since the k value was determined for the first time are within the standard deviation of the average. APPLICATIONS
The preference of a single comparator method over the more used relative method is mainly a matter of experimental facilities available and problems under study. Evidently if only a reactor with a high epithermal flux and nonsteady operation is available, the method cannot be used simply. A large variety of elements to be determined in a small series of samples may not warrant the preliminary assessment of k values and continuous control of the constancy of the experimental parameters. The method is, on the contrary, ideally suited for large scale activation analysis by means of steady and well thermalized reactors. Under these conditions the method is as precise and accurate as the relative method and simplification of the analytical technique leads to a higher number of analyses per unit time, possible use of nonspecialized personnel for many operations, and possibility of establishing a standard routine procedure in which the chance of
accidental errors is minimized. The coupling of this method with the evaluation of photopeak activity and calculation of weights by means of a n electronic computer (the IBM 7090 in our case) ( I I ) , further enhances the possibility of a routine service on a semiautomated basis. This method has been used satisfactorily for routine analysis in our laboratory for more than a year. Applications include nondestructive determination of trace elements giving shortlived isotopes (Mn, C1, Na, and Cu in terphenyls, Mn in biological materials) and destructive analysis of trace elements giving both short-lived and longlived isotopes (Rb, Cs, Sr, Fe, Co, Zn, U, Th, Sc, Cr, Hg, etc., mainly in biological specimens). LITERATURE CITED
(1) Adams, F., Hoste, J., Speecke, A., Talanta 10. 1243 (1963). (2) Anders, ’0.U.; ANAL. CHEM. 36, 564 _ _ -11964). (3) Borella, A,, Guzzi, G., Euratom Rept. EUR 531.e (1964). (4) Bresesti, M.. Neumann.. H.,, Zbid., EUR 374.e (1963). (5) Brume, D., Jirlow, K., Reactor Sci. Technol. 17, 350 (1953). (6) Covell, D. F., ANAL.CHEM.31, 1785 (1959). \ - -
- 1 .
’
(7) Covell, D. F., Nuclear Instrumentation Methods 22, 101 (1963). ( 8 ) Crouch, D. F., Heath, R. L., Proceedings of Symposium on Application of ComDuters t o Nuclear and Radiochemistiy, Gatlinburg, Tenn., Oct. 1719. 1962. - - - ~ (9)Fukai, R., Meinke, W. W., Nature 184, 815 (1959). (10) Girardi, F., Guzzi, G., Pauly, J., ANAL.CHEM.36. 1588 (1964). (11) Girardi, F., Qerlini, M., ’“L’analyse par radioactivation et ses applications aux sciences biologiques,” p. 32, Presses Universitaires de France, Paris, 1964. (12) Hughes, D. J., Harvey, J. A., “Neutron Cross Sections,” McGrawHill, N e w York, 1955. (13) Hughes, D. J., Magurno, B. A., Brussel, M. H., “Neutron Cross Sections,” Suppl. 1 BXL 325, Brookhaven Natl. Lab., Upton, N. Y., 1960. (14) Macklin, R . L., Pomerance, H. S., Progr. Nucl. Energy, Ser. I, 1 (1955). (15) Monnier, D., Haerdi, W., Vogel, J., Helv. Chim. Acta 44, 897-903 (1961). (16) Morzek, P., Kernenergie 5, 839 (1962). (17) Pauly, J., Girardi, F., Bull. SOC. Chim. France 1963, 244. (18) Schute, D. F., Yale Univ. Geochemistry Techn., Rept. 9 (1964). (19) Wainerdi, R. E., Fite, L. E., Steele, E. L., “L’analyse par radioactivation et ses applications aux sciences biologi ues,” p. 171, Presses Universitaires de #ranee, Paris, 1964. RECEIVEDfor review October 5, 1964. Accepted April 14, 1965. I
Determination of Oxalate in Beer by Cerate Oxidimetry and the Application of isotopically-Derived Correction Factor GILBERT H. KOCH Department of Chemistry, Universify of Wisconsin-Milwaukee, Milwaukee, Wis.
F. M. STRONG Department o f Biochemistry, University o f Wisconsin, Madison, Wis. The oxalate content of most beers is below 20 mg. per liter and usually below 15 mg. The oxalate can be more readily and completely precipitated as calcium oxalate after a known amount of pure sodium oxalate is added to the beer sample. The precipitated calcium oxalate is determined by cerate titration in 1N perchloric acid. By use of oxalic-C14 acid, the best conditions for the precipitation of calcium oxalate have been established. From the radioactivity of the beer filtrates and the amount of precipitated oxalate found by cerate oxidimetry, a correction factor has been calculated for unprecipitated oxalate. The oxalate content of all beers, whether high or low in oxalate, can now be determined with an accuracy of i0.5 mg. per liter.
1092
a
ANALYTICAL CHEMISTRY
T
of oxalate in relation to some of the problems encountered in the brewery has been reviewed by Burger and Becker (5) and by Brenner (3,4). The formation of calcium oxalate crystals in packaged beer contributes to haze, while the presence of microcrystalline nuclei of calcium oxalate promotes gushing and overfoaming. Hence, the oxalate content of beer has been of interest to the brewing industry for a long time. The usual oxalate level, ca. 7-20 mg. per liter, is too low to be determined quantitatively by the conventional method of precipitation of calcium oxalate. I n the method of Bau ( I ) , which has been commonly used in the brewing industry, the beer sample, after the addition of calcium acetate, is allowed to stand for 40 hours in the refrigerator a t approximately 4’ C. Brenner (3) HE ROLE
has shown by an isotope dilution technique that 19-60% of the oxalate failed to precipitate under these conditions from seven 4-liter beer samples, and 20-48y0 from eleven 300-mI. beer samples. Filtration of the precipitate and the determination of the recovered calcium oxalate by ashing or by titration with dilute permanganate present numerous problems to the most careful analyst. Finally, the correction factor (5) suggested to compensate for the oxalate remaining in the beer filtrate and wash water, based on the solubility of calcium oxalate in water, is meaningless and arbitrary since the solubility in beer is different from that in water and is further modified by excess of calcium ions from the precipitant. The present method incorporates conditions for the precipitation of calcium oxalate which were established by
the use of 0xa1ic-C'~ acid. I n addition, improved filtration and washing techniques are used. From the radioactivity of the beer filtrate and the determination of precipitated calcium oxalate by cerate oxidimetry, a correction factor for unprecipitated oxalate has been calculated.
EXPERIMENTAL
Apparatus. For the filtration of calcium oxalate a filter tube with a 30-mm., porosity B , glass filter disk (Ace Glass, Inc., Vineland, N. J., catalog No. 8566) was most suitable. A slurry of medium-size asbestos fibers was added to the filter tube to give a 1-mm. asbestos pad which must be sucked down tightly with a vacuum filtration apparatus. d 1-liter filter flask is necessary to accommodate the 25- x 200-mm. test tubes used later in the procedure. A Baird-Atomic Xodel 123 Abacus scaler with an EWH-108 Geiger-Muller tube (iltomic Accessories, Inc., Valley Stream, N. Y.) was used for counting the radioactivity. All samples for counting were plated on 1'/4 X '/8-inch stainless-steel planchets with concentricring bottoms. Class A volumetric pipets were used and the cerate titrations were done with a class -4 microburet, graduated in 0.02-ml. subdivisions. Reagents. A 2-mg./ml. solution of primary standard sodium oxalate from Thorn Smith (Royal Oak, Mich.) was used for addition to the beer samples and for standardization of the cerate solution. Cerate ammonium nitrate and nitro-ferroin indicator were from G . F. Smith Chemical Co. A 0.02N cerate solution in I N HC104 was prepared according to the procedure of Diehl and Smith (6). This was standardized by adding 10 ml. of 2N HC104 to 10 ml. of the NazCz04solution, diluting with 50 ml. of lhr HC1O4, and titrating with the cerate solution as described under Recommended Procedure below. The 0xa1ic-C'~ acid was supplied by the New England Nuclear Corp. and had a specific activity of 1.25 mc. per mmole. Solutions of the acid in distilled water were prepared as needed a t a concentration of 0.36 mg. (5 pc.) per 3 ml. The gelatin used for plating the radioactive calcium oxalate was a bacteriological grade. Other chemicals were of reagent grade. Wash water sat'urated wit,h calcium oxalate (7) was prepared by suspending pure calcium oxalate powder in several liters of distilled water, in a suitable container for siphoning, and allowing the mixture to equilibrate for a t least one day. After the calcium oxalate had settled, the wash water was filtered by suction through a sintered-glass filter tube containing a n asbestos pad as described above. A siphoning arrangement was connected from the water reservoir to the filter tube. The calcium oxalate remaining in the reservoir was resuspended with distilled water for
an additional supply of wash water. A commercial beer known to be low in oxalate (Beer I) was used to invesA cerate titration of a portion of the filtered wash water showed that it was tigate the completeness of calcium saturated with calcium oxalate after 24 oxalate precipitation a t four levels of added oxalate-equivalent to 0.54, 2.0, hours of equilibration with the solid 4.0, and 8.0 mg. of Na2C204 per 120 phase. Procedure. A. RECOMMENDEDml. of beer-after refrigeration periods METHOD FOR DETERMINATIONOF of 2, 3 , 4 , 6, and 9 days-Le., 20 samples in all. OXALATEI N BEER. Ten milliliters (20.0 mg.) of primary standard NazT o each of five samples was added 3 ml. of the oxalic-C14 acid solution and C 2 0 4solution is pipetted into 300 ml. the solutions were thoroughly mixed. of decarbonated beer contained in a 400-ml. beaker, and 5 ml. of 10% (w./v.) This set of samples represented the Ca(OAc)2.Hz0 is slowly added mith lowest level of added oxalate-Le., stirring. The beaker is covered with 0.54 mg. (as Na2C204) per 120 ml. Parafilm and refrigerated for 4 days a t Two milliliters of 10% Ca(OAc)z.HzO 4 ' C. The calcium oxalate is then was then mixed into each of these filtered off and the beaker, funnel, and samples and three 1-ml. aliquots of each precipitate are washed with calciumwere immediately pipetted into planoxalate-saturated wash water. The chet&. No precipitation of calcium beer filtrate is discarded, a 25- x 200oxalate had occurred at this time. mm. test tube is inserted into the filter After 2 days of air-drying, the planchets flask, and the filter tube is replaced with were counted as described below. The its outlet in the mouth of the test tube. observed counts, minus the background The calcium oxalate is then dissolved count, represented the initial radiowith hot 1 N HC104 (wash bottle) by activity of all 20 samples. rinsing the beaker and filter tube and T o each of the remaining 15 samples, slurrying the asbestos in the filter tube. 3 ml. of oxalic-Cl4 acid solution was This procedure is repeated a number of added with stirring and the 15 were divided into three sets of five each. times until about 50 ml. of solution has been collected. The contents of the T o each sample was then added 2 ml. of test tube are transferred to a 150-ml. a sodium oxalate solution of such conbeaker, 3 drops of 0.0025M nitrocentration that the amount of NazCz04 ferroin indicator is added, and the introduced was 1.46 mg. in the first set, mixture is titrated with 0.02N cerate 3.46 mg. in the second, and 7.46 mg. in until the solution remains decolorized the third. Then 2 ml. of 10% Cafor a t least 30 seconds. The asbestos ( O A C ) ~ . H ~was O added with stirring to in the filter tube is reslurried with diseach solution and all 20 beakers were tilled water, sucked down firmly, thorcovered with Parafilm and refrigerated oughly washed, and is then ready for at 4' C. reuse. Four samples, one at each level of added oxalate, were filtered after each Apparent milligrams of Cz04-2per liter of the refrigeration periods indicated (2' - S ) X N X 44 X 1000 above. The first 100 ml. of filtrate of beer = was discarded, and the last portion was ml. beer collected in a 25- X 200-mm. test tube. Three 1-ml. portions of each beer where T = ml. of cerate for beer filtrate were plated on planchets for sample containing 20 mg. counting of residual radioactivity. The of added Na2C2O4 planchets, after air-drying, were counted S = ml. of cerate equivalent to for 10 minutes with 950 volts applied 20 mg. of Na2Cz04 to the G-M tube. The planchet to N = cerate normality Geiger tube distance was about 10 mm., and the counting efficiency of the scaler The amount of oxalate determined by and G-M tube were frequently checked the cerate oxidimetry is referred to as with a C14 reference source. A back"apparent oxalate" since it does not ground count was made with a planchet include the unprecipitated oxalate ions containing the residue from 1 ml. of remaining in the beer filtrate. For beer. No correction was made for most beers the unprecipitated fraction coincidence loss because the counts were represents a very substantial portion of relatively low. The percentage of the the total oxalate. According to the initial radioactivity remaining in each radioisotope studies described below, beer filtrate was then calculated. this fraction amounts to 2.6 mg. of A second commercial beer (Beer 11) CZO4+ per liter of beer when the was treated in the same manner with the precipitation is carried out as specified exception that 24 samples were set up above. Therefore : in order to include a 1-dtiy refrigeration Corrected milligrams of Cz04-2per period. liter of beer = 2.6 apparent oxalate C. CORRECTIONFACTOR FOR UNPRECIPITATED OXALATE. From the B. EFFECT OF ADDED SODIUM d a t a obtained in part 13, the condiTIME. tions selected as optimum for t h e OXALATEA N D REFRIGERATION T o conserve 0xa1ic-C~~acid and to precipitation of calcium oxalate in expedite the handling of a large number beer were first, the addition of 8 mg. of samples in the experimental proceof Na2C204per 120 ml. of heel. (or 20 dures below, the beer sample volume mg. per 300 ml.) and second, 4 days of was reduced from the customary 300 ml. refrigeration. to 120 ml. The quantities of reagents T o determine the value of the correcwere reduced proportionately and 150tion factor, five different commercial ml. beakers were used. beers were analyzed. The precipitated
+
VOL 37, NO. 9, AUGUST 1965
1093
Table 1.
Beer sampleh A- 1 A-2 A-3
B- 1 B-2 c-1 c-2 c-3 D-1 D-2 D-3 D-4 E-1 E-2 E-3 a
Radioactivity of Beer Filtrates and Correction Factor for Unprecipitated Oxalatea
Precipitated oxalatec mg./300 ml. 14.57 14.71 13.83 14.68 _14.75 _ 14.85 15.09 15.03 15.24 1.6 25 I5 66 15 27 16 47 16 76 16.42
Per cent radioactivity in filtrate 5.48 5.32 4.82 5.78 5 36
5.8i
4.37 4.04 5.52 4.89 3 86 4 19 5 24 3 82 4.48
Calcd. total oxalated mg./300 ml. 15.41 15.54 14,53 15.58 15.59 15 77 15 78 15 66 16 13 16.03 16 29 15 94 17 38 17 43 17.19
Correction factor" mg./300 ml. 0.84 0.83 0.70 0.9.0 0.84 0.92 0 69 0 63 0 89 0.78 0 63 0 67 0 91 0 67 0.77 Av. 0.78
Refrigeration time 4 days; added Na2C2O4,20 mg./300 ml. beer.
* Letters designate different commercial beers and numbers indicate separate analyses.
All analyses were in duplicate except for A-3 and D-4 which were single determinations. c Determined by recommended procedure, part A, and including added Na2C201. Values expressed in terms of C204-2. d (Precipitated oxalate x 100) + (100 - % radioactivity in filtrate). Unprecipitated oxalate. 6
-1-A
I
0
I
I
I
I
I
2
4
6
8
IO
REFRIGERATION TIME (DAYS)
Figure 1 , Radioactivity remaining in beer filtrate as function of refrigeration time I and II are commercial beers. Beer I, 0-0; beer 11, 0.. 0.0.36 mg. (5 MC.)of ~ x a l i c - C ' ~ acid added to all beer sampler before precipitation of calcium oxalate. Total oxalate a d d e d to 120 ml. beer, expressed os NozCzOr: I-A and L A , 0.54 mg. 1-6 and 11-6, 2.0 mg. I-C and 11-C, 4.0 mg. I-D and 11-D, 8.0 mg.
II. Apparent
Table
Oxalate Correction Factor
and
Refrigeration time: 2 days Milligrams C20r-* per liter beer Appar- CorrecBeer ent tion Corrected sample oxalate factor oxalatea A-2 3.9 3.4 7.1 ._ 6.8 3.6 3 4 B-2 3.7 8.8 5.6 c-2 2.8 9.3 D-2 6.1 2.8 14.6 E-2 11.4 Av. 3 . 2 Beer sama Apparent oxalate + 3.2. ples designated as in Table I, footnote b. Table 111. Radioactivity in Calcium Oxalate Precipitates and in Beer Filtrates"
Per cent radioactivitv found CaC?Od precipi- Beer filtrate Total tate 5.48 98.9 93.40 5.32 96.9 91.54 91.22 97.0 5.78 B- 1 97.2 91.91 5.36 B-2 92.83 5.69 98.5 c-1 99.1 94.68 4.37 c-2 4.04 95.6 91.60 c-3 92.25 97.8 5.52 D- 1 4.89 97.3 92.47 D-2 94.1 90.26 3.86 D-3 5.24 99.3 94,06 E-1 90.50 3.83 94.3 E-2 92.24 4.20 96.4 E-3 ___ Av . 97.1 a Conditions as in Table I, footnote a. All values are averages of duplicate determinations. Beer samples designated as in Table I, footnote b. Beer sample A- 1 A-2
1094
ANALYTICAL CHEMISTRY
oxalate was first determined by the procedure in part A. Three additional samples of each decarbonated beer were then treated according to the radioisotope procedure of part B to determine the per cent of radioactivity remaining in the filtrates. From the precipitated oxalate values and the per cent radioactivity left in the filtrates, the total oxalate content of the different beers and the correction factor for unprecipitated oxalate were calculated. D. RADIOACTIVITY I X CALCIUM OXALATEPRECIPITATES. -4s a check on the determination of radioactivity remaining in t h e beer filtrates, determinations of the radioactivity in the calcium oxalate precipitates were carried out. The precipitates from the filtrations in part C were washed with calcium-oxalate-saturated water and the washings were discarded. The precipitates were dissolved from their respective beakers and filter tubes with about 40 ml. of hot 0.2N HC104 in the general manner described in part A. The oxalate solutions were made to volume with 0.2N HC1O4 in 50-ml. volumetric flasks, and 15-ml. portions were transferred to 25-ml. volumetric flasks. A solution of 2.57, gelatin in 0.5N NaOH was added until the mixture became just pink to phenolphthalein and the mixture was then slightly acidified with 2 drops of 0.2.47 HC104. The solutions were diluted to 25 ml. with 2.5y0 gelatin in 0.02N NaOH (2). The resulting 1% gelatin solutions of the radioactive oxalate were mixed and immediately 1-ml. portions were plated in triplicate on planchets. At the same time that the beer samples were set up in part C, a reference sample (containing the same amount of radioisotope as in each beer sample) was prepared as follows: 3 ml. of oxalicC14 acid stock solution and 2 ml. (7.46
mg.) of Na2C204solution were pipetted into a 50-ml. volumetric flask. T o precipitate the oxalate, 6 ml. of CaClz solution (0.4 mg. Ca+2 per ml.) was added and the reference mixture was refrigerated. When the above calcium oxalate precipitates were dissolved and plated, 2.75 ml. of I N HC1O4was added to the reference mixture and the solution was made to volume with 0.2N HC104. Two 15-ml. aliquots were transferred to 25-ml. volumetric flasks and 1% gelatin solutions were prepared and plated as described above. The planchets from the beer and reference samples were counted for 5 minutes as previously described and the per cent activity in the calcium oxalate precipitate from each beer was calculated. No correction was made for coincidence loss. RESULTS A N D DISCUSSION
The rate and completeness of calcium oxalate precipitation in a beer sample after the addition of excess calcium acetate will depend on the amount of oxalate ions present and the refrigeration time. Figure 1 shows the results with two commercial beers after adding 0xa1ic-C'~ acid and unlabeled sodium oxalate and measuring the radioactivity of the beer filtrates after varying refrigeration periods. The two beers contained 3.5 and 7.0 mg. per liter of apparent oxalate. The pronounced and continued precipitation of calcium oxalate in samples I-A and 11--4 illustrate the importance of refrigeration time and the necessity for the addition of sodium oxalate. These results further empha-
size the unreliability of the oxalate values obtained by the conventional methods ( f , 5 ) . It should be noted that it is the change in slope of the curve in Figure 1 that is important and not the actual percentage of radioactivity remaining in the filtrate. The percentage of radioactivity will naturally decrease as more unlabeled sodium oxalate is added to the beer samples. The addition of 2 mg. of sodium oxalate per 120 ml. of beer was not very effective (Figure I ) , but the higher levels, particularly 8 mg., caused most of the calcium oxalate to precipitate in 24 hours. Four days of refrigeration is preferred to ensure more reliable results when applying a common correction factor to oxalate values from beers that may vary somewhat in composition. Later results with five different beers showed that the steeper slope of curve 11-D was more typical than that of curve I-D. Table I presents the data for the calculation of the correction factor for oxalate ions remaining in the beer filtiate after the precipitation of calcium oxalate under the conditions specified. 1he residues from 1 ml. of beer filtrates gave an average count of 75 c.p.m. compared to an average of 1550 c.p.m. for the initial count of the treated beer samples. The assumption is made that the percentage of radioactivity remaining in the filtrate equals the percentage of the total oxalate not precipitated. The average correction factor was 0.78. mg. of C204-2 per 300 ml. of beer (Table I), or 2.6 mg. per liter. The standard deviation was 0.11 mg. of CZO4-*per 300 ml., or 0.35 mg. per liter. A comparison of the correction factors for beers A and E (Table I) with the oxalate content of these two beers (Table IV) indicates that a common correction factor can be used regardless of the oxalate content of the beer. Duplicate samples of the five beers were also analyzed exactly as described in part C except that the refrigeration time was reduced to 2 days. The results are summarized in Table 11. A somewhat higher correction factor was found. Table I11 shows the percentages of the added radioactivity which was found in the calcium oxalate pre-
Table IV.
Beer sample A- 1
Milliliters standard cerate Standardization Total (20 mg. Net titration Na&204) titration
Milligrams CzO4-2 per liter Apparent Corrected*
1.66 4.7 15.39 15.39 1.73 4.9 7.4 15.84 1.89 5.2 A-2 15.84 1.91 5.3 7.9 B-1 15.39 1.79 5.1 15.39 1.83 5.2 7.8 1 96 5 6 15 43 B-2 15 43 1 81 5 2 8 0 1 95 5.5 1 15 39 15.39 2.07 5.9 8.3 c-2 15.43 2.28 6.5 6.5 9.1 2.29 15,43 D- 1 15 39 2.43 6.9 7.1 9.6 15.39 2.51 7.1 2.56 D-2 15.84 15.84 2.54 7.0 9.7 11.2 3.95 15.39 E-1 13 7 3 87 11 0 15 39 E-2 15 43 4 21 12 0 12 2 14 7 4 27 15 43 Conditions as in Table I, footnote a. Beer samples designated as in Table I, footnote
c-
b.
Apparent and Corrected Oxalate Values of Five Beers.
17.05 17.12 17.73 17.75 17.18 17.22 17 39 17 24 17 34 17.46 17.71 17.72 17.82 17.90 18.40 18.38 19.34 19 26 19 64 19 70
Apparent mg. C2Oa-2/liter plus 2.6.
cipitates. The planchets prepared from the reference samples (100% activity) and from the recovered calcium oxalates gave counts in the range of 3000 to 4000 c.p.m. The average total recovery of radioactivity from the precipitates plus the beer filtrates was 97.1%. The determination of the radioactivity from the precipitates is more complicated and subject to more errors than in the filtrates. For this reason the values obtained from the beer filtrate radioactivities were preferred in calculating the correction factors. The addition of 20 mg. of Na&zOd to 300 ml. of beer prior to the precipitation of the calcium oxalate may seem to be an unjustifiably large amount of oxalate ions (13.1 mg.) compared to the 2 to 5 mg. present in the beer itself. Nevertheless, as shown in Table IV, the replicate results of many beer samples are in close agreement. The following factors were important for securing satisfactory results using the method described. The filter tube with asbestos pad provided excellent retention of calcium oxalate and speed of
filtration compared to other filtration methods tried. The calcium-oxalatesaturated wash water assured no loss of calcium oxalate during the washing procedure. The titration with cerate in HC1O4 a t room temperature using nitro-ferroin indicator gave easily recognizable end points. LITERATURE CITED
( 1 ) Bau, A., Wochschr. Brau. 35, 31, 70 (1918); C.A. 12, 2037 (1918). (2) Bomstein, R., Johnson, M. J., J . B i d . Chem. 198, 145 (1952). ( 3 ) Brenner, M. W., Am. SOC.Brewing Chemists PTOC.1957, 5. (4) Brenner, M. W., European Brewing Convention, Proc. 1957 Congress, p. 349, Elsevier, New York, 1958. (5) Burger, M., Becker, K., Am. SOC. Brewing Chemists Proc. 1949, 102. (6) Diehl, H., Smith, G. F., “Quantitative Analysis,” p. 275, Wiley, New York, 1952. ( 7 ) Peech, M., ANAL. CHEM. 13, 436 (1941).
RECEIVED for review February 10, 1965. Accepted June 14, 1965. Su ported in part by National Science Poundation Grant (2-22249.
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