Occurrence and Determination of Aluminum in Foods I. Determination of Aluminum in Organic Materials GERALD J. Cox, E. W. SCHWARTZE, RAYMOND M. HANIV, AND RICHARD B. UNANGST, Mellon Institute of Industrial Research, Pittsburgh, Pa., AND J. L. NEAL,,4luminuni Co. of America, New Kensington, Pa.
D
URING the course of A l u m i n u m in organic tissues is determined l n o n o s o d i u m phosphate a n d an investigation of the colorimetrically as a lake with aurin tri2 cc. of 0.04 per cent bromosuitability of v a r i o u s phenol blue are added and then carboxylic acid. Exact details Of ashing pro;M a m m o n i u m h y d r o x i d e metals for the construction of cooking utensils and food concedure and conditions for lake formation are until a permanent precipitate tainers, it has been necessary for gizien. Simplification of technic and increased is f o r m e d , N e x t t h e p H is precision ocer. previously described methods are brought to 4.2 by the addition the authors to determine very attained by use of a color standard of 3 i v sodium acetate. The small amounts of the respective metals that have been taken up mixture is c e n t r i f u g e d for 5 and a combined reagent for lake developmenl. by foodstuffs from the surfaces minutes at 1800 r, p, m., and the with which they have been in A~erCaptOUcetiC(thioglycolic) acid iS used as a liquor is discarded. contact. These analytical prolest for iron. The p r e c i p i t a t e of ferric cedures have been applied both and a l u m i n u m phosphate is to the foodstuffs as prepared for the table and to the tissues dissolved in 0.5 cc. of 6 T .I hydrochloric acid and 1.25 cc. of of animals that have received known concentrations of metal- glacial acetic acid, and approximately 1.5 cc. of hot water lie salts in their daily diet. This paper describes the pro- are added. Five cc. of 6 J4 sodium hydroxide are stirred cedure that has been followed for the determination of in with a glass rod, and the mixture allowed to stand for an aluminum. The concentrations of aluminum thus deter- hour with frequent stirring. The glass rod is washed, and mined have been as small as 0.1 p. p. m. the ferric hydroxide thrown down by centrifuging for 5 A colorimetric method for aluminum, based on the pro- minutes a t 1800 r. p. m. The solution is decanted through cedures of Myers, Mull, and Morrison ( 2 ) ,and of Winter and two 9-cm. filter papers prepared by washing with warm 1.2 M his associates (7, 8, 9), has been used. The method, in brief, sodium hydroxide, followed by hot water until practically consists of ashing the material a t as low a temperature as all alkali is removed. The filtration is made into a 100-cc. possible, solution of the ash, precipitation of aluniinum as volumetric flask containing 1 cc. of 6 .If hydrochloric acid. the phosphate adsorbed on an excess of ferric phosphate, The precipitate remaining in the centrifuge tube is washed removal of iron, development of a color by means of the aurin once by stirring with 20 cc. of hot water and centrifuging. tricarboxylic acid lake, and determination of the aluminum The washings are decanted through the by comparison with a fixed color standtird. filter. The ferric hydroxide is not transferred to the paper. The filter paper is PROCEDURE thoroughly washed with water, the soFresh washed tissue ( 5 to 150 grams) is dried overnight in a lution cooled, made faintly acid to litplatinum dish a t 110" C. It is then placed on a nichrome- mus, and diluted to 100 CC. triangle in a cold electrically heated muffle, and the temperaTwenty cubic centimeters of the soture is gradually raised during the day to a faint red heat. lution are measured into a dry 250-cc. The ashing process is completed overnight while a slow g l a s s - s t o p p e r e d E r l e n m e y e r flask. current of oxygen is passed into the muffle to hasten the Then there are added 25 cc. of a solution containing, in 1 liter, 1 mole of combustion. The ash is treated with 10 cc. of concentrated hydrochloric ammonium acetate, 1 mole of ammoacid and 2.5 cc. of water. The resulting solution is evaporated nium chloride, 80 cc. of 0.1 per cent to dryness to dehydrate silica. To the residue are added ammonium aurin tricarboxylate (alu10 cc. of 3 M hydrochloric acid and 25 cc. of water, and the minon), and 60 cc. of 6 M hydrochloric solution is boiled for 5 to 10 minutes. The solution is cen- acid. The pH a t this point must be trifuged a t 1800 r. p. m. for 5 minutes and decanted into a between 4.5 and 5.5. A snugly fitting 100-cc. Erlenmeyer flask. The residue, if any, is washed and test-tube condenser (Figure 1) is placed discarded; but if carbon remains in the residue, the latter is in the neck of the flask, and the solu- FIGURE 1. FLASK transferred to a platinum crucible and dried. The silica is tion is boiled for 1 minute, timed from AND CONDENSERFOR volatilized with hydrofluoric acid and sulfuric acid, followed the first appearance of steam bubbles. LAKE I h m L o P M E N T by gentle ignition. The residue is then fused in a small The solution is cooled for 1 minute amount of equal parts of sodium and potassium carbonates under the condenser and then to room temperature in running water. and dissolved in the main solution. To the solution of the ash are added 1 cc. of concentrated Enough 1.6 J4 ammonium carbonate (4.8 to 5 cc.) is added nitric acid (specific gravity, 1.42) and 1 cc. of 0.1 JI ferric with gentle shaking to give a final pH of 7.1. The flask is sulfate. The solution is evaporated to about 10 cc. and stoppered and shaken up and down twenty times. Carbon diluted with water to approximately 60 cc. Five cc of 1 -11 dioxide is released by cautiously removing the stopper, and 403
I N D U S T R I A4L A N D E N G I N E E R I N G C H E YI I S T R Y
404
the flask is allowed to stand for 20 minutes, with the stopper loosened, for the excess dye to be decolorized. The pH should now be within the limits 7.0 and 7.3, but preferably a t 7.1. The concentration of the aluminum is determined immediately in a Duboscq colorimeter by comparison of the intensity of the lake color with a standard containing 5 cc. of 0.04 per cent thymol blue and 8 cc. of 6 M hydrochloric acid in 500 cc. of solution. The aluminum lake solution is set at a constant depth of 30 mm., and the depth of the standard is varied for the color matching.
prolonged, the pH will be high enough to result in appreciable fading of the lake color. The thymol blue permanent standard used by T h n (6) in the study of the stability of aluminum lakes is very convenient and preferable to standards prepared from aluminum solutions. The pink color is stable over periods of a t least 6 months. The color match is not perfect, but readings are readily duplicated to give thoroughly reliable analyses. The precision of the readings is such that the probable error of sixteen random observations at about 30 mm. is +0.10 mm. A random selection of some early readings, made by another trained observer , of aluminum lake against aluminum lake shows the probable error of *0.25. All reagents were made up from large stocks of chemicals of the same lot number in order to maintain a uniform blank in all analyses. hlallinckrodt’s c. P. quality sodium hydroxide crystals (NaOH HzO) have been found to be practically aluminum-free. Its use obviates the tedious preparation of the alkali solution from aluminum-free sodium. The 6 M solution was prepared by direct weighing without subsequent standardization. The 6 M hydrochloric acid was prepared by distilling an approximately 6 M solution and collecting the last three-fourths of the distillate. The aluminon of the Fales Chemical Company was wed. The empirical curve employed is an inverse curve compared with that of Winter, Thrun, and Bird (9), as the depth of the standard in the colorimeter was varied rather than that of the unknown. The curve automatically corrects for the blank of the reagents, as i t is a record of the colorimeter readings obtained when definitely known amounts of aluminum enter the analytical system. The curve shown in Figure 2 represents the net colorimeter readings obtained in a series of observations of the intensity of color produced by known amounts of pure aluminum salts added a t the beginning of the procedure, The reagents used produced color equivalent to 12.5 to 14 mm. of the thymol blue standard. In routine analyses a blank control was invariably carried through to secure the net reading of the colorimeter, a reading which is obviously altered by the size of the aliquot required. The curve shown in Figure 2 is not reproduced for use, &B its form and position depend on the quality of all reagents. It is necessary to prepare a curve for each set of reagents, ~b process that is simply a matter of “blanking” the reagents. I n a system which involves the use of standards prepared from known amounts of aluminum, the reagents involved in the preparation of those standards always introduce an additional unknown amount of aluminum. An inherent property of an empirical curve based on the use of a permanent standard and the values of aluminum entering the system is that it corrects for the entire blank of the procedure. At the beginning of this investigation two general methodb were available-the chemical and the spectrographic procedures. The latter presented the advantage of speed and the complete absence of contamination of the ash by the addition of reagents, but a t that time was only roughly quantitative and required the installation and maintenance of expensive equipment, The advances made by Myers, Mull, and Morrison (6),and, during the period of the present studies, by Winter and associates (5,7 , 8 , 9 ) and by Underhill and Peterman ( 6 ) , have influenced the choice of the chemical method here used. After an exhaustive investigation of ashing by the wet method-i. e., by sulfuric acid digestion with the addition of perchloric acid or nitric acid-and of the alternative ashing by ignition, the latter was chosen. I n the wet-ashing procedure, some aluminum was introduced by the reagents, and containers were etched to give insoluble residues which were difficult to handle. These objectional features were absent
+
I
I
0005
RDlO
0015
I
I
a020
a025
I 0030
&WMINUM,MO
FIGURE 2. NETCOLORIMETER READ-
INGS OBTAINED WITH ADDED ALUMINUM
The amount of aluminum present in the aliquot is determined by reference to a curve plotted from the colorimeter readings obtained when varying amounts of aluminum are subjected to the entire scheme of analysis beginning with the ash (Figure 2). DISCUSSION OF TECHNIC AND REAGENTS In the initial solution of the ash of heavy carcasses it is necessary to use 25 cc. of concentrated hydrochloric acid and 25 cc. of water and to aid the process by trituration of bone ash with a flattened glass rod. The second solution is effected by 20 cc. of concentrated hydrochloric acid and 150 cc. of water. An aliquot, usually one-fifth, is taken for the rest of the procedure. No addition of monosodium phosphate is necessary in the analysis of carcasses. The reaction of pH 4.2 should be very closely controlled in the precipitation of the phosphates of iron and aluminum, to effect a quantitative removal of aluminum and insure its freedom from calcium phosphate. Carl Schleicher and Schiill No. 589 white ribbon filter paper was used in preparing the iron-free filtrate from the ferric hydroxide precipitation. These filtrates must be entirely iron-free, and the mercaptoacetic acid test has been found most satisfactory. When 10 cc. of the solution are treated with a drop of mercaptoacetic acid (1) and then made alkaline with ammonium hydroxide, there should be no pink color developed. The writers have found that iron is not completely removed by centrifuging. Filtration through one paper removes part of the residual iron; two papers remove all of it. The process seems to be one of adsorption rather than of filtration. The reagent used for development of the aluminum lake is based on the separate solutions used by Winter, Thrun, and Bird (9). The combined reagent obviously reduces the manipulations and insures more uniform conditions during the lake development. Because of variations in the escape of carbon dioxide in the decolorization of the excess dye after lake development, the given procedure for shaking must be rigidly adhered to in order to insure a final pH of 7.1. If the solution is not shaken enough, the pH will be too low for complete change of the dye to the yellow form; if the shaking is too vigorous or
Vol. 24, No. 4
April, 1932
I \ D C S T R I 4 L A4hD E N G I S E E H I N G
from the dry-ashing procedure. The authors were not able to demonstrate a loss of added aliiiiimum in smoke evolved in ignition of various materials. An investigation was made of the removal of iron, which interferes by Of The effected very completeJ but the 'lethod was cumbersome and required an ashing procedure to remove the excess of cupferron. Iron is not removed completely by centrifuging, but, by adsorptive filtration through two filter papers, the mercaptoacetic acid test becomes uniformly negative. A study of the interference of silica in colorimetry reveals that it is sufficiently removed by dehydration so that hydrofluoric acid treatment is unnecessary. The absolute quantity of aluminum present in a given tissue, of course, cannot be determined by any method. Recourse must therefore be had to experiments on the recovery of added aluminum to prove the accuracy of a method. A well-mixed sample of ground beef liver showed, after the manipulations of grinding and mixing, 5.0 p. p.m. of aluminum. When 0.05 mg. of aluminum was added to 25 grams of the beef liver (yielding 0.125 mg. of aluminum), making a total apparent aluminum content of 0.175 mg. before asbing, 0.1785 and 0.1725 mg. were recovered. When the aluminum was added to the ash, 0.1725 and 0.1715 mg. were recovered. The reader is referred to the articles cited (2, 5 , 6, 9) for other details of technic and for the underlying principles of the method. Other articles in this series ( 3 , 4) haye been
CHE3fISTRY
105
published, and further contributions will follow. Copies may be obtained by addressing the senior author. ACKNOWLEDGMENT In the developnlent of this method W. N, Bradley, Alary L. Dodds, L4,D. filelavan, F. J , Murphy, and Ilelpn B. Jvigman have rendered technical assistance. The authors also wish to acknowledge their indebtedness to G. D. Beal, R. R. Bridges, and F. C. Frary for many helpful suggestions. . LITERATURE CITED (1) Lyons, E., J . Am. Chem. SOC.,49, 1916 (192i). (2) Myers, V. C., Mull, J. TV., and Morrison, D. B., J. H d . Chern , 78. 595 (192s). (3) Schwartze, E. W:, Murphy, F. J., and Cox, G. J . , J N u t r i t i o n , 4, 211 (1931). (4) Schwartze, E. IT., Murphy, F. J., and Hann, R. M., Ibid,, 2, 325 (1930). (5) Thrun, W. E., J. Phys. Chem., 33, 977 (1929). (6) Underhill, F. P., and Peterman, F.I., B m . J. Physiol., 90, 1 (1929). (7) Winter, 0. B., J. Assoc. Oficial A g r . Chem., 13, 220 (1930). (8) Winter, 0. B., and Bird, 0. D., J. Am. Chem. Soc., 51, 2964 (1929). (9) Winter, 0. B., Thrun, W. E., and Bird, 0. D., Ibid.. 2721 (1929).
RECEIVEDJanuary 18, 1932. Present addresses of authors are a8 f o l l o w : E. W. Schwartze, hledical Department, University of Georgia, Auguata, Ga.; R. M. Hann, National Institute of Health, Washington, 13. C.;R. B. Unangst, Aluminum Co. of America, New Kensington, Pa.; J. L. Neal. 1125 W Onondaga Si..Syracuse, N. Y
e,...
11. Aluminum Content of Foodstuffs Cooked in Glass and in Aluminum GEORGED. BEAL,RICHARD B. UNANGST,HELENB. WIGMAN,AND GERALDJ. Cox Mellon Institute of Industrial Research, Pittsburgh, Pa. XECESSARY qualificaFoods cooked in glass and in aluminum hate pected (phosphorus starvation) been analyzed for aluminum, and the average when overwhelming doses of tion of all materials that soluble aluminum salts are fed come into contact with increase has been calculated. The taking up of have also been described ( 3 ) . I n food during its production, disaluminum by neutral foods is negligible: acid the present paper a study of the tribution, cooking, or service is that they must not in any way and alkaline foods are relatively more corrosive. amounts of a l u m i n u m which depreciate the q u a l i t y of the I n no case, howecer, is suficient aluminum disenter foods by contact with alufood. Primarily, it is not persolced from utensils to interfere seriously with minum in a variety of culinary missible that any essential conphosphorusabsorption. average daily intake practices will be recorded. In every case the amount of alumiof the food be removed of aluminum in case all foods are cooked in or any harmful substance added. num added to foods by utensils Of secondary importance is the aluminum is estimated at mg.9 of which made of it has been found to be 5 mg. is deriz\ed,fromthe utensils. requirement that a specific unsmall and far below that necesdesirable taste or odor should sary to produce p h o s p h o r u s not be imparted and that a discoloration should not be pro- -starvation and its sequelae, the only abnormal conditions which duced in foods by containers during any of the operations the authors were able to cause by administering to experimentioned. It is, of course, also of importance that the utensil mental animals excessively large doses of aluminum. be inert to corrosion by foods in order that it survive use in PREVIOUS STUDIESOF ALUMINUM CORROSION cooking. Aluminum is today the most popular of all materials for the A few quantitative and many qualitative studies of the construction of cooking utensils because of its light weight, corrosion of aluminum by foods have been made, especially in bright attractive appearance, durability, high heat conduc- European laboratories. The latter have consisted of observativity, and ease of cleaning and maintenance for constant use. tions of pitting, polishing, or staining of aluminum surfaces This wide popularity of aluminum after about thirty years exposed to foods; of precipitates in water and clear liquids; of employment is convincing evidence that it is satisfactorily of discoloration of foods; and of alleged changes in taste. resistant to food attack, and that no harm results from eating Occasionally, chemical demonstrations of the presence of foods customarily prepared in aluminum vessels. It has aluminum in the food have been mentioned. Most of these been shown elsewhere (14) that only barely (detectable reported findings, both qualitative and quantitative, are of amounts of aluminum appear in the tissues, following diets little value now, as they were obtained by the use of articles containing large amounts of aluminum; the results to be ex- constructed of various grades of early commercial aluminum
A
