Spectrophotometric Study of Dichromate-Chromic Salt Mixtures Applicution t o the Determination of Glycerol in Vinegar D. T. ENGLIS AND LOUIS A. TFOLLERRIAN University of Tllinois, Crbana, Ill.
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colors of chromium ronipounds in the various oxidation states are characteristic and lend themselves t o the estimation of thrs clement in the several statcs by spectrophotometric methods. Iiasline and 1IelIori ( 2 ) have prepared and published absorption curves for rhroniatc :iiid dirhroniate solutions in the visual range. Sandell ( 4 ) give:; an absorption curve for chromate through the ultraviolet r n i i y i ~IThicki W I . ~ established by Rossler (3)wing a ppectrogrophic. tec,liiiique. T h absorption maximum n - : :it ~ 370 nip. Perhaps. hccnuse iliis ip belo~vthe range of many early spectrophotometer?, certain appliratioiis which it offers have not been realized. 8ar:dell ,stresses the fact that the optical density values yielded by mail. filter iiistruinents inay not he proportional to concentr:itioii ciiire tlie filter is not usually sufficiently selectiw and r n e a ~ : i i ~ ~ are ~ ~ ~frequently mts made a t the etlgr of an adsorption ljantl. lIon-L~vc~r, h a u s e of the fact that the sensitivity is so niuc~hgreater in the shorter wave length region aiid gootl iiistrumriir* for Ivorbing in this range are now geiierally availablr, :i~ t i i t l yof posible applications of Ivork in thc ultraviolet region n-as undertaken, The determination of small amounts of glycerol and certain other products of microbiological action has bceii frcclrieiitly arcoinplished by oxidation with an excess of dichronilitr follov;ed h ~ a- volumetric titration of the execs oxidant with .srJmi suit:ihle reducing agent. It n-a3 of intrrwt t o 1e:irn if thir niight be (lone morc conveniently by a colorimetric procedure. .\iter the glycerol oxitlatioii. Imth hexavalent and trivalent chromium are presciit in tlie fiii:iI s o l u t i ~ n ,which is commonly strongl>- acidic. The b:izicity iicccos for conversion of the dichroniate ion to chromate would cnause tlir precipitation of the chromic ion and require removal of the insoluble chromic hydroxide or basic salt. Such a precipitatioii would require addi-
tional time and possible loss of some chromate in the gelatinous precipitate. Hence, the dichromate is the preferred form for estimation of the excess oxidant. Accordingly it was necessary to know not only the absorption characteristics of the dichromate in the ultraviolet region, but also t h a t of the chromic salt to determine whether the latter would interfere with the estimation. Another factor t o be established was the effect of the presence of sulfuric acid in the reaction mixture.
Equipment. A Cary I f ode1 11 recording spectrophotometer was employed where complete absorption curve3 were to be established. When evaluations a t selected single wave length positions were adequate, a Beckman Model DU spectrophotometer was used. For limited work in the visual range, some tests v,we made with a Lumetron filter-type photoelectric colorimrter, Uodel 400A. ABSORPTION CHARACTERISTICS OF CHROMIUM COMPOUSDS
Aqueous solutions of chromate, dichromate, and chromic conipouuds were prepared eo t h a t each xould contain t h e same concentratiori of chromium (32.06 p.p.ni.). I n a 1-em. cell this solution gives an optical density of about 1 unit for the dirhroiiiate form. T o simulate conditions which would be characteristic of the solutions after the glycerol oxidation, the acidity \vas adjusted so as to be 0.09 in sulfuric acid for the dichromate and chroinic compounds. The solutions were examined in the range 200 to 800 nip using the Cary instrument. When the acid n-as added t o the chromium solutions, it was added in identical amounts t o the water which served as the reference solution. The curves, Figure 1, show that t h e dichromate and chromate lire similar in character in t h a t each shows two maxima. These lippc:tr at 257.5 and 350 mp for the dichromate and at 2 i 4 and 3iOp for the chromate. In the case of the dichromate theae are of slightlv lower optical density per unit neight of chromium than the correqponding maxima for the chromate. The chromic sulfate ~olutionshows no significant absorption at this concentration (32.95 p p.m as chromium) betneen 250 and 800 mp. Hence, under these conditions it will not intern feie n i t h the estimation of chromate or dichromate. A much more concentrated solution of the chromlc salt, containing the equivalent of 5 272 mg. per milliliter of 20s 250 3w 350 400 450 5w 550 chromium, shon s (Figure 2) U A ~ E LENCT, (mp) peaks a t 416 and 587 mp. Figure 1. Absorption Curves of C h r o m i u m Compounds, Each Containing 32.95 P.P.31. The presence of the hexavaof C h r o m i u m lent forms would inteifere to 1. Potassium chromate some degree in the 416 mp 2. Potassium dichromate 3. Chromic sulfate range but would have no in+ i
1983
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ANALYTICAL CHEMISTRY
fluence upon the determination of the chromic compound a t 587 mp since they do not absorb in this region. I n solutions of adequate concentration the measurement of the chromic form may be advantageous since dilution errors necessary for the estimation of the excess reagent as dichromate will be eliminated. Furthermore, the quantity of chromic salt is directly proportional t o the material oxidized. I n the determination of an excess oxidant, such as the dichromate forms, the desired value is secured by difference from the total quantity added. I n this sense the method is indirect, but the increased sensitivity of the indirect procedure is imperative for low concentrations. An application of the foregoing information and principles to the determination of glycerol was investigated. DETERMINATION OF GLYCEROL BY DICHROMATE OXIDATION
The process of separation and purification of the glycerol in vinegar, wines, liquors, and flavoring extracts is a major problem in itself. The method of the iissociation of Official Agricultural Chemists for vinegar ( I ) , which has been official for years, is tedious and time-consuming. It involves extractions and precipitations to eliminate Eugars, proteins, organic acids, and other oxidizable substances. T h e method is generally conceded t o be far from satisfactory. The authors have given attention t o this phase of the determination with some success and efforts t o improve the procedure are being continued. The work which follows has to do with the testing of the method based upon the absorption characteristics of the dichromate-chromic salt mixture and the comparison of the results with those obtained by the official titrimetric procedure. I n order to eliminate all errors inherent in the purification process, the analyses TTere made upon solutions of pure glycerol in water.
Table I. Preparation of Potassium Dichromate-Chromic Sulfate Mixtures Standard Chromic Sulfate, 111
Standard Dichromate, 311.
Concd. HzS06, RI1.
Finala Volume of Diluted Sample, RI 1
a For photometry a t 587 ms. For measurement a t 350 mp, 10 ml. of these solutions were diluted t o 500 ml.
The glycerol content of vinegar varies from 0.0 t o 0.5 gram per 100 ml. and only one half of the 100-ml. portion specified for treatment is finally subjected to oxidation. Therefore, the actual quantity determined is usually less than 0.25 gram. I n the official procedure, this is oxidized with 30 ml. of potassium dichromate solution, 1 ml. of which is equivalent t o 0.01 gram of glycerol and contains 26.36 mg. of chromium. Oxidation Procedure. The oxidation procedure for the glycerol solution is essentially the same as is represented by the final steps in the official method and may be carried out as follows: Measure into a 250-mi. volumetric flask a quantity of glycerol in water not t o exceed 0.25 gram of glycerol. Add 30 ml. of the standard potassium dichromate solution and follow with the careful addition of 25 ml. of concentrated sulfuric acid. Heat in a boiling water bath for 20 minutes, cool, and make t o volume nith distilled water. This may be called solution 8. Estimation of Reagent Consumed. For the volumetric method the excess of dichromate is estimated as specified in the official method by titration of a portion of solution S with ferrous ammonium sulfate 1%-hichhas been standardized against the dichromate. For the spectrophotometric methods, the amount of chromic salt is estimated from the color value of another portion of solu-
tion S a t 587 mp, and the residual dichromate is estimated by dilution of a 10-ml. portion of the solution S t o 500 ml. with distilled n-ater and evaluation of the optical density a t 350 mp. A sulfuric acid blank or reference solution must be employed in each case. For measurement a t 587 mp the acid should be 4.4X , and for measurement a t 350 mp, it should be 0.1 N . Before the spectrophotometric evaluations may be made a standard curve must be constructed. Preparation of Standards. I n the present study a solution of chromic sulfate was prepared so as to contain the same amount of chromium (26.36 mg. per ml.) and sulfuric acid as the standard dichromate solution. Standards were then made up which contained varying amounts of the chromic sulfate and dichromat? but always in quantities such that the total amount was equal to 30.00 ml. Thus each mixture contained the same amount of chromium, but the quantities of trivalent and hexavalent forms varied over the desired range. T o each standard was added 25 ml. of concentrated sulfuric acid and the resulting solution was cooled and made up to 250 ml. in a volumetric flask. Such a series of solutions as has been described would be characteristic of the mixtures which result from reduction of dichromate xvith varying amounts of glycerol according to the official method. Table I shows the quantities used in the preparation of the standards.
I
I
I ~
A
I
GO 3w
500
Ai0
602
7m
LCO
W A V E L E N G T H (‘7,U)
Figure 2.
Absorption Curve of Chromic Sulfate Containing 5212 P.P.M. of Chromium
As was indicated in Figure I, an evaluation of the optical density of the solution a t 350 mp gives a measure of the dichromate without interference by the chromic ion, Because of the high absorption a t this wave length it is necessary to dilute 10 ml. of solution S t o 500 ml. in order t o secure optical denpities in the working range of the Beckman instrument. These values, when plotted, show good conformance to Beer’s law and establish the freedom from interference by chromic ion in the dichromate determination. The fact that chromium can be estimated as the dichromate a t 350 mp in a range of 0 t o 50 p.p.m. indicates a sensitivity approaching that of the diphenyl carbazide method.
Table 11. Determination of Glycerol by Dichromate Oxidation (Evaluation of reagent consumed by different methods) Glycerol, Grains By Spectrophotometric Evaluation By titration Sample A t 350 nip At 587 ,m# of excess Iid?rzO7 so. for excess KzCr207 for chromic salt (b) (C) (a) 1 0 029 2 0 033 3 0 044 0 057
9
10
11 12
0 0 0 0 0 0
069
082
09-1 113 120 13‘4
0 145
0 159
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200
250
300 W A V E LENGTH
contained traces of moisture, so the results for the analyses cannot be expressed with reference t o an absolute value, but as a comparison of values by the different methods. These values are given in Table 11. Since 1 ml. of potassium dichromate is equivalent t o 0.01 gram of glycerol, the values in column ( a ) result from a subtraction of the excess dichromate from the total (30 ml.) and multiplying by 0.01. The values indicated in column ( b ) were found by noting the value in micrograms per milliliter (or parts per million) of dichromate for the observed optical density at 350 mp and inserting the value in the foliorving expression:
4
350
(mp)
Figure 3. Curves Showing Effect of Addition of Sulfuric Acid upon .4bsorption of Potassium Dichromate Solution Containing 80 P.P.II. as Chromium 1. In water 3. In 0.1 iVHzSO4 5. In 5.0
[30 50 X micrograms per milliliter 4 X 26.36 X 0.01 = grams of glycerol
2. In 0.01 N H ~ S O I 4. I n 1.0 N H?SOa HzS04
In a siniilar way, the chromic salt concentrations were related t o optical densities at 587 mp but without the additional dilution. T~ test the performaIlce with less refined equipment, a of observations were made n i t h the Lumetron colorimeter using the orange (580) filter. This gave satisfactory results also. For the preparation of the staridard curves and for the examination of samples t o be referred t o them, the concentration of the sulfuric acid should be the same in both standards and samples. I t was observed thai. the adsorption at 587 mp by chromic sulfate was decreased slightly as the concentration of sulfuric acid was increased. In the case of the dichromate an increase in concentration of the sulfuric acid from 0.01 t o 4.4 lowered the absorption peak at 350 mp (Figure 3 ) about 40%. However a change in acidity, 0.01 to 0.1 N , had an almost insignificant effect. Glycerol of the highest purity available was used to prepare the polutions subjected to analy-is. However, it probably
>I
(
Similarly the values in column (c) nere calculated tiy taking the mjlligrams per milliliter of chromium indicated by the optical at mfl and using the Milligrams per milliliter X 250 = grams of glycerol 2636 LITERATURE CITED
(1) Assoc. Official Agr. Chem., “Methods of Analysis,” 7th ed., p. 489, TTashington, D. C., 1950. ( 2 ) Kasline, C. T., and hiellon, hI. G., ISD. ENG.CHEBI.,!.~NAL. ED., 8, 463 (1936). (3) Rossleri G., Ber., 5 9 ~2605 (1926). (4) Sandell, E. B., ”Colcrimetric Determination of Traces of Metals,’’ p. 191. Kew York, Interscience Publishers, Inc., 1914. RECEIVED for review June 23, 1962.
Accepted August 23, 1952.
2- (o-Hydroxyphenyl)benzoxazole as a Volumetric Reagent for Cadmium JOSEPH L. WALTER AND HENRY FREISER Department of Chemistry, University of Pittsburgh, Pittsburgh 13, Pa. I Y C E the publication of the paper by JValter and Freiser involving the compound 2-(o-hydrouyphenyl)benzoxazole as a gravimetric reagent for the determination of cadmium (Z), investigation has been carried out with this compound as a possible volumetric reagent for the determination of cadmium. The fact that the compound is a phenol and also that the compound and chelate are readily soluble in glacial acetic acid was utilized. These facts suggested the use of the bromate-bromide method for the determination of phenols. I t was found that an accuracy of f 0 . 2 mg. could be expected xhen determining from 2 to 80 mg. of cadmium. The dead-stop indicator was used to determine the end point. The rapid dissociation of the chelate in glacial acetic acid, and the fact that the compound is a phenol led to the use of the bromination technique as described by Siggia (1 ) substituting acetic acid for hydrochloric acid as the solvent. From the experimental
results and also from a microanalysis performed on the purified bromo compound, it was found that two bromine8 substituted on the ring, most probably ortho and para on the phenol portion of the molecule. The following equations best show the steps involved in the volumetric procedure: First the cadmium is precipitated as the chelate, Cd++
+ 2C13H902N +Cd(CiaHa0zN)z + 2Hf
Then the cadmium chelate is dissolved in glacial acetic acid,
+ 2 H 0 . 4 ~--+
Cd(Ci3H80~N)z
Cd++
+ 2Ci3HgOzS + 2 0 - 4 ~ -
The dissociated chelate is brominated,
+ 4Br2 +2CI3H7O2NBrZ+ 4HBr
2Cl~H902N
1 atom of Cd
4 Rr?
