ANALYTICAL CHEMISTRY
1336 discussion of this problem is necessarily vague, the ultimate decision depending upon the composition of the particular solutioii with which one is working. T h e particularly troublesome ahsorption due to nitrate ion a t 280 to 300 mM should be noted. Perchlorate, sulfate, and chloride, on the other hand, do not interfere. T h e occasional interference of copper serves as an example of the care which must be taken in the selection of wave lengthz. At 265 nip, copper does not interfere, as stated above, b u t it, w a ~ found troublesome in a titration a t 285 mp; a t the latter n a v e length, the copper-ethylenediamine tetraacetate complex ha? almost the same absorptivity as the bismuth complex, and hence the titration curve, instead of becoming horizontal, s h o w only 3 small change in slope a t the bismuth end point. Lead, cadmium. zinc, cobalt, and nickel ions were found not to absorb strongljenough either a t 265 mp or higher ivave lengths to interfere in t,he bismuth titration. Tin may be considered more or less by itself as an interfering ion. Because of its tendency to precipitate as the hydrous oside, its beharior in the titration of bismuth has not been studied. Tin is conveniently eliminated by volatilization a8 stannic hromide according to the standard procedure (1j , modified only h y omitting phoPphoric acid from the recommended solutions. -1. this procedure also eliminates arsenic and antimony, the interference of these elements in the bismuth titration has not heen investigated. L I T E R i T U R E CITED (1) Am. SOC.Teating
Materials, Philadelphia, “AST.\I Method, for Chemical Analysis of Metals,” 1950. ( 2 ) Corning Glass Works, Corning, S . Y., “Laboratory Glassware.“ Catalog No. LP-34, 1954. (3) Goddu, R. F., and Hume, D. S . , ANAL.CHEM.,26, 1740 (1954j. (4) 1\Ialiiistadt, H. V., and Gohrbsndt, E. C., Ibid., 26, 442 (1951)
P; BISMUTH J END POINT
I 1
O
A
Figure 3.
I
1
2 3 4 ML OF EDTA
5
Simultaneous titration of bismuth and lead
0.5 nig. of Bi and 0.5 rng. of Pb titrated at 240 m p
(Sj Sweetser, P. R., and Rricker. C. E . , Ibid., 25, 253 (195.3) (6) Ihid., 26, 195 (195-4). (7) Underwood, 1.L., Ibid., 25, 1910 (1953). ( 8 ) I h i d . . 26, 1322 (1954). (9) Underwood, -i.I,.,J . C‘hem. EdTic., 31, 394 (1954). RECEIVED for review January 13, 1933. Accepted M a y 2, 1955. Presented beiore the Division of Analytical Chemistry a t the 126th Meeting of the A I I E R I C A N C H E . \ I I c A L SOCIETY, New York, N. Y.. 1954. Taken from a thesis submitted by Richard N. Wilhite in partial fulfillment of the requirements for the master of science degree, Departnient of Chei~iistry,Emory University. Emory University G a .
Spectrophotometric Determination of Copper with Salicylaldoxime Application to Analysis of Aluminum Alloys S. H. SIMONSEN and H. M. BURNETT D e p a r t m e n t o f Chemistry, The University o f Texas, Austin 72, T e x .
A simple, rapid procedure for the spectrophotometric determination of copper in aluminum and zinc-base alloys has been developed. The method is based upon the formation of copper(I1) salic?-IaIdoxiniatewhich is extracted into n-amJl acetate from a well buffered aqueous solution of pH about 4.4. A t this pH there are no interferences bj- the elements usually found in aluminum alloys. The copper salicylaldoximate in 1 1 amyl acetate exhibits a n absorption maximum at 3 t l m p . The range of maximuni photometric accuracy is 2.5 to 8.5 X 10-6 mole per liter in the extracted phase. The method was tested by the analysis of a number of RBS certified standards.
D
URIKG a n investigation of the chloroform extraction of some metal salicylaldoximates (5)it was noted that copper salicylaldoximate was extracted quantitatively from aqueous solutions Because the extracted copper complex was colored and because separation from elements usually found in aluminum :tllo>s was easily effected by control of the p H of the aqueous solution, the method, with slight modifications, was applied to the analysis of aluminum alloys for copper. When separating cations by the liquid-liquid extraction of metallo-organic complexes, two variables must be controlled: the reagent concentration in the organic solvent, and the hydrogen ion concentration in the aqueous phase. Kolthoff and Sandell
(4)showed that if the formation
of the metallo-organic complex in the aqueous phase is given by the equation
+ nHR
=
1IRn
+ nH+
(1)
the extractability of the complex into a n immiscible solvent can be expressed quantitatively by
E = K[HR];![H+]: (2) where E is the extractability of the metal complex, K is the estractability constant, [HR]o is the concentration of the reagent is the hydrogen ion concentrain the organic phase, and [“I1. tion in the aqueous phase. The derivation is based upon the mass-action and distribution laws, and the assumptions are made that the reagent and complex are present in both the aqueous and organic phases in the nonassociated forms, and t h a t the metal ion is present only in the squeous phase. Irving and Williams ( 3 ) w o t e Equation 2 in the logarithmic form and then differentiated it:
(blog E j b p H ) ( a ~ i a= (blog Elblog [ H R l o j p ~ (31 Equation 3 shows directly the effect of changing the reagent and hydrogen ion concentrations: Thus, a change in extractability due to an increase (or decrease) of one p H unit can be exactly offset by a tenfold decrease (or increase) in the concentration of excess reagent in the organic phase. Practical considerations limit the permissible variations in reagent concentration: The upper limit is determined by the solubility of the
V O L U M E 2 7 , NO. 8, A U G U S T 1 9 5 5
1337
organic reagent in the solvent; if the metal complex is t o be determined spectrophotometrically, a large excess of reagent would be undesirable if i t had an appreciable absorptivity a t the wave length used for measurements; and an adequate excess of reagent must be used if sharp separations are t o be made, so that the concentration of the reagent does not change appreciably during the extraction. Because of these factors, changes in hydrogen ion concentration are the more important in practical applications of the extraction of metals as complexes, and the reagent concentration should be kept a t a constant value.
i
///
a n d then making the solution slightly acid with 311.1 nitric acid. This solution was then diluted t o 1 liter. Standards of lower concentrations were prepared as needed by volumetric dilution. Buffer Solutions. The buffer solutions used to investigate the extraction a t various pH values were prepared with the reagents and concentrations as suggested by Clark and Lubs (8). The final pII of the buffer solutions !vas checked with the pH meter. PROCEDURE
Extractions were carried out in 30-ml. ground-glass-stoppered bottles, which have certain advantages over separatory funnels: There is no contact of the system with stopcock grease, and mechanical shakers for multiple extractions can be more readily utilized. Ten milliliters of the standard copper(I1) solution, suitablj- buffered to the desired pH, n-as transferred to the bottle and 10 ml. of the salicylaldoxime solution in n-amyl acetate added with a volumetric pipet. Five minutes of shaking was found sufficient for complete extraction (only one extraction is necessary). After t h e phases had been allowed to separate for 15 minutes, the n-amyl acetate phase was transferred, using a medicine dropper or pipet, to an absorption cell for transmittance measurement. If the n-amyl acetate phase n-as transferred immediately, erratic results n-ere obtained due t o very small droplets of n-ater still remaining in the organic phase. Best red t s were obtained by allowing a t least 15 minutes for the separation of phases.
1007
1
I
I
I
l
l
8070-
20
60 -
ooooo1
000002
650000010
330
340
350
360
370
380
390
400
WAVE LENGTH mp
Figure 1. Absorption spectra of copper(I1) salicylaldoxiniate in n-amyl acetate
%Ow 0
z
‘
30-
2
I n this investigation a 0.0231 solution of salicylaldoxime in n-amyl acetate was used as the extraction reagent. Complete extraction of copper(I1) was obtained in the pH range 3 5 to 9.5. The n-am? 1 acetate solution of copper salicylaldovimate exhibited an absorbance maximum a t 344 mp which vias suitable for anal) tical measurements. The range of mavimum photometric accuracy, determined by a Ringbom plot as recommended by ilvres ( I ) ,was between 2 5 and 8 5 X 10-5 mole per liter in the n-am? 1 acetatil phase APPARATUS AND REAGENTS
Instruments. Transmittance measurements Ivere made with a Beckman quartz spectrophotometer, Model DE, using a tungsten lanip and blue-sensitive photocell. (Measurements in the range below 330 mp were made using a hydrogen discharge lamp. j The instrument was operated a t constant sensitivity corresponding to 2 . 5 turns counterclockn-ise from the maximum position and with slit yidths of 0.60 t o 0.70 mm. corresponding t o a nominal band width of about 4 mp. Matched, stoppered Cores absorption cells n-ith optical paths of 1.000 cm. were used for all measurements above 330 mp: silica cells were used belov 330 mp. A Beckman Model H-2 p l l meter with a glass-calomel electrode assembly as used t o check the pH of all buffer solutions. Salicylaldoxime Solution. h 0.02M solution mas prepared l>y dissolving 2.7426 grams of salicylaldoxime (Eastman No. 2956; in exactly 1 liter of ,n-amyl wetate. Standard Copper(I1) Solutions. A 0.009997-11 stock solution of copper(I1) was prepared by dissolving 0.6355 gram of electrolytic copper in 8-11 nitric acid, diluting, boiling t o expel oxides of nitrogen, neutralizing the excess acid wit,h 8M sodium hydroxide,
MOLES COPPERD) SALICYLALDOXiIvIATE PER LITER X IO’
Figure 2. Calibration curve for determination of copper(I1) salicylaldoximate in n-amyl acetate
Absorption and Calibration Curves. Standard 1.0,2.0, 4.0, 6.0, 8.0, and 10 0 X lO-5M copper(I1) solutions n-ere prepared by volumetric dilution of the standard stock solution. The dilutions were made r i t h a buffer solution of pH 4.0. Ten milliliters of the standard solution were extracted with 10 ml. of the reagent solution. A blank n-as prepared bv carrying I0 ml. of the buffer colution through the extraction procedure. T h e spectral characteristics of the system n-ere evaluated by measuring the trammittance, a t frequent n nve-length intervals, over the range 320 to 420 mw. All transmittance measurements Tvere made against the reagent solution nhich hnd been carried through the extraction procedure. The absorption curves are shown in Figure 1.
ANALYTICAL CHEMISTRY
1338 An absorbance maximum suitable for analytical measurements occurred a t 344 mp. A calibration curve was plotted using transmittances measured a t 344 mp (against the reagent solution blank). The color system conforms to Beer's law over the concentration range investigated, as shown by Figure 2. RESULTS
Stability of Color. Samples extracted from a standard copper solution gave constant transmittance readings over a period of measurement of 48 hours. Several solutions were stoppered and read after a period of several weeks with little change in transmittance.
Effect of Buffer Concentration. To prevent interference by nickel the extraction was made from solutions of p H 4.4; but the Clark and Lubs buffers did not have sufficient capacity when used with samples that had been dissolved in concentrated acids. A suitable buffer was prepared by adding 75.0 ml. of 0.4Msodium hydroxide to 500 ml. of 0.4.V potassium acid phthalate and diluting to 1 liter. The amount of this buffer needed was determined by extracting solutions of p H 4.4 containing a constant concentration of copper(I1) with varying amounts of the above buffer solution. The results are given in Figure 4. For each 50 ml. of solution t o be extracted, a t least 10 ml. of the above buffer must be present to ensure adequate capacity to maintain a constant p H during the extraction. A high capacity acetic acid-sodium acetate buffer of pH 4.0 was also tried with completely unsatisfactory results; the per cent transmittance of the extracted copper salicylaldoximate solution increased linearly with increasing buffer concentration showing no tendency toward leveling off a t higher concentrations.
Table I.
Elements Present in Samples Analyzed Maximum Percentage
Element Chromium Iron Magnesium Manganese Nickel Silicon Titanium Zinc
30
Major constituent in sample 94a
I ( I ( I I I I ( I I I 1 2 3 4 5 6 7 8 9 l O l l pH OF AQUEOUS PHASE
Figure 3. Effect of pH on extraction of copper(I1) salicylaldoximate into n-amyl acetate
t3
0.24 0.90 1.58 0.81 2.00 0.88 0.10
1
E ,L
/-
NAOH- KHCsH404
I
5
10
15
20
25
Effect of Temperature. iill extractions were made a t room temperature, and the spectrophotometer was maintained at 25"C. The transmittance of the copper salicylaldoximate solution in n-amyl acetate showed only the usual temperature coefficient due to the change in volume of the organic solvent with temperature. Because the temperature coefficient of expansion is rather large, it is advisable to make all extractions and transmittance measurements a t approximately the same temperature as the calibration. Effect of Diverse Ions. KO specific interference studies were made, but the method was applied t o the analysis of various aluminum alloys. Copper was satisfactorily determined in the presence of the elements given in Table I by controlling the pH of the aqueous phase. The percentages of the elements given in the table are the maximum values found in all the samples analyzed.
I
MILLILITERS OF BUFFER IN 50 MiLLILITERS OF SOLUTION
Figure 4. Effect of buffer concentration on extraction of copper(I1) salicylaldoximateinto n-amylacetate
Table 11. Analysis of Standard Samples NBS Sample 85a 86c 601 603 a
16 12 12
5
Range
Found, % Average
2 . 4 8 -2.52 7 . 8 0 -7.90 4 . 3 0 -4.39 0.274-0.275 3 . 9 6 -3.98 1 . 0 8 -1.09
2.49 7.83 4.35 0.275 3.97 1.08
6 8 Zinc-base alloy containing 3.90% aluminum.
604 940
Effect of pH on Extraction of Copper Salicylaldoximate. Standard 5.0 X 10+M copper(I1) solutions were prepared by volumetric dilution of the standard stock solution with buffers of varying pH. Ten milliliters of these standard solutions were extracted with 10 ml. of the reagent solution. The pH study showed that, using 0.02M salicylaldoxime in n-amyl acetate, copper(I1) could be extracted quantitatively in the p H range 3.5 to 9.5. The results are shown in Figure 3. Above pH 9.5 extraction is not quantitative, apparently because salicylaldoxime acts as a dibasic acid, and copper monosalicylaldoximate is formed in alkaline solutions.
No. of Samples
Av. Deviation,
Certificate Value of Copper, A
0.01 0.04 0.02
2.48 7.92 4.38 0.29 3.98 1.08
%
0.001 0.01 0.01
When silicon is present in large amounts, it is sometimes necessary t o remove the silica by filtration prior to extraction in order to get a good separation of phases. Kickel can also be quantitatively extracted, but no interference occurs if the aqueous phase is maintained a t a pH below 5.0. Application to Samples. The method was applied to a number of National Bureau of Standards certified aluminum alloys and to one zinc-baee alloy. Sample weights of approximately 0 1 gram
V O L U M E 27, NO. 8, A U G U S T 1 9 5 5
1339
were used for analysis of hamples containing 2 to 47, copper. Larger sample veights or suitable dilutions were made to adjust other samples to the desired range. The aluminum alloy samples were dissolved by the addition of 5 to 10 ml. of aqua regia, added in small portions to prevent loss of sample. When dissolution was complete, the solution \vas heated almost t o dryness, and then quantitatively transfrrred t o a 250-ml. volumetric flask. The solution was diluted to the mark with distilled water and mixed thoroughlv. Twenty-five milliliters of this solution was diluted to 100 ml. with 50 ml. of the buffer solution and distilled water. A 10-nil. aliquot was eatracted with 10 ml. of the reagent solution. After waiting 15 minutes for complete separation of phases the n-amyl acetate solution was transferred to a n absorption cell and the transmittance measured a t 344 mp against a blank prepared by carrying 10 ml. of the buffer solution though
the extraction procedure. in Table 11.
The results of the analyses are given
ACKNOWLEDGMENT
The authors are greatly indebted to J. B. Martin for his preliminary work in the quantitative investigation of the chloroform extraction of copper and nickel salicylaldoximates from aqueous solutions. LITERATURE CITED
(1) Ayres, G. H., ABAL.CHEM.,21, 652 (1949). ( 2 ) Clark, W.AI., and Lubs, H. il., J . B i d . Chem., 25, 479 (1916). (2) Irving, H., and Williams, J. P., J . Chem. Soc., 1949, 1841. (4) Kolthoff, I. XI., and Sandell, E. B., J . Am. Chem. Soc., 63, 1906 (1941). ( 5 ) Martin, J. B., Thesis, University of Texas, 1951.
RECEIVED for review J u n e
19, 1953.
Accepted hlarch 16, 1955.
Determination of Traces of Wickel in Malt Beverages MORRIS KENIGSBERG' and IRWIN STONE Wallerstein laboratories, New York 16,
N. Y.
A precise colorimetric method for the determination of traces of nickel in beers and ales is presented. The technique involves ashing the sample, taking up the ash w-ith dilute acid, and shaking out with dithizone in carbon tetrachloride. The soluble, colored nickel complex is formed in the aqueous layer by treatment with dimethylglpoxinie, bromine water, and ammonia. The color formed is proportional to the nickel present. Recovery of added nickel is good and copper and iron do not interfere.
A
SEKSITIVE and precise colorimetric method for the determination of traces of nickel in worts, beers, and brewing materials LYas developed for investigations on the effect of this metal on fermentation and wildness. This is a continuation in part of the studies conducted in this laboratory on the effects of trace metals in the brewing process. The presence of traces of nickel in worts and beers is assuming greater importance with t,lie increasing use of nickel or nickelconhining equipment in bren-eries. T h e contamination of worts with a few parts per million of nickel map lead to toxic effects on the yeast and slowing of the fermentation. Traces of nickel in the finished beer tend to form gas-evolving nuclei producing the condition of m-ildness, or gushing beer. This paper presents the details of the technique and data relating to the determination of nickel. T h e results of tests on the effects of traces of nickel on ferment ation and the production of wildness will be presented elsewhere (8). Hagues ( 5 )in 1931, in a paper on the contamination of beer by copper and nickel. proposed a method which involved dry ashing the sample and finally visually matching the color produced by strongly alkaline dimethylglyoxime wit,h standards in Sessler tubes after standing overnight. The method lnclis sensitivity, as 0.6 p.p.m. appears to be the lowest detectable limit. I n 1945, Essery ( 3 ) noted the many difficulties encountered in the determination of trace metals in mort. After an elaborate dry ashing and solubilization of the ash, he determined nicLrel by visual comparison of the color produced by oxidation with bromine and treatment with ammonia and dimetb>-lglyoxime. Citric acid \vas used to prevent interference of phosphates. b u t i t isstatedthat, if matching is delayed for a short time, precipitation interferes with the color Comparison. The author states he obtained a recoverv of 93%. The presence of 3 t o 4 p,p.m, of 1
Present address. T h e Toni Co., St. Paul, hlinn.
iron produced no interference, b u t thc: effect of the presence of traces of copper was not mentioned. After the present method was in use, the paper of A n d r e w and Harrison ( 1 ) appeared in 1954. Their met,hod ut'ilizes a wet digestion to destroy organic matter and the nickel is determined colorimetriczlly with afurildioxime in chloroform. The interference of copper is eliminated bv extraction of the chloroform solution xvith dilute sulfuric acid. Very good recoveries are reported. The levels of nickel found in the beers and brewing materials are in line with those found by the authors. The method reported here is a refinement of the reaction involved in the method of Essery. The reaction was first reported by Feigl(4), who found t h a t nickel and dimethvlglyosime, when oxidized by lead peroxide, produced a red colored compound in alkaline solution. Rollet ( 6 ) substituted bromine mater as the oxidizing agent and used the reaction to determine nickel quantit,atively in steel and various organic compounds. H e achieved a n accuracy of TTithin 5% and reported that copper interferes. Sandell ( 7 ) recommended the separation of nickel from copper by chloroform extraction of t,he nickelous dimethylglyoxime. Babco (2) investigated the reaction of nickel with dimet,hylglyoxime in the presence of bromine water and concluded t h a t the mechanism involved oxidation of the dimethylglyoxime and t h a t the order of mixing reagents is important. The direct determination of nickel in beer, without ashing, was tried, b u t i t is not satisfactory in the range of desired sensitivity. Colored materials are produced from the beer constituents during the test and the colored nickel complex is not readily extractable by solvents from the dark reaction mixture. The method, as here presented, involves dry ashing of the beer or wort. The ash is taken up in dilute hydrochloric acid and extracted with a carbon tetrachloride solation of dithizone to remove the interference of copper and iron. The aqueous layer is treated with dimethylglyoxime and bromine and t,hen made alkaline. developing the bromn-to-red nickel color. The intensity of the color, which is directly related to the concentration in the range 0.0 to 0.5 p.p.m., is then read in a photometer. After the ashing step the reactions are conducted in a stoppered 15-ml. graduated centrifuge tube and then transferred to a 10-ml. volumetric flask for color development. REAGENTS
Hydrochloric acid, 1 to 1 and 1 to 9. Dithizone (diphenyl thiocarbazone), 0.05% in carbon tetrachloride. Keep refrigerated in brown bottle.
