,
%
PH
Orig. Zn Content of S o h , 173.92 hfg. Zn/100 M1. 6.60 7.00 7.64 8.03 8.72
61.69 73.36 96.97 99.26 99.97
Orig. Zn Content of Soln., 86.96 Mg. Zn/100 MI. 6.48 7.00 7.63 8.05 8.50 8.72
61.17 82.53 95.53 99.63 99.89 96.83
digested for 10 minutes a t 90" to 100" C. before filtration, filtered and washed, and dried a t 110" C. to constant weight. The precipitates were weighed and analyzed for zinc. In each case the composition of the precipitate corresponded t o that of the zinc monoPalicylaldoximate. The results are given in Table VI. DISCUSSION
Principal Lines of Zinc Mono- and Disalicylaldoximates
Zinc hlonosalicylaldoximate, Digested d Intensity"
Table VI.
S, strong: h l , medium: W, weak: VW, very weak.
The composition of the product of the reaction between zinc and salicylaldoxime depends upon the pH of the solution in which the precipitate is formed and upon the temperature of digestion. Depending upon the conditions of the precipitation, zinc monosalicylaldoximate, zinc disalicylaldoximate, or a mixture of the two is formed. The monosalicylaldoximate is evidently the more stable because digestion of the precipitate a t 90" to 100" C. always transforms the disalicylaldoximate into the monosalicylaldoximate, regardless of the pH a t which the precipitate was formed. This is shown by the data in Table 11. In fact, Pearson's ( 4 ) table shoFing the loss of weight during heating of the precipitate at various temperatures shows that the transformation occurs in the dry, solid state, being complete a t 140" C. When precipitated from solution having a p H 8.50, the monosalicylaldoximate is always obtained; below pH 8.50 a mixture of the monosalicylaldoximate and disalicylaldoximate is obtained. This is probably due to an equilibrium between two ionic species of the oxime:
H EFFECTOF Low TEMPERATURE ON COMPOSITION OF PRECIPISolutions of zinc and salicylaldoxime were thoroughly mixed and placed in an ice bath. When the temperature of the solutions had reached 9" to 11" C. the pH was adjusted by the slow addition of dilute ammonia solution. The first turbidity appeared a t p H 5.3 to 6.0, just as in the cases when the precipitation was carried out a t room temperature. The precipitate was filtered and washed immediately after precipitation. Filtration and washing were difficult because the precipitate was very fine and passed through the filter. Complete removal of the excess reagent was finally accomplished by washing with a solution containing 20% alcohol. The precipitate was dried to constant weight a t SO" C. and analyzed for zinc. The results are given in Table V.
H
TATE.
HC
The oxime group is a very weak acid ( 5 ) ,so that in solutions of lower pH, form I is present in appreciable quantities and the disalicylaldoximate is formed; in solutions of higher pH, form I1 predominates and the monosalicylaldoximate is formed exclusively; and in solutions of intermediate pH, mixtures are formed. From the data in Table I1 the amounts of each compound formed a t the different p H values can be calculated by the following relationships:
% Zn Table
&-0-
\C/ H
=
fm
X
70 Zn,
+
fd
X
5%
Znd
(2)
V. Composition of Precipitates Obtained by Precipitation at 10' C. Final pH 7.50 7.54 7.60
Temp., 11
10 9
C.
Zn, % 32.69 32.46 32.79
EFFECTOF pH O N COMPLETENESS OF PRECIPITATION. Precipitations were carried out by mixing the standard solutions of zinc and salicylaldoxime thoroughly and regulating the pH by slow addition of dilute ammonia solution. The precipitates were
where % Zn is the percentage zinc in the mixture; % Zn, and % Znd are the percentages of zinc in the monosalicylaldoximate and the disalicylaldoximate, respectively; and f m and fd are the fractions of the mono- and disalicylaldoximates, respectively, in the mixture. The results of such calculations are given in Table VII. When the dilute ammonia solution is added to the solution of zinc and salicylaldoxime, there is a localized area a t the point of addition where the pH is higher than that of the bulk of the BO-
683
V O L U M E 2 6 , NO. 4, A P R I L 1 9 5 4 Table VII. Zinc Monosalicylaldoximate Content of Precipitates as a Function of pH Zinc Blonosalicylaldoximate, PH
%
7.20 7.50 8.00 8.50
66.9
76.0 89.4 99.0
lution. Thus, some monosalicylaldoximate i s formed even when the final p H of the solution is kept low. This localized, high pH area is minimized when the p H of the solution is adjusted by the addition of a sodium acetate solution, so that in this case a precipitate of pure zinc disalicylaldoximate can be obtained. When precipitated from a solution of p H 7.50 a t a temperature of 10" C pure monosalicylaldoximate was obtained, whereas when precipitated from a solution of the same p H a t room temperature a mixture was obtained. .4n explanation for this is possible by considering the formation of the disalicylaldoximate 8 s taking place in two steps g*ith the formation of a soluble intermediate. H C HC'
Zn-++
I
H C \C'
HC \C/ H
II
'NOH
In an experiment investigating the feasibility of the reaction for the determination of zinc amperometrically (in a sodium acetatebuffered solution) it was found that 15 to 20 minutes was r e quired to reach a steady current state after each increment of salicylaldoxime had been added. This indicates a slow rate-determining step, which could be step b. If the temperature were decreased, the rate could be slowed down to such an extent that the formation of the monosalicylaldoximate predominates, because of the equilibrium shown by Equation 1. The range of complete precipitation was found to be between pH 8.1 and 8.7, the range changing slightly with the original concentration of the zinc. This is essentially in agreement with Flagg and Furman ( 3 ) and Biefeld and Ligett ( 1 ) who found precipitation complete between pH 7 and 8. The difference is probably due to the slightly different methods used; in this investigation the pH measurements were made on the supernatant liquid prior t o digestion, whereas the p H measurements made by Biefeld and Ligett were on the filtrate obtained after digestion of the precipitate. Undoubtedly, some ammonia was lost. during the digestion of 10 minutes a t 90" t o 100" C. ACKNOWLEDGMENT
The authors wish to thank J. F. Flagg and W. B. Ligett for some excellent suggestions after reading the original manuscript.
(a)
C-0-
LITERATURE CITED
(*) HC
P\
C
H
(1) Biefeld, L. P., and Ligett, W. B., IND.ENG.CREW,. 4 x a ~ .ED., 14, 359 (1942). (2) Ephraim, F., Ber., 64, 1210, 1215 (1931). (3) Flagg, J. F., and Furman, N. H., IND.ENG.CHEM.,AKAL.ED., 12. 663 (1940). (4) Pearson, T h . G : ,2.anal. Chem., 112, 179 (1938). (5) Sidgewick, N. V., "The Organic Chemistry of Nitrogen," p. 107 Oxford, Clarendon Press, 1910. RECEIVED for review February 13, 1953. Accepted January 13, 1954.
Determination of Small Amounts of Niobium in Pure Tantalum and Its Oxide JANE HASTINGS and THOMAS A. McCLARlTY Materials and Processes Laboratory, Transformer Division, General Electric Co., Pittsfield, Mass.
A sensitive chemical method was needed for the analysis of pure tantalum metal. The spectrophotometric method for the determination of niobium by measuring the intensity of the ether extract of the yellow thiocyanate complex of niobium has been reported but there has been no extension of the method to a material containing a preponderanceof tantalum which reduces the intensity of the color. Details of concentration, manipulation, and timing must be strictly observed. A method is presented for the determination of niobium in pure tantalum metal in the range of 0.01 to 0.307' niobium with good reproducibility. No separations are necessary except for the extraction of the thiocyanate complex of niobium with ether. Contamination from large amounts of reagents is avoided.
0
F T H E minor constituents known to be present in pure tantalum metal (99.8y0tantalum), niobium was shown to
be present to the extent of 0.1% or less from qualitative spectrographic analysis. There are over 30 references in the literature to the determina-
tion of niobium and tantalum. Most of them are concerned with the analysis of ores, minerals, and steels and a few refer to pure salts. None of the methods are directly applicable to the determination of niobium in the presence of a preponderance of tantalum. The spectrophotometric determination of the yellow thiocyanate complex of niobium in solution appeared to offer the best solution t o the problem. The previous work of Alimarin (1)in Russia, Hume and his coworkers (a),and Freund and Levi t t ( 8 ) all furnished valuable information on the problem. The details of the method as presented here had to be worked out, in some cases as the investigation proceeded. GENERAL PRINCIPLES
The tantalum metal is dissolved in hydrofluoric acid and a solution of potassium bisulfate-oxalic acid is added to compensate for that present in the solutions used in preparing the calibration curve. After evaporation to a moist salt, hydrochloric acid is added and finally, in quick succession and in precise amounts, solutions of boric acid, stannous chloride, and ammonium thiocyanate are added. The thiocyanate forms a yellow complex with niobium which is extracted with ethyl ether, and the absorb-
