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 an 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. The 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. The 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. but it 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, but 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 that 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. He achieved an 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 that the mechanism involved oxidation of the dimethylglyoxime and that the order of mixing reagents is important. The direct determination of nickel in beer, without ashing, was tried, but it 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.
1340
ANALYTICAL CHEMISTRY
Dimethylglyoxime, 1% in 95% ethyl alcohol. Bromine water, a saturated solution of bromine in distilled water. Ammonium hydroxide, concentrated (28% ammonia). Standard nickel solution, nickel ammonium sulfate, NiSO,.(NH,),SOa.6H20,6.730 grams per liter: 1 ml. = 1 mg. of nickel. CALIBRATION OF PHOTOMETER
To graduated 15-ml. centrifuge tubes, add 1 ml. of 1 to 9 hydrochloric acid and aliquots of freshly prepared, appropriately diluted standard nickel solution, keeping the total volume under 6 ml. Cover the range from 0.0 to 0.5 mg. o$ nickel in suitable increments. Dilute to the 6-ml. mark with distilled water, add 4 ml. of dithizone solution, and continue as described below. The intensity of color developed is proportional to the concentration of nickel in the above range, so if the response of the colorimeter or photometer employed is linear, a straight-line calibration plot should be obtained and an average calibration factor converting scale readings to parts per million may be calculated. If the response of the photometer is nonlinear, a graphic method with a calibration curve may be used. .METHOD
Evaporate 100 ml. of wort or “degassed” beer to dryness in a silica dish previously cleaned by boiling with 1 to 1 hydrochloric acid, Char with a Bunsen flame, then ash in a muffle furnace a t 500O to 550’ C. until ash is white. ( I n the case of dry materials such as malt, weigh out a suitable charge, such as 10 grams, and char and ash as above.) To the ash add2 ml. of 1to 1hydrochloric acid and 2 ml. of distilled water, taking care to wash down the sides of the dish. Eva orate to dryness on a boiling water bath, and while dish is still got, take up residue with 1 ml. of 1 to 9 hydrochloric acid and 2 ml. of distilled water. Carefully transfer the acid solution to a graduated 15-ml. centrifuge tube (graduated in 0.1-ml. divisions). Wash out the silica dish with small increments of water, transferring t o the tube until the 6-ml. mark is reached.
-
__
Table I. Sample Beer A
-
--
_. .
i.’O
-_
~~~
Reco\,ery of Added Nickel Nickel Found, P.P.M.
Added Sichel Recovered, P.P.M.
0 05 (1 10 0.40
0.02 0.044 0.07 0.13 0.43
0 : Oi? 0.05 0.11 0 41
0.40 c 40
0.41 0.41
0.40 0 40
Added Metal, P.P.hI. Copper Iron Sickel
..
.
..
i.’O
0’025
d d d 4 ml. of dithizone solution, cork tube tightly, and shakc vigorously for 30 seconds, and allow layers to separate. Repeat shaking four times. If color of dithizone turns brownish, draw off spent dithizone and add a fresh 4-ml. portion. Repeat shakeouts and dithizone additions until no change in original dithizone color occum. Using a 5-ml. volumetric pipet, carefully remove 5 ml. of the aqueous (top) layer and transfer to a 10-ml. volumetric flask. Add 0.5 ml. of dimethylglyoxime solution, mix, and then add 1.0 ml. of saturated bromine water. Mix and let stand 10 minutes. (The solution should be brownish, owing to the excess bromine. If not, add more bromine aTater.) Then add 0.5 nil. of ammonium hydroxide, mix thoroughly, and diluted to the 10-ml. mark with distilled ivater. Transfer to a centrifuge tube and centrifuge for 3 minutes a t approximately 2500 r.p.m. Read the supernatant liquid (within 15 minutes) in a photoelectric colorimeter, using a green filter and a suitable cell. Set the “zero” reading of the instrument x i t h water. The authors have used a 13-mm. cell and a filter photometer with a composite glass filter having a maximum transmittance at 500 mp. A reagent blank is run by mixing 1 ml. of 1 t o 9 hydrochloric
acid and 5 ml. of distilled water in a graduated centrifuge tube and proceeding from the step where dithizone is added. The results are calculated in the usual manner for a colorimetric procedure. If the calibration data are a linear function, multiply the factor (converting photometer readings to parts per million of nickel) by the difference obtained by subtracting the blank photometer reading from the sample photometer reading. If a curvilinear function is obtained for the calibration, pick off the values for the sample and blank from a curve constructed on graph paper relating photometer readings t o parts per million of nickel. Subtract the blank results from the sample value t o obtain the parts per million of nickel to be reported.
Table 11.
Replication of Results Nickel, P.P.31. 2
Beer Beer Beer Beer Beer
A B C D
E
0.01 0.02 0.02 0.04 0.84
0.01 0.02 0.02 0.04 0.85
3 0.01 0.02 0.01 0.04 0.84
FOIbeeis containing nickel in excess of 0.5 p.p.m., reduce the charge of sample used for ashing. Select a charge such that the color intensity of the solution read in the photometer falls within the value of the calibration curve and gives readings of acceptable precision. CALIBRATION DATA
+4plot of the calibration data obtained on the Illett-Summereon photoelectric colorimeter gives a straight line. indicating that Beer’s lan- is obeyed in this range of concentrations. DATA ON RECOVERY 4 S D ISTERFERENCE
To determine the precision and reliability of the method, a series of tests measuring the recovery- of added nickel was carried out.. Table I gives the results of tests for the determination of the recovery of added nickel in a series of beers. Recovery tests mere also conducted on beers to which 2 p.p.m. of copper and 5 p.p.m. of iron were added, to see whether the presence of theee levels of copper and iron interfere. Two parts per million of copper and 5 p.p.m. of iron are much above the level of contamination generally found in beer. Inspection of the data shows that excellent recovery of added nickel can be expected from this method and that there is no interference from copper and iron a t levels much beyond that ordinarily encountered. REPLICATION
T’ery good replication of results is indicated. Table I1 cont:iins results of tests conducted in triplicate on a series of beers. Replication is good both a t levels of nickel generally encountered from normal beers (beers A to D) and a t levels indicating nickel pickup (beer E ) LITERATURE CITED
(1) Andrews, J.. and Harrison, G. A. F., J . Inst. Brewing, 60, S o . 2, 133-5 (1954). (2) Babco, A. K., Zhur. Anal. Khim., 3 , 284 (1948). (3) Essery, R. E., J . Inst. Brewing, 51, 185-8 (1945). (4) Feigl, F., Ber., 57, 758 (1924). !
RECEIVED for reriew January 3,
1955.
Accepted .4pril 5, 1955.
