Microdetermination of Chromium with Dipheny lcarbazide by Permanganate Oxidation .
Improved Method of Oxidation and Color Development BERNARD E. SALTZMAN Division of Occupational Health, Public Health Service, Federal Security Agency, Cincinnati 2, Ohio Existing methods for the colorimetric determination of microgram quantities of chromium with diphenylcarbazide gave poor recovery, unstable colors, and unreliable results. Investigation revealed that the methods of oxidizing the chromium to the required hexavalent state and of destroying the excess oxidizing reagent were of critical importance. Oxidation in alkaline media was subject to losses because of precipitation, while oxidation in acid media with energetic agents such as persulfate or bismuthate could generate traces of hydrogen peroxide which
I
N A recent industrial hl giene survey of the chromate industry,
an analytical method was required for the microdetermination of chromium in large numbers of samples. Diphenylcarbazide is generally regarded as one of the best reagents for the colorimetric determination of minute amounts of chromium and has been used for the analysis of rocks, minerals, iron and steel, water, soil, air, leather, and biological materials. A comprehensive review of the use of diphenylcarbazide as a chromium reagent has been made by Welcher (12), and an excellent discussion by Sandell (8) is available. Under the proper conditions. hexavalent chromium gives an intense red-violet color with the reagent. Many metals such as mercury, copper, cadmium, silver, lead, nickel, cobalt, manganese, magnesium, zinc, and iron react with the reagent, giving colors ranging from led through violet, blue, and blue-black, but under the conditions of low pH required for the determination of chromium, the sensitivity of the color reaction with these metals is so low that the method may be regarded as practically specific for chromium. -4 vital step in the method is the oxidation of the chromium to the hexavalent form. Generally, samples require ashing to destroy reducing materials, after which a powerful oxidizing agent must be used to convert the chromium to the hexavalent form and the excess oxidant must be destroyed without reducing the chromium. This step critically affects the recovery and final color stability. In acid solution, persulfate has been general11 recommended as the oxidizing agent, the excess being destroyed by boiling (8). I n alkaline solution, hydrogen peroxide (11, 1 2 ) may be used and the excess boiled off, or bromine may be used, followed by acidification and removal of excess bromine n ith phenol (10). In applying the method to urine samples, oxidation in alkaline solution was found to be impracticable because the large precipitate of calcium and magnesium phosphate resulted in lois of chromium. The persulfate method gave unstable colors and 4 a i unreliable when the amount of chromium to be determined PT as of the order of 1 microgram, owing to the trouble in boiling off the last trace of excess reagent or the hydrogen peroxide formed from it. Recognizing these difficulties, Urone and Anders, in one of the few papers to be found in the literature on the microdeterminatiorl of chromium in urine ( I O ) , proposed sodium bismuthate ds the oxidizing agent because the excess could later be removed 1)) filtration. Although this reagent \?-as more satisfactory than persulfate, the color faded so rapidly as to require reading within 5 minutes, and turbidities from bismuth hydrolysis appeared in many samples.
would later reduce chromium recoveries. A new method was developed for oxidizing chromium in acid medium with permanganate; the excess oxidant was destroyed with sodium azide. The color with diphenylcarbazide was stabilized by the addition,of a phosphate buffer. The procedure is conveniently applied to air, water, and urine samples, yielding excellent recovery and color stability, with a sensitivity of 0.03 microgram of chromium in a volume of 25 ml. The method should be applicable in many fields.
An investigation was undertaken, therefore, to find a method giving reliable recovery and stable color. It was felt that a milder oxidizing reagent would avoid formation of hydrogen peroxide, which would lead to the reduction of chromate in later steps. Permanganate has been used in relatively few instances for the microdetermination of chromium-in feces (6) and in steel (1). The excess permanganate may be destroyed without reducing chromate by careful heating with hydrochloric acid ( 1 , 5 ) or by precipitating as manganese dioxide in alkaline solution (6). Both methods are time-consuming and require filtration. Sodium azide has been recommended for qualitative use by Feigl (3) to destroy permanganate without reducing chromate, and it was applied to the present problem. Sodium azide was found to remove excess permanganate completely and rapidly without necessitating filtration. The successful quantitative application of the permanganate azide combination to the determination of microgram quantities of chromium by diphenylcarbazide forms the subject of this r e port. Reliable results with the development of stable colors were obtained when the method was applied to urine, air, and water samples. REAGENTS
Sitric acid, concentrated reagent grade acid redistilled in an -~ all-glass still. Potassium permanganate, 0.1 N , 3.16 grams made up to 1 liter with double-distilled water. Decant after a few d a w if neceesary. Sodium azide, 5%, 5 grams, made up t o 100 ml. with doubledistilled water. Sulfuric acid, 0.5 N , 13.9 ml. of concentrated sulfuric acid diluted to 1 liter with doubledistilled water. Sodium bisulfate, 40%, 20 gram? made up to 50 ml. s-Diphenylcarbizzide. Dissolve 10.0 grams of phthalic anhydride in 175 ml. of redistilled 95y0 ethyl alcohol, warming t o rffect solution. Cool, add a solution of 0.625 gram of s-diphenylcarbazide in about 50 ml. of redistilled 95% ethyl alcohol, and make to 250 ml. with the ethyl alcohol. Keep in a brown bottle in the refrigerator, This solution is usable for several months if standardized occasionally. Sodium dihydrogen phosphate, 4 M, 138.01 grams of sodium dihydrogen phosphate monohydrate, made up to 250 ml. nith double-distilled water. Standard Potassium Dichromate. Dissolve 0.2263 gram of pure potassium dichromate in 1 liter of doubledistilled water. This stock solution contains 80 miorograms of chromium per ml. Prepare a working standard solution (2 mivrograms per ml.) by diluting 5 ml. to 200 ml. with double-distilled water just before use. Standard Trivalent Chromium. Pipet 5 ml. of stock potas-
1016
V O L U M E 2 4 , NO. 6, J U N E 1 9 5 2
1017
sium dichromate solution (80 micrograms per ml.) into a Phillips beaker. Reduce by adding about 15 mg. of sodium sulfite and 0.5 ml. of nitric acid. Evaporate t o dryness gently, as strong heating may reoxidize the chromium. Add 0.5 ml. of nitric acid and again evaporate t o dryness gently to destroy any excess sulfite. Dissolve in 1 ml. of nitric acid with warming and make to 200 ml. with distilled water. Test a portion for the absence of hexavalent chromium by adding some diphenylcarbazide. This solution contains 2 micrograms per ml. and is stable for a fe\T dRys. I
23
I
a W
ing 5% sodium azide a t the rate of 1 drop every 10 seconds (3 to 5 drops required), swirling after each drop. Use sufficient azide t o destroy any brownish tint, but avoid excess. Remove promptly from the steam bath and place in a tray of cold water. Filter, if necessary, through a Selas crucible or transfer to a 25-ml. volumetric flask using double-distilled water to avoid possible reduction of chromate. Add 1 ml. of diphenylcarbazide. Mix and allow 1 minute for color development. Then add 2.5 ml. of 4 M sodium dihydrogen phosphate, mix, and make t o mark with double-distilled water. Read the pink color a t 540 mk, using a 0.02-mm. slit, within 30 minutes. A tube containing distilled water may be used as the reference. Run a blank with each batch of samples. Colors too intense t o read may bediluted with the reagents. Standardize the procedure by preparing a series of beakers containing trivalent chromium standard solution in amounts varying from 0 to 16 micrograms of chromium. Carry through the entire procedure (including ashing), and use the zero standard R S the reference for reading the colors. CHROMlUM RECOVERY AND COLOR STABILITY
-% series of colors was prepared by adding graduated amounts of
standard dichromate solution t o 25-ml. volumetric flasks, adding 10 ml. of 0.5 ,V sulfuric acid and 1 ml. of diphenylcarbazide reagent, mixing allowing l minute for color development, then adding 2.5 ml. of 4 M sodium dihydrogen phosphate, making t o mark, and mixing. This series was read in the spectrophotometer at varying times after mixing up t o 1.5 hours (Table I). FIGURE
I
-
DENSITY CONCENTRATION PLOT
X
o
SERIES I
DICHROYATE
STDI.
”
2
Or THRU
‘
3
URINE STDS. THRU PROCEOURE
PROCEDURE
Table I.
Pure Dichromate-Diphenylcarbazide
(Optical density a t 540 ma, 0.02-m1;1. slit, a t various times after mixing) Minutes after Mixing Cr, Y 4 10 21 30 60 92 0 0.003 0.002 0.002 0.002 0.002 0,002 2.60 0.167 0.168 0,165 0.165 0.165 0,165 5.20 0.326 0,326 0.325 0,325 0.324 0.321 10.40 0,649 0,649 0.646 0.647 0.641 0.639 15.60 0.965 0.965 0.966 0.965 0.960 0.958
u
Distilled Water. Ordinary distilled water did not contain any appreciable amount of chromium. Double-Distilled water, redistilled in an all-glass still. Doubledistilled water was used for all steps following the oxidation of chromium t o the hexavalent form, t o avoid possible reduction which might occur from the impurities in ordinary distilled water. Wash Acid. Add 50 ml. of concentrated nitric acid t o 150 ml. of concentrated hydrochloric acid, mix, and add 200 ml. of distilled water. Wash acid was used over and over again for rinsing glassware. After a wash acid rinse, the glassware was filled completely with t a p water to flush out vapors, followed by t a p water and distilled water rinsings. APPARATUS
Hot Plate. -4 Lindberg hot plate with a variable heat control was used for ashing. The plate was capable of reaching 400’ C. at full heat and was covered with a thin sheet of asbestos paper. Filter Crucibles. Selas crucibles of 20-ml. size. Clean after use with 1 to 1 nitric acid follon ed by water. Manganese dioxide may be readily dissolved with hydroxylamine hydrochloride solution followed by thorough washing. Spectrophotometer, Beckman Model DU. A set of matched test tubes, 22 X 175 mm., was used. The cuvette holder in the instrument was replaced by a wooden block specially made to fit the tubes. Glassware. Borosilicate glass Phillips beakers of 250-ni1. capacity were used for ashing. To avoid contamination, the standard chromic acid cleaning solution was not used. When necessary, alcoholic potassium hydroxide was used to remove grease. All glassaare was given final rinsings with wash acid followed by tap and distilled water. DETERMINATlON OF CHROMIUM
To the ashed sample add 10 ml. of 0.5 -V sulfuric acid. Swirl Add 0.5 ml. of 0.1 N potassium permanganate, cover with a watch glass, and heat on steam bath 20 minutes. If the pink color disappears, add more permanganate as required to maintain a slight excess. Decolorize the permanganate by addto dissolve ash.
FIGURE 2
RECOVERY
a coux
STABIUTY
w
8
SODIUM MSYUTWITL METHOD 70
[URINE
aios.)
Excellent color stability was observed. The initial figures were taken as the theoretical colors to be obtained for a given amount of chromium. The molecular extinction coefficient based on the molarity of the dichromate was found to be 8.0 X lo4 (freshly prepared diphenylcarbazide reagent gave slightly higher results). A second series was prepared with trivalent chromium standards and taken through the entire procedure. A recovery of 99% of the theoretical color was obtained (Table 11) with good color stability. Beer’s law is followed, as will be seen from Figure 1 showing the optical densities 10 minutes after mixing (corrected for the blank),
Table 11. Procedure Applied to Trivalent Chromiuni Standards (Optical density a t 540 nip, 0.02-nim. slit, a t various times after n i n m g ) Minutes after Mixing Cr, Y 8 16 30 99
1018
ANALYTICAL CHEMISTRY
plotted against the chromium concentrations. The sum of the corrected densities a t each time for each series was divided by the theoretical density for the total amount of chromium added to the series to give the average per cent of the theoretical color, which is plotted in Figure 2. Series 1shows practically no loss of color for the first 30 minutes; the line for series 2 is parallel to that of series 1 and about 1% lower.
Table 111. Procedure Applied to Dichromate-Urine Standards (Optical density a t 540 mp, 0.02-mm. slit, a t various times after mixing) Cr,y Minutes after Mixing 0 2.60
0.009 0.168 0.327 0.641 0.960
5,20 10 40 15.60
0.009 0,167 0.326 0.640 0.960
0.009 0.165 0.324 0.633 0.955
0.009 0.164 0.320 0.628 0.938
0.009 0.161 0,315 0.618 0,920
0.009 0.160 0.311 0.608 0.905
APPLlCATlON OF METHOD TO URINE SAMPLES
Limitations of muffle-furnace space and platinum ware seriously restrict the use of dry-ashing procedures. Wet-ashing procedures with sulfuric, nitric, and perchloric acid mixtures or with sulfuric acid and hydrogen peroxide (8, 12) were considered; however. the use of sulfuric acid introduced the problem of removing the excess and of heating the ash to a . sufficiently high temperature, whereas perchloric acid which was required for effective oxidation caused severe losses of chromium, probably because of the formation of chromyl chloride CrO2C12 (boiling point 118' (3.). The combination of wet-ashing with nitric acid followed by dry-ashing at 500" C. in the same 100-ml. borosilicate beaker as recommended by Urone and Anders (10) was satisfactory. It wae found that the use of a furnace could be eliminated by operating the hot plate a t full heat (400" (3.). The use of Phillips beakers reduced the danger of loss by spattering and permitted the nitric acid to reflux and wash down the sides, giving complete oxidation. Chromium was oxidized to chromium acid, which, however, decomposed on stronger heating to the trivalent form. Hydrolysis of Polyphosphate in Urine Ash. In developing the method for urine samples, several factors were investigated. The presence of phosphate in urine resulted in erratic recovery of chromium. I t was found that this was due to polyphosphate formed from phosphate at the end of the ashing process by the strong heating. Polyphosphate compleses chromium, making it unreactive to permanganate. Polyphosphate may be readily hydrolyzed by heating in nitric acid solution to phosphoric acid, which does not interfere with the oxidation of chromium. When this solution was taken to dryness on a hot plate to eliminate the nitric acid, however, sufficient polyphosphate was regenerated to cause loss of chromium even if the teniperature was kept low and the beaker removed promptly when it became dry. The literaNa2HzP201 ture indicated ( 7 ) for the reaction, 2XaH,POa HzO, dissociation pressures as follows:
*
t,
c.
P H , o , mm.
110 17.9
120 36.1
130 66.1
+
150 750.0
I t thus appeared essential that the escess nitric acid be evaporated on the steam bath, as even a few degrees of heat above 100' C. had a marked effect of regenerating undesired polyphosphate. Chromiumwas completely recovered in this manner if the beaker was removed promptly as it nent dry. (Leaving the beaker on the steam bath 2.5 hours a f t n it went to dryness resulted in 90% recovery of the chromiuni, showing some pyrolysis of the phosphate even a t steam-bath temperature.) Ashing Procedure for Urine. The follovririg procedure was developed for the ashing of urine and thc hydrolysis of polyphosphate in the ash. Measure 50 ml. of the fresh urine sample into a 250-ml. borosilicate Phillips beaker. If the urine contains a precipitate, empty the sample bottle, dissolve the precipitate adhering t o the glass,
and rinse out with two small portions of nitric acid, followed by one of water. Mix the urine and rinsings cautiously and take a volume of the mixture equivalent to 50 ml. of urine. Evaporate to dryness overnight on the steam bath. Add 1 nil of nitric acid and heat on the hot plate a t a moderate heat for a few moments, then swirl the beaker to dissolve and mix the residue. Continue heating until the reaction ends. Repeat with small portions of nitric acid,,gradually decreasing the portions to 0.5 ml. and gradually increasing the heat of the hot plate, charring as much as possible and cooling the beaker before each addition. Overheating causes violent flashing and should be avoided to prevent loss of sample. After about 7 ml. of nitric acid have been added, the hot plate should be a t full heat (400" C.). Following the production of a white ash, add a few more portions of nitric acid to ensure complete ashing. About 10 ml. of nitric acid suffices for the whole process. To the ashed sample add 2 ml. of nitric acid and reflux for 1 to 2 minutes on the sides of the beaker on a hot plate at moderate heat without boiling the acid off. Add 10 ml. of water and swirl to dissolve ash. Evaporate on a steam bath (not a hot plate!) until the bottom and sides are dry, and remove from heat promptly. The eva oration of the last few drops at the top of the beaker may be spee&d by gentle blowing of air. Repeat this step, if the ash did not completely dissolve in the acid, Recovery of Chromium from Urine Standards. A series of samples was prepared by adding standard dichromate solution to 50-ml. portions of urine and carrying them through the procedure. A recovery of 98% of the theoretical color was obtained (Table 111). Figure 1 shows the results obtained by reading the colors of this series 10 minutes after mixing. Beer's law is followed. Figure 2 shows the stability of the colors. For comparison the results obtained by the sodium bismuthate method (10) are shown; the recovery and stability of the permanganate method are much superior. The persulfate method of oxidation was found to be unreliable, as the colors faded very quickly.
Table IV. Standard Trivalent C h r o m i u m Solution Added to Air S a m p l e Cr Added, 0 2.60 5.20 9.10 13.00
y
Cr Found,
y
1.34 4.11 6.50 10.41 14.04
Net Cr y 0 00 2 77 5,16 9.07 12.70
OTHER APPLICATIONS OF METHOD
The method should be applicable to many other types of samples containing small amounts of chromium, provided that they are adequately ashed and brought into solution and are free from excessive amounts of interfering substances. Tests of two other applications of the method are given below. To test chromium recovery, standard trivalent chromium solution was added t o equal aliquots of a nitric acid solution of an air sample collected on a paper filter in a chromate plant. Each aliquot, representing several cubic fe'et of air, was ashed in a 250-ml. borosilicate Phillips beaker with 0.5-nil. portions of nitric acid 011 the hot plate unt,il no further change occurred. I t x a s necessary to add a small amount of sodium bisulfate (0.25 ml. of 40% sodium bisulfste) wit,hthe first portion of nitric acid to avoid low results, possibly by preventing the formation of insoluble oxides o i chromium during the strong heating of the process. Table Is' shom the degree of the recovery obtained. A sample representing 10 cubic feet of chromium-contaminated air is adequate for the currently accepted maximal allowable concentration in air for healt~hfulconditions (0.1 nig. of CrOI per cubic meter). Other work by the author indicates that if chromite ore dust is present in an air sample, i t will not be dissolved by the ordinary treatment, but it may be attacked by fusion at 900' C. for 30 minutes with 4 to 1 magnesium oxide-sodium carbonate mixture in a platinum crucible ( I I ) , and then leached and filtered from the residue. Anot.her series vxs prepared by adding standard trivalent chromium to 25-ml. portions of Cincinnati tap water (treated Ohio
1019
V O L U M E 2 4 , N O . 6, J U N E 1 9 5 2 Table 1’.
Standard Trivalent Chromium Solution Added to 25 311. of Tap Water Cr Found, 0.00 2.62 7.87
Cr Added, 7 0 2.60 7.80 13.00
y
12.98
River water). Each portion was ashed as given above for the air samples. Table V shows the recovery obtained. A 25-1111. xater sample suffices for froni 0 to 0.6 p.p.m. of chromium. Blood was successfully ashed by the method given above for urine, but because of the high concentration of iron, the permanganate oxidation procedure was not used. The work of Urone find Anders (10)indicates that iron may be precipitated with sodium hydroxide and filtered off after the oxidation of the chromium to chromate without loss of the latter; however, the comhination of the above-mentioned ashing procedure with the alkaline-bromate oxidation method recommended by these author3 was found to be satisfactory and removed the iron without estra steps. Agnew’s work ( 1 )indicates that sodium carbonate may be used to remove the iron from a sample of steel after the permanganate oxidation. As a general rule, it is best to ash in such 3. manner as to volatilize chloride as early as possible and to finish with an ash containing minimum amounts of acid. Perchloric acid may not be used. The preliminary ashing and solution of other types of samples are, of course, too involved to be covered in this presentation; existing methods for preparing each type of material may be adapted. DISCUSSION
Acidity during Oxidation of Chromium. The optimal acidity of the permanganate oxidation was investigated. For the reaction
3 Mn04-
+ 5 C r + + ++ 8 H20-.,3 M n + + + 5 G O 4 - - + 16 H +
the Ian of mass action would indicate that the lower the acidity the more complete the reaction; a t lower acidity, however, there IF: greater likelihood of precipitating undesirable manganese dioxide. On the other hand, high acidity is detrimental to the stability of the permanganate and also of the color formed with diphenylcarbazide in the subsequent step. An acidity of 0.5 A’ sulfuric acid was a satisfactory compromise, giving complete oxidation on the steam bath in 20 minutes. Reduction of Excess Permanganate by Sodium Azide. .hother factor investigated was the effect of the sodium azide upon the analysis. rl few drops of 5% sodium azide solution rapidly decolorized the hot permanganate solution after the oxidation had I)een completed on the steam bath. In order to determine if anv chromate was reduced in the process, a 2-ml. escess of sodium azide was added and the solution was heated on the steam bath an additional 15 minutes; about 90% of the chromium was recovered. Complete recovery was obtained, however, if no more than 1 or 2 drops excess was used, followed by heating for 20 minutes on the steam bath. It was thus concluded that no risk of 1098 of chromate would be incurred in decolorizing the permanganate by adding 5% sodium azide dropwise to the hot solution, followed by cooling without delay. I n the hot acid solution the slight excess hydrazoic acid would be readilv volatilized. Sodium azide also dissolves small amounts of manganese dioxide, resulting from decomposition of the permanganate in incompletely ashed samples; larger amounts should be filtered off. Turbidity. ,4fter the oxidation step a few urine samples required filtration, although most displayed only a faint turbidity, and air and water samples generally yielded clear solutions. Other acids were substituted for the sulfuric acid in the oxidation step to avoid the formation of calcium sulfate, but no great improvement in clarity was obtained; perchloric acid resulted in a larger precipitate of potassium perchlorate, and nitric acid tended to
cause the permanganate to decompose and also reduced the final color stability. Stability of Chromate Solution. Long standing before adding diphenylcarbazide affects the results. Thus, standing 1 hour lowered the results of uiine ctandards about 2 9 $ , 3 hours, about -5%. Losses with air anti water standards Rere less. The color should be developed piomptly to avoid error. Diphenylcarbazide Reagent. The diphenylcarbazide reagent used was that of Ege and Silverman ( 2 ) . It was stable for several months if kept in a brown bottle in the refrigerator. Phosphate Buffer. Nitrates were found to be detrimental t o acid solution. 1-rine ash the stability of the final color in 0 2 had an appreciable nitrate content. Attempts were made t o eliminate the nitrate bv using hydrochloric or sulfuric acid for the hydrolysis of the polyphosphate, but the former caused decomposition of the permanganate, and the latter did not give complete hydrolysis with the amount of acid that could be used. It was found that the detrimental effect of the nitrate could be greatly reduced by the addition of buffer to raise the pH to 2 Best results were obtained bv adding the diphenylcarbazide first, allon ing rapid color development (1 minute is ample time), and then stabilizing the color with the buffer. Addition of the buffer before the diphenylcarbaxide retards the rate of color development (12 minutes are required for completion) and gives slightly less color. Standardization. Standard dichromate n-as used for the urine procedure as the most convenient reagent available, since tests showed the results obtained were identical n-ith those using standard trivalent chromium. During the urine ashing process the hexavalent chromium is reduced t o the trivalent form by the organic matter and the prolonged intensive heating. During the ashing of air and water samples, on the other hand, standard dichromate may not he completely reduced, and hence trivalent chromium standard solution is preferred because it gives a check on the efficiency of the oxidation step. IYTERFEREYCES
Iron and vanadium react with diphenylcarbazide to produce yellow-brown colors, but a t a much lower sensitivity than chromium. Mercury produces a violet precipitate very slowly. Addition of a small amount of sodium chloride before the diphenylcarbaxide forms molecular mercuric chloride and readily prevents this interference. Reasonable amounts of molybdenum, copper, and cadmium may be tolerated. Table VI shows the results obtained by adding interfering metals to 50-ml. urine standards, and Table VII, for trivalent chromium standards, both carried through the complete procedure. A4fterthe oxidation of chr+ mium to the hexavalent form, extraction with 8-quinolinol in chloroform a t pH 4 will remove larger amounts of iron, molybdenum, and copper ( 4 ) as well as vanadium (8, 9); precipitation with sodium carbonate will separatr iron in a steel analysis (1). Table VI. Interference 2 me;. Fe 2 mp. Fe 0 O5mg. V 0 . 0 5 mg. \’ None
Interfering hIetals Added to 50-MI.Urine Standards Cr Added, y 0 13 0
n
i.8 0
Cr Found, 1.1
7
13.7 0.1 8.0 0.0
In the absence of chromium, interfering metals give slight colors which lead to positive errors; in the presence of appreciable chromium low results are obtained. This phenomenon may be due to the reaction of the large amount of interfering metal with the diphenylcarbazide to prevent its combination with chromium. The addition of the phosphate buffer before addition of the diphenylcarbazide reduces the interference of iron. Fifteen minutes should be allo~-edfor complete color development in this case.
ANALYTICAL CHEMISTRY
1020
Table VII. Interference 5 mg.Fe 0 . 2 5 mg. V
0.5mg.V
Interfering Metals Added to Trivalent Chromium Standards Cr Added, 0.0 10.4
y
0.5 10.0
0.0
0.9 9.3
1 mg. Mo
10.4
9.9
5 mg. Mo
0.8
10.4
0.1 8.8
5 mg. Cu
0.0 10.4
0.1 9.2
5 mg. Cd
0.0 10.4
0.0 10.3
5 mg. H g
10.4 0.0 10.4 20 mg. of NaCl added before diphenylcarbaeide. a
y
ACKNOWLEDGMENT
9.9
0.0 10.4 10.4
a
Cr Found, 1.0
oxidation in alkaline media. The method is proposed for general use in the determination of microgram quantities of chromium by proper adaptation of existing procedures.
19.4 0.0 10.4
CONCLUSIONS
.4 new method for the oxidation of chromium, using permanganate and sodium azide, and the development of a color with diphenylcarbazide using a phosphate buffer to improve stability, gave higher recovery and more stable colon than the more energetic oxidizing agents previously used, such as penulfate or bismuthate and avoided losses due to precipitation which occur 17-ith
The author wishes to thank N.A. Talvitie for his many valuable suggestions during the course of the investigation and D. H. Byers and H. E. Stokinger for their review and helpful criticism of the work. LITERATURE CITED
Agnew, W. J., Analyst, 56, 24-8 (1931). Ege, J. F.,and Silverman, L., IND.ENG.CHEM.,ANAL. ED.,19, 693-4 (1947).
Feigl, F.,“Qualitative Analysis by S p o t Tests,” 3rd ed.,pp. 2623,New York, Elsevier Publishing Co., 1946. Gentry, C. H. R., and Sherrington, L. G., Analyst, 75, 17-21 (1950).
Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorganic Analysis,” pp. 411-14,New York, John Wiley &Sons, 1929. Maunsell, P. W., New Zealand J. Sci. Techml., 26,9443 (1945). Perry, J. H., “Chemical Engineers’ Handbook,” 3rd ed., p. 174, New York, McGraw-Hill Book Co., 1950. Sandell, E. B., “Colorimetric Determination of Traces of Metals,’’pp. 189-99,New York, Interscience Publishers, 1944. ENG.CHEM.,ANAL.ED.,8,336 (1936). Sandell, E. B., IND. Urone, P. F., and Bnders, H. K., ANAL. CHEM.,22, 1317-21 (1950).
Urone, P. F.,Druschel, M.L., and Anders, H. K., Ibid.,22,472-6 (1950).
Welcher, F. J., “Organic Analytical Reagents.” Vol. 111. 433-6,Ken, York, D. Van Nostrand Co., 1947.
pp.
RECEIVEDfor review January 7, 19.52. Accepted l p r i l 14.1952.
Determination of Thiamine in Rice and Rice Products Rapid and Simple Method CARL M . LYMAN, ROBERT ORY, MARY TRANT, AND GENE RICH Texas Agricultural Experiment Station, Texas Agricultural and Mechanical College System, College Station, Tex.
ICE constitutes a major part of the diet of a large percentage
’
of the world’s people, and beri beri is still a problem among many oriental peoples. Most of the thiamine in rice is lost during the milling process because it is concentrated in the brown outer coating, which is removed during the polishing, and unpolished rice is subject to rapid deterioration during storage. It thus becomes a practical necessity to polish rice in o r d s to prevent loss. It would appear possible to polish rice to the point where the product could be safely stored and still leave enough thiamine to supply the dietary needs for this vitamin, if thiamine could be determined without use of expensive scientific equipment. The present procedure was developed with the hope that it might meet this need. In contrast t o many other foodstuffs, most of the thiamine in rice is in the free form-approximately 85%, as indicated by experiments of the present investigators. This makes it possible to omit the enzymatic digestion of the sample, whereby thiamine is freed from cocarboxylase. The method reported here is based on the color developed with diazotized paminoacetophenone. The use of this reaction for the determination of thiamine was first described by Prebluda and McCollum ( 2 ) . The first part of the problem consisted in the establishment of conditions for the rapid preparation of extracts which may be used for the direct development of the color without
preliminary adsorption and elution procedures. The removal of impurities by the precipitation of barium sulfate in the extract proved important in this step. Before the establishment of the structure of thiamine, Williams and coworkers ( 3 ) found the use of barium hydroxide an effective means of eliminating certain impurities during the isolation of the crystalline vitamin from rice polish. Tests next established the conditions for the quantitative development of the color using reagents prepared and used a t room temperature. Because the amounts of the several reagents have been adjusted for use on rice extracts prepared under specific conditions, the procedure reported here may not be widely adaptable to other types of foodstuffs without further modification. The direct application of the color reagents t o rice extracts resulted in the development of a small amount of colored material extractable with xylene, which was not due to thiamine. This difficulty was eliminated by the use of mixtures of iso-octane and xylene in the place of xylene alone for the extraction of the colored compound. REAGENTS AND MATERIALS
p-Aminoacetophenone Solution. Dissolve 2.5 grams in 9 ml. of concentrated hydrochloric acid, and dilute to 100 ml. Sodium nitrite solution, 23 grams in 100 ml. of water.
