1985
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?
ANALYTICAL CHEMISTRY
1986 From this, then, one can readily calculate the per cent cadmium present in the sample. REAGENTS AND APPARATUS
Bromate-Bromide Mixture. T o make a 0.1 S bromatt4)romide solution, 2.780 grams of C.P. potassium bromate and 10.00 grams of C.P. potassium bromide is dissolved in about 600 ml. of distilled water and dilutcd to 1 liter in a volumetric flask.
Table I. Cadmium Taken, Gram 0 0425 0 0425 0 0425 0 0425 0 0425 0 0435 0 0435 0 0435 0 0435 0 0435 0 0211 0 0108 0 0049 0 0013
Volumetric Determination of Cadmium Cadmium Found, Gram
Vol. RrOa-Br Used,
M1.
27 27 27 27 27 28 28 28 28 28
0 0427
9 9 4 6
0427 0419 0421 0423 0439 0435 0433 0437 0 0433 0 0213 0 0111 0 OOjl 0 0017 Average error 0 0 0 0 0 0 0 0
7 7 4
4 6 3 14 0 12 6
3 4 1 2
Error, Grain +o 0002 +o 0002 -0 0006 -0 0004 -0 0002 -0 0004 0 0000 0 0000 -0 0002 -0 0002 -0 0002 + O 0003 -0 0002 + 0 0004 zk0 0002
Standard Thiosulfate Solution. Exactly 24.820 grams of grade sodium thiosulfate is weighed into a 1-liter volumc,tric flask, and the volume is brought u p to 1000 ml. n ith distilled water. This solution is standardized against the bromrttebromide solution. Dead-Stop Indicator. .I description of this apparatus is avails1)le (3,4).
A.R
PROCEDURE
The chelate, containing about 50 mg. of cadmium, precipitated a t approximately p H 11 with the reagent, is filtered through a medium-porosity sintered-glass crucible with suction using the procedure previously described ( 2 ) . It is n-ashed thoroughly with 50$& w./v. alrohol to which a trace of ammonia had been added. The chelate is then dissolved in about 50 ml. of hot glacial acetic acid, transfcrred to a n iodine flask, and diluted with 20 ml. of distilled water. Thirty-five milliliters of a 0.1 -V bromate-bromide mixture is pipetted into the flask. The flask is immediately stoppered, and 2 ml. of potassium iodide solution is added to the reservoir of the iodine flask to prevent any loss of bromine. The reaction is allowed to proceed for a period of 1 to
1.25 hours. A solution containing about 1.5 grams of potassium iodide is then added to the reaction. The liberated iodine is titrated with the standard 0.1 LV sodium thiosulfate solution t o the end point as detected with the dcad-stop indicator. This apparatus consists of tR-0 platinum electrodes, connected in series with a potential just sufficient to balance the back e.m.f. (about 20 mv.). During the titration, current nil1 flow to give a full deflection of the galvanometer. \Then the end point isreached the needle returns to zero. Further addition of thiosulfate causes no deflection. This apparatus was used because of the difficulty encountered in detecting the end point visually with solid precipitate of the brominated compound present during the titration. DISCUSSION
It was somewhat disappointing to find that less than the theoretical amount of bromine was taken up by the cadmium chelate. Despite the variation of the reaction temperature and the lengthening of the reaction time, only 9Opc of the theoretical bromine uptake naq observed. HoTvever, while the reaction did not go to completion, the iesults nere easily reproducible. Cpon the application of an empirical factor, it was found that an accuracy of i 0.2 mg. of cadniium could be obtained by this procedure in the presence of, or in the absence of, most metallic ions ( 2 ) . Some of the rmults obtained can be seen in Table I. The number of grams of cadmium found in the samples (Column 3) was calculated by multiplying a quantity, the volume of thiosulfate used for a blank minus the volume used for the sample, by the normality of the thiosulfate, and finally by the empirical factor 0.0163. A C K \ O I LEDGJIENT
The authors wish t o express their appreciation to the United States Atomic Energy Commission for financial support. LITERATURE CITED
(1) Siggia, S., “Quantitative Organic Analysis via Functional Groups,” p. 111, New l o r k , .John K l e y 8: Sons, 1949. (2) Walter, J. L., and Freiser, Henry, .%S.LL. CHEX.,24, 984 (1952). (3) I b i d . , in press. (4) Willard, H. H., et uL, “Instrume~.talMethods of Analysis,” pp. 212-13, New York, D. Van S o s t m n d Co., 1951.
RECEIVEDf o r review July 7, 1952. .iccepred September 26. 1952. Contribution h’o. 867 froni the Departnient of Chemistry. University < i f Pittnburgh, Pittsburgh 13, Pa.
Extraction and Determination of Total Pectic Materials in Fruits R . &I.MCCREADY AND E. A. BICCOMB Western Regional Research Laboratory, Albany, Calif.
approach to the analysis of fruits for total pectic sub0SEstances is t o extract the tissue in turn with hot water, dilute mineral acid ( p H 2), and cold alkali or ammonium oxalate (3). Three or four frwtions of pectic substances are isolated, each presumed to have unique solubility characteristics. The highm e t h o y l pectins are lvater-soluble; acid promotes solution of the protopectin and alkali or oxalate dissolves loa-methoxyl pectins and pectates. Difficulties encountered in this procedure are due t o overlapp’ng solubilities of the various pectic substances. For example, sodium pectate is soluble in water, alkali, and ammonium oxalate, and partly soluble in hot dilute acid. Low-methoxyl pectins may be soluble in all extractants in some fruits and not in others, depending upon the cation composition. The quantitative multiextraction procedure is long and tedious and the final interpretation may be questionable. A seemingly more reasonable approach t o this problem in
view of the present knowledge of the physical and chemical properties of high polymers assumes the existence in fruits of pectins of wide ranges of methoxyl contents and molecular lveights and the possibility of extraction of all of the pectic substances in fruit tissue a t pH i and below with very mild heating, provided t h a t the tissue is sufficiently macerated and that the polyvalent cations are sequestered. Pectin problems can be attacked by determining the total pectic mbstances as anhydrouroriic acid (.$Uri) and bv characterizing pectic substances extracted by mild means ( 7 ) . Such information may permit deductions of the possible role of the pectic substances as they exist in the tiysue under investigation. Some k n o d e d g e of the plant’s cation composition is necessary because cations play significant roles in maintaining texture of fruits. Pectic enzymes were used for thc extraction of pectin from
