Colorimetric Determination of Zinc and Cadmium with 8-Quinolinol W. L. MEDLIN Field Research laboratory, Magnolia Petroleum Co., Dallas, rex.
b Colorimetric methods are described for the determination of trace amounts of zinc and cadmium in synthetic anhydrite samples, 8-Quinolinol is the color reagent. In the pH 5 to 13 range both metals form insoluble complexes, Zn(CgH6ON)p and Cd(CgH6ON)2, which are soluble in chloroform. The chloroform extracts have absorption peaks at 390 to 400 ml. The maximum absorbance for the zinc complex occurs when the pH of the aqueous phase is in the 8.8 fo 9.5 range and for cadmium in the 7.6 to 8.6 range. The use of an ammonium hydroxide-ammonium chloride buffer has adverse effects on the precision of the me!hod because of the formation of NH&HeON. The use of other buffers i s prohibited by the presence of calcium. However, the pH was adequately controlled for both determinations b y the addition of a small amount of ammonium hydroxide. Beer’s law is obeyed with good precision up to at least 10 p.p.m. for zinc and from 10 to at least 40 p.p.m. for cadmium. Improvement of the extraction procedure would probably extend the upper limit in both cases. The effects of large quantities of calcium and interference by trace amounts of iron were investigated. Possible interference by other cations i s discussed but was not investigated.
T
HE colorimetric determination of
trace amounts of zinc in anhydrite presents a special problem because of the presence of calcium. None of the available methods of determination is applicable, but the use of 8-quinolinol as a color reagent provides a satisfactory means of determination in the presence of large quantities of calcium. At least three reliable methods have been developed for the colorimetric determination of zinc. The best known involves the use of dithizone (diphenylthiocarbazone) as the color reagent ( I $ ) . More recent methods use Zincon (2carboxy - 2’ hydroxy - 5’ - sulfoformazylbenzene (IO) and a,b,y,btetraphenylporphine (8) as the color-forming reagents. None of these methods was satisfactory for the present application. The Zincon method, the most straightforward of the three, requires a buffer to maintain a constant pH in the 9 to 10 range. The use of ammonium hydroxide and ammonium chloride is precluded by. the adverse effect of the ammonium ion, which partially destroys the color due to zinc (whether this results from a separate complexing action by the ammonium ion has not been determined). None of the other buffers which are effective in this p H range, including borates, phosphates, etc., could be used here because they form precipitates with calcium ion in basic
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solutions. The alternative procedure of adjusting the pH by using successively more dilute solutions of sodium hydroxide and hydrochloric acid was impractical. The dithizone procedure was also unsatisfactory because of interference by calcium. The cu,p,y,btetraphenylporphine method was discarded because of the difficulty in preparing the reagent (9). The procedure described here using 8-quinolinol elimnates all these difficulties in the determination of zinc. It is also applicable in determining cadmium in anhydrite samples. APPARATUS AND REAGENTS
A Beckman Model DU spectrophotometer with matched 1.0-cm. Corex absorption cells waa used to determine the absorbance of all samples. A filament source was satisfactory for use with the 1P28 multiplier phototube as detector. Standard solutions of zinc and cadmium were prepared by dissolving the metals (reagent grade) in hydrochloric acid. It was convenient to prepare solutions containing 1.00 and 0,100 mg. per ml. of each cation. The color reagent was prepared by dissolving 5.00 grams of 8-quinolinol in 15 ml. of glacial acetic acid and diluting to 100 ml. with distilled water. The chloroform was reagent grade. DETERMINATION OF ZINC
I n the presence of 8-quinolinol zinc forms the complex Zn(CoH60N)2,which is insoluble in aqueous solution but soluble in chloroform in the p H range 4.6 to 13.4 (4). In chloroform the I
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Figure 1. Absorption spectra for reagent blank and zinc complex showing the effect of calcium
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Wave length in mp 632
ANALYTICAL CHEMISTRY
Figure 2. Absorbance of zinc complex as a function of pH of the aqueous phase
complex results in an absorption peak a t 390 to 400 mp, which is superimposed on the strong absorption peak of the reagent blank itself a t 360 mp. The absorption spectra for a blank and for a sample containing 12 p.p.m. of zinc ion are shown in Figure 1. The effect of a large concentration of calcium ion is also illustrated. The absorbance was measured against a chloroform blank in all three cases. The effect of calcium is not interference in the normal sense, because its presence does not alter the absorption spectrum of the reagent blank. The increase in the zinc ion absorption therefore depends on the presence of both zinc and calcium ion. The solubility of the calcium quinolinolate complex in chloroform has not been determined, but it is certainly less than 0.1 mg. per ml. ( I I ) . It would be expected, then, that the effect of calcium should be approximately independent of its concentration a t levels greater than a few milligrams per milliliter. This result was observed. The effect of calcium was found to be approximately independent of its concentration above 2 to 3 mg. of calcium per ml. of chloroform. Figure 2 shows the effect of p H on the zinc absorption peak. These results were obtained by adding increasing amounts of ammonium hydroxide to solutions containing fixed concentrations of hydrochloric acid and calcium and zinc ions. The absorbance was measured against a chloroform blank. By carrying out successive extractions it was determined that the increased absorbance in the pH 8.8 to 9.5 range is not merely due to an increase in the extraction efficiency. The absorbance of the reagent blank is constant over the p H range of Figure 2. The presence of ammonium hydroxide in the standards and in the reagent blank resulted in the formation of N H ~ C Q H ~ O NThis . yellow complex is insoluble in the aqueous phase over the pH range 4 to 10 and is soluble in
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the chloroform phase. It was demonstrated, however, that its presence in the chloroform layer did not affect the absorption. Blanks prepared with and without the addition of hydrochloric acid and ammonium hydroxide exhibited identical absorption spectra. Nevertheless, the ammonium ion uses up a portion of the 8-quinolinol reagent in the aqueous phase and thus affects the formation of the zinc complex (other bases such as sodium hydroxide, potassium hydroxide, etc., produce the same result). Therefore its concentration must be kept a t a minimum. For this reason, the use of an ammonia buffer such as NH,OH-NH&l for maintaining the pH in the 8.8 t o 9.5 range would not appear feasible. It was observed that the presence of this buffer resulted in a severe loss in precision. However, it was readily demonstrated that a buffer is not actually necessary, because the pH can be easily controlled within the necessary limits by the addition of a small amount of ammonium hydroxide. No effort was made to determine the most efficient procedure for the extraction of the zinc complex with chloroform. The method used here consisted of shaking the precipitated complex with one 10-ml. portion for 60 seconds. The results of absorbance us. concentration measured a t 400 mp are shown in Figure 3. The data for samples containing 0.030 gram of calcium ion in the aqueous phase (corresponding to 0.100 gram of anhydrite) are also shown. Because the absorption was measured against a chloroform blank, the absorbance a t zero concentration is that of the blank reagent. Beer’s law is obeyed up to a t least 10 p.p.m. of zinc ion. At concentrations greater than this the precision of the data becomes unacceptable. The precision to be expected from the method is illustrated in Table I. Each sample contained 50 mg. of calcium.
Table 1.
Zn Present, 2.00 4.00
6.00
Precision of Method
Zn
y
Found,
y
Standard Deviation
2.22 2.00 2.18 3.79 4.00 3.98 6.02 5.87 5.94
0.12
0.12 0.08
When the determination is carried out in the presence of calcium, it becomes necessary to filter the chloroform extract before determining its absorbance. This eliminates small particles of calcium quinolinolate which are inadvertently transferred to the chloroform layer while shaking. The filtration process introduces an error into the determination because o chloroform evaporation. The error can be minimized by using small filters and keeping the filtration time constant for all samples. Interference by cations other than calcium was not a serious problem here, because most of the synthetic anhydrite samples contained negligible quantities of other impurities. However, a small group of samples contained trace amounts of iron added along n-ith the zinc impurity. The Fe(CQH60N)acomplex has absorbance maxima a t 300 and 460 mp (8) which result in appreciable interference with the zinc determination a t 400 mp. The iron interference could be accounted for a t concentrations no greater than about 7 p.p.m., if its concentration was determined independently by another procedure. The ophenanthroline method ( I ) was satisfactory for this purpose.
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b Figure 4. Absorption spectra for reagent blank and cadmium complex showing the effect of calcium
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Figure 3. Absorbance as a function of concentration for the zinc complex showing the effect of cadmium
Wove length in rnp VOL. 32, NO. 6, MAY 1960
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Figure 5. Absorbance of cadmium complex as a function of pH of the aqueous phase
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Interference by other cation impurities was not investigated here. However, the 8-quinolinol reagent has been used in colorimetric determinations of several other metals, including aluminum (6), copper ( 1 4 , gallium ( 7 ) , indium (16), vanadium (6), and titanium (11). A survey of these results indicates that interference by small amounts of aluminum, vanadium, indium, and titanium will be eliminated in this case because their quinolinolates can be extracted only from acid solutions. However, copper and gallium will interfere a t concentrations approaching that of zinc. It appears that small amounts of cobalt, lead, bismuth, manganese, nickel, and uranium will also interfere on the basis of their precipitation with 8-quinolinol in the 8.8 to 9.5 pH range and the solubility of the precipitate in chloroform ( 3 ) . DETERMINATION OF CADMIUM
The cadmium determination is neither as sensitive nor as precise as the zinc method. The absorption spectrum for Cd(C9HeON)z shown in Figure 4 is strikingly similar to the spectrum for the zinc complex. The effect of calcium is again observed as an increased absorbance. The variation in the cadmium absorption peak with pH is illustrated in Figure 5. The absorbance was measured against a chloroform blank. In this case the absorption is almost negligible in the acid range. The range of complete precipitation of cadmium in the aqueous phase is 5.7 to 14.6 ( 4 ) . As in the case of zinc, the addition of a small amount of ammonium hydroxide provided adequate pH control. Figure 6 shows the absorbance (measured against a chloroform blank) as a function of concentration. Beer’s law fails below 10 p.p.m. and the precision of the determination is poor above 40 p.p,m. The remarks of the preceding section concerning interference by other cations apply to the cadmium determination also. ANALYTICAL CHEMISTRY
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ZINC DETERMINATION
Add 0.100 gram or less of anhydrite sample containing no more than 100 y of zinc to a 10-ml. volumetric flask and fill to the mark with 7% hydrochloric acid. Agitate until all of the solid has gone into solution. Transfer the contents to a 60-ml. separatory funnel. Wash the volumetric flask with two 5-ml. portions of distilled water and add the washings to the separatory funnel to ensure a quantitative transfer. Add 2.5 ml. of concentrated ammonium hydroxide and shake. This should bring the pH into the 8.8 to 9.5 range. Then add 2 ml. of the color reagent and shake again. The formation of yellow precipitates of Ca(C9HeON)2,Z~(C~HP,ON)Z, and NHICsHaON mill be observed immediately. Without delay, add 10 ml. of chloroform and shake for 60 seconds. Filter the chloroform extract to remove colloidal particles of calcium quinolinol and measure the absorbance a t 400 mp. Determine the zinc concentration from Figure 3. The concentration in this case is in parts per million of zinc in solution and must be multiplied by 100 to obtain parts per million in the solid. PROCEDURE
FOR
CADMIUM DETERMINATION
This procedure is identical to the one for zinc, except that only 2.2 ml. of ammonium hydroxide is added to the sample (after transfer to the separatory funnel) to bring the p H into the 7 . 6 to 8.6 range. The concentration is determined from Figure 5 in parts per million in solution. As in the zinc determination, this value must be multiplied by 100 to obtain parts per million of cadmium in the solid. CONCLUSIONS
The Ebquinolinol reagent provides the only straightforward colorimetric method now available for the determination of Einc in the presence of large quantities of calcium. For other applications the method is less sensitive than the Zincon determination (0.14 as
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P P ~ C. d ” Figure 6. Absorbance as a function of concentration for the calcium complex
opposed to 0.0035 y per sq. em.) (10) and is infer 3r z this method in general. In the detern: ,tion of cadmium, the method has no particular advantages over present techniques (IS), as the presence of calcium is not a serious problem in this case. ACKNOWLEDGMENT
The author thanks H. G. Bufforcl and J. J. LIcAlpin for their assistance and the Magnolia Petroleum Co. f o y permission t o publish this paper. LITERATURE CITED
( 1 ) Bandemer, S. L., Schaible, P. J.,
IND.ENG.CHEXI..ANAL. ED. 16. 317 (1944). (2) Banks, C. V., Bisque, R. E., ANAL. CHEM.29,522 (1957). ( 3 ) Fleck, H. R., Analyst 62, 378 (1937). (4) Fleck, H. R., Ward, A. J., Zbid., 58,3888 (1933). (5) Gardner, K., Zbid., 76,485 (1951). (6) Kenyon, 0. A,, Bewick, H. A., A s . 4 ~ . CHEM.24,1826 (1952). ( 7 ) Lacroix, S., Anal. Chim. . I d a 1 , 200 (1947). (8) Lavollay, J., Bull. SOC. chim. biol. 17, 432 (1935). (9) Priesthoff, J. H., Banks, C. Y.,J. Am. Chem. SOC.76,937 (1954). (10) Rush, R. M., Yoe, J. H., ANAL. CHEM.26,1345 (1954). (11) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 2nd ed., 115, Interscience, New York, 1950. (l!‘ Ib id., p. 619. (131 Snell, F. D., Snell, C. T., “Colorimetric Methods of Anal sis,” 3rd ed., Vol. 11, p. 140, Van &strand, New York, 1949. (14) Sudo, E., Sci. Repts. Tohoku Univ. 4A, 268 (1952). (15) Talvitie, N. A,, ANAL. CHEY.25, 604 (1953).
RECEIVED for review September 8, 1959. Accepted January 29, 1960.
