ANALYTICAL EDITION
November 15, 1941
Acknowledgment The author wishes to thank R* "'. Schayer for his assistance in carrying out most of the experimental detail.
Literature Cited (1) Fales, H. A., and Kenny, Frederic, "Inorganic Quantitative Analysis", p. 47, New York, D. hppleton-Century Co., 1939.
791
(2) Goldman, F. H., Pub. Health Repts., 52, No. 48, 1702-12 (Nov.
26, 1937) Reprint 1882. (3) Goldman, F. H., and Dalla Valle, J. hf., Am. .Mineral., 24, 40-7 (1939). (4) Hatch, T., and Moke, C. B., J. Ind. Hug. TozicoZ., 18, 91 (1936). ( 5 ) Knopf, Adolph, Pub. Health Repts., 48, No. 8, 183-90 (Feb. 24, 1933) Reprint 1560. ( 6 ) Moke, C. B., J.Ind. Hug. Tozicol., 18, 299 (1936).
Determination of Traces of Tin in Malt Beverages IRWIN STONE, Wallerstein Laboratories, New York, N. Y.
T
HE presence of traces of tin in beer, due to solution from
tin surfaces in brewery and dispensing equipment, has long been known to produce haziness and turbidity in the finished beverage. The relatively high intensity of the colloidal haze produced seems out of all proportion to the minute quantities of tin involved. As little as 0.1 part per million will affect the clarity of the beverage. Goob (3) in 1912, writing on tin turbidities, noted the unsatisfactory state of the quantitative methods for the chemical determination of traces of this metal. A search of the literature failed t o reveal any specific methods or improvements of methods for the chemical determination of these traces of tin in beer since that time. There are many published chemical methods for determining small amounts of tin in food and related products, but these, without exception, are for quantities greatly in excess of those which may cause trouble in beer. Some of these methods might be adapted to determining traces of tin by using extremely large volumes of sample, b u t this is not a completely satisfactory expedientfor instance, i t would be necessary t o examine 4 liters of a beer containing 0.25 part per million of tin in order to recover 1 mg. of tin. Large charges necessitate increased volumes of reagents and solutions and the effective increase in sensitivity thus obtained is not great. In most cases, the minute traces of tin involved would be lost in the elaborate separations used. The development of spectrographic analysis (4) has brought a new tool which may be applied to this problem, but this method requires special training and expensive apparatus and is beyond the means of the average analyst. Nearly all the methods in the literature for determining tin in biological materials utilize a tedious wet digestion for destroying the organic matter prior to separating and measuring the tin and avoid the more convenient ashing. During ashing, the tin changes t o tin oxide, which is extremely refractory and resists solution in all common acids. The widespread use of the wet digestion has probably been due to a desire to avoid the formation of this refractory tin oxide. Tin oxide readily becomes soluble when fused with a number of different substances. A method for determining tin in foods, which utilizes a n ashing and a fusion of the ash, was described ( 2 ) in 1928. This method is not sufficiently sensitive for determining tin in beer, but the treatment of the sample prior to actual analysis is of interest. B u n t i 1 recently, there has been a notable lack of good specific sensitive organic reagents for detecting tin. The older tests utilized, in one way or another, the reducing properties of the stannous ion or the oxidizing properties of the stannic
ion. I n 1936, Clark (1) recommended the dimercaptobenzenes as sensitive and specific reagents for tin. Tin combines with these organic reagents to give red precipitates. By combining the ashing and fusion technique for the preliminary preparation of the sample with the new organic reagent, i t was possible to develop a simple, rapid, and accurate method for the determination of traces of tin in beer. This quantitative determination also serves as a qualitative test, thus ensuring that the measured factor is actually tin. The availability of the reagent (l-methyl-3,4-dimercaptobenzene, obtainable from the Organic Products Company, 17 Thompson St., Xew k'ork, X.Y., or the British Drug House through the Eastman Kodak Company, Rochester, N. Y.) and the perfection of a stable solution add further to its convenience.
Reagents FUSIOSMIXTURE. Quickly grind (to avoid local evolution of hydrocyanic acid by atmospheric carbon dioxide) with a mortar and pestle 12.5 grams of sodium cyanide with 37.5 grams of anhydrous sodium carbonate to give a homogeneous powder. HYDROCHLORIC ACID. Dilute 1 volume of concentrated hydrochloric acid with 1 volume of water. 1-METHYL-3,4-DIVERCAPTOBESZESE (DITHIOL) SOLUTION. Warm the dithiol slightly so that it Iiquefies, dissolve 0.28 ml. in 10 ml. of thioglycolic acid, and dilute to 200 ml. with 95 per cent ethyl alcohol. Store in full tightly corked small bottles in the dark. The reagent is stable if protected from air, but in a bottle that has been repeatedly opened will frequently become oxidized and no longer give the reaction with tin. Thus, the reagent from an opened bottle should be tested against a known tin solution before adding it to the sample. (The reagent has a strong sulfidic odor and may be deodorized, if spilled, by washing with a dilute iodine solution.) TIS ST.ASDARD. Dissolve 1.90 grams of stannous chloride dihydrate in 20 ml. of the hydrochloric acid solution and dilute to 1 liter with water. This solution should be prepared fresh, as it is not very stable. GUM ARABIC SOLUTIOS. Dissolve 100 grams of powdered gum arabic (U. S. P.) in 1 liter of hot water containing 100 ml. of 0.1 per cent phenyl mercuric acetate solution. When dissolved, filter through paper in a hot water funnel. This solution is stable and will not get moldy.
Method Char and then ignite (preferably in a muffle at about 550" C.) 100 t o 200 ml. of well-mixed, degassed beer in a silica dish to give a white fluffy ash. Avoid too high a temperature during ashing to prevent fusion of the ash. Transfer the ash by brushing to a No. 00 Coors high-form porcelain crucible, tamp down ash in the crucible, and cover with 1 gram of the fusion mixture. Fuse over a MBker burner for about 15 seconds, holdin< the crucible with a pair of tongs and sn-irling, so that the melt is given
192
INDUSTRIAL AND ENGINEERING CHEMISTRY TABLE I. RECOYERV OF ADDEDTIX
Sample Taken for Analysis
M1. 200 200 200 200 100 100 100 100 100 100 100 200 100 200 100 100 100
T i n Added M g . per charge P. p . m. Nil
0.05 0.03 0.03 0.04 0.075
Nil
0.25 0.15 0.15 0.40 0.75
Tin Found Mg. Nil
0.05 0.03 0.04 0.04 0.076
P. p . m. Nil
0.25 0.15 0.20 0.40 0.75
Xi1
Xi1
Xi1
Xi1
0.05 0.05
0.5 0.5
0.05 0.04
0.5 0.4
Xi1
Nil Nil
Nil Nil
Nil
0.01 0.02 0.015 0.03
0.025 0.05 0.05
0.1 0.1 0.15 0.15 0.25 0.5 0.5
0.01 Nil 0.015 0.025 0.04 0.04
07.,05 hll
0.08 0.25 0.4 0.4
a rotating motion. Do not overheat or fuse too long. Cool and place the crucible upright in a 50-ml. Pyrex beaker which is covered with a watch glass. All further operations, because of the poisonous character of the liberated hydrocyanic acid gas, must be conducted in a well-ventilated hood. Put 5 ml. of the hydrochloric acid solution directly into the crucible by means of a pi et introduced through the space between the watch glass a n t t h e beaker spout. Allow it t o react until the evolution of gas ceases. Remove the watch glass and wash down its undersurface with water, permitting this wash water to run into the crucible. With a thin stirring rod overturn the crucible and heat. This will generally cause further evolution of gas from the crucible. Continue heating and stirring until no further evolution of gas takes place, pick up the crucible by means of the stirring rod, and wash with water. Cover the beaker with the original watch glass and boil down the combined solution and washings to a volume of less than 10 ml. Transfer and wash this solution into a 15-ml. centrifuge tube graduated at 10 ml., make up volume to 10 ml., mix, and centrifuge at high speed. Transfer the clear supernatant liquid to a test tube, add 0.5 ml. of the tin reagent, mix, and heat for 1 minute in a slowly boiling water bath. Cool, add 2 ml. of the gum arabic solution, cork, and shake thoroughly. Compare the resulting colored turbidity with standards by reflection, using daylight. PREPARATION OF STAXDARDS. Dilute 20 ml. of fresh tin standard solution and 10 ml. of hydrochloric acid to 200 ml. with water. Pipet the following suggested quantities of this standard solution into a series of test tubes: 0.0, 0.1, 0.25, 0.50, 0.75, and 1.0 ml. (equivalent to 0.0, 0.01, 0.025, 0.05, 0,.075, and 0.1 mg. of tin, respectively). Add 1 mI. of hydrochloric acid solution and make volume up to 10 ml. with water. Add 0.5ml. of tinreagent, mix, and heat in a slowly boiling water bath for 1 minute. Cool, add 2 ml. of gum arabic solution, cork, and shake thoroughly. Standards thus prepared are stable for a month or more if kept corked and in the dark. After standing, they should be vigorously shaken before using.
Vol. 13, No. 11
the insoluble Prussian blue. The Prussian blue is then removed along with the other insoluble salts. Table I1 shows the effect of the presence of 10 parts per million of interfering heavy metals on the determination of tin in a beer containing 0.5 part per million of tin. Except for iron, the results in Table I1 are of no practical interest. Excess iron is removed in the course of the analysis and the copper content of beer is effectively limited by the normal removal of excess copper by the yeast. None of the other metals is present in beer in quantities sufficient. to cause interference. I n the case of a rare sample in which large amounts of the interfering metals may be present, they may be dissolved out of the ash by dilute acids, leaving the insoluble tin oxide behind. This purified precipitate containing the tin oxide, after filtration, washing, and re-ashing, can then be treated as in the regular method. This procedure tends to reduce the sensitivity of the method and great care is needed to prevent the fine tin oxide precipitate from mechanically passing into the acid-soluble filtrate. I n this laboratory, where many different samples representing all types of beers and ales have has been encountered 'Onas yet no been taining a sufficient quantity of interfering metal to necessitate the use of this longer and less sensitive modified procedure. This method has been tried On number Of different types of materials including several foods, water, and other beverages and the indications are that it may readily be adapted to materials other than beer. It is especially useful where only
amounts Of
TABLE
Ir.
are
EFFECT OF
OF DETERMINATION OF TIN
METALS OK
(Beer containing 0.5 p. p. m. of tin) Metal Bdded
Results
(10P. P. M.)
None Bismuth Cobalt Copper Iron Lead lllanganese Nickel Zinc
0.5p. p. m.tin, red turbidity
N o tin, yellow precipitate S o tin, dirty yellow precipitate No tin, black precipitate 0.5 p. p. m. tin, no interference 0.5 p. p. rn. tin, no interference 0.4 p. p. rn. tin, orange turbidity 0.1 p. p. rn. tin 0.25p. p. rn. tin, orange turbidity
Summary
Since there is no other reliable method available against which the accuracy of the method could be checked, recourse was had to a determination of added tin. Beers that were brilliant on chilling are found to give a negative test for tin by this method. Such beers were assumed to be tin-free and tin was added in varying amounts. After standing some time, they were analyzed and the results are given in Table I.
Minute traces of tin, much below the practical limit of sensitivity of existing chemical analytical methods, produce haziness in beers and ales. A method is described for determining such traces of tin, wherein the beer is ashed and the ash fused with a sodium carbonate-sodium cyanide mixture. After solution in acid, the tin is determined by the intensity of the color of the red precipitate produced by a solution of the specific organic reagent 1-methyl-3,Cdimercaptobenzene (dithiol). Recoveries and interferences are noted. This method may be adapted to the determination of traces of tin in ot1:er foodstuffs, water, and biological materials.
Interferences
Acknowledgment
As shown by Clark, numerous heavy metals give varicolored precipitates with this reagent, but the red color of tin is characteristic; the only metal which approaches this color is bismuth, which exhibits a completely different shade of red. Other metals, if present to excess, can interfere in the method by masking the color of the tin precipitate. Iron, the only metal that is liable to be encountered in excess in beers, will not interfere in the test because during the cyanide fusion i t is converted to compounds which, on later acidification, form
The writer wishes to thank Roy TV. Seaholm for conducting many of the determinations reported here.
Recovery of Added Tin
Literature Cited (1) Clark, R. E. D., Analyst, 61, 243 (1936). (2) Glassmann, B., and Barsutkil, S., 2. Untersuch. Lebensm., 56, 208
(1928). (3) Goob, G. L., Orig. Commun. 8th Intern. Cow. Appl. Chem., V , 14, 81 (1912). (4) Staud, A. H., Glass Packer, 15, 731 (1936).
