1110
ANALYTICAL CHEMISTRY
For 100-mm. immersion. The graduations were in 0.1" intervals from 98" to 120" C. with 25 graduations per inch. When in use the thermometers were calibrated a t least once a day a t the steam point. The precision of measurement with these thermometers, as determined on six identical samples of known freezing point, was 1 0 . 0 1 " C. The thermometers proved completely satisfactory. The only change contemplated for future thermometers is a 75-mm. immersion line instead of 100 mm.
Analysis of Known Mixtures. I n order to piove this method, known mixtures of dry gamma and alpha isomers were analyzed by the freezing point depression, using the value of 0.0148 for the cryoscopic constant. The known samples were heated to 118" to 120" C. and mixed thoroughly, and the jacket heater and cell pressure were regulated to give a cooling rate of not more than 0.5' C. per minute. The freezing point was taken as the temperature plateau that occurred immediately after recovery from undercooling. The thermometer was read to zt0.01" C. with the aid of a small magnifying glass. The gamma isomer concentration was calculated from the equation : Log mole %gamma = 2 where At = 112.86
-
X At - 0.0148 2.30259
(4)
freezing point of sampk.
The results obtained on known samples are given in Table 111. The cryoscopic analysis of commercial sample? of lindane is complicated by the possible presence of moisture and/or residual solvents. Complete drying can best be effected by dispersing dry nitrogen through the melted material. The possibility of change in composition was investigated by preparing known samples of
the dry alpha and gamma isomers, passing dry nitrogen through the melted samples held a t 115' C. for varying lengths of time, and then determining the gamma isomer content. These data are summarized in Table IV. ACKNOWLEDGMENT
The authors are indebted to Frederick D. Rossini and iZnton J. Streiff of the Carnegie Institute of Technology for their many helpful suggestions while this work was in progress. The preparation of the high purity isomers by Robert H. Cundiff of this Iaboratory is also gratefully acknowledged. LITERATURE CITED
( 1 ) Aepli, 0. T., Rlunter, P. d.,and Gall, J. F., ANAL. CHEM.,2 0 , 610 (1948). ( 2 ) hrceneaux, C. J., Ibid., 23, 906 (1951). (3) Bowen, C. V.. and Pogorelskin, M. A , , Ibid., 20, 346 (1948). (4) Chent. Eng. A-ews, 27, 2210 (1949). ( 5 ) Daasch, L. K., Ax.4~.CHEY., 19, 779 (1947). (6) Glasgow, A. R., Krouskop, K. C . , Beadle, J., Axilrod, G. D.. and Rossini, F. D., Ibid., 20, 410 (1948). (7) Glasstone, S.,"Textbook of Physical Chemistry," p. 454, Sew York, D. Van Nostrand Co., 1940. ( 8 ) Harris. T. H.. J. Assoc. Oifc. Am. Chemists. 32. 684 (1949).
(9) Kauei. K. B., DuVall. R: S., ind dlquist, F. K., I n d . Eng. Chem., 39, 1335 (1947). (10) SIcCione, IV. C., A N ~ LCHEM., . 21, 862 (1949). (11) Mali, B. J., Glasaoi3, -1.R.. and Rossini, F. D., J . Research .Yutl. B u r . Standards, 26, 591 (1941). (12) Taylor, IT.J., and Rossini, F. P., Ibid., 32, 197 (1944). RECEIVED January 18, 1951.
Presented before the Division of hnalytical Chemistry a t the 118th Meeting of the AIIERICANCHEMICALQ O C I E T Y , Chicago. 111.
Separation of Chromium from Vanadium By Extraction of Perchromic Acid with Ethyl Acetate ROBERT K. BROOKSHIER, Northwest Electrodevelopment Laboratory, Bureau of Mines, Albany, Ore., AND HARRY FREUND, Department of Chemistry, Oregon State College, Coroallis, Ore. Research by the U. S. Bureau of Mines on the recovery of vanadium from low grade ferrophosphorus required the determination of small amounts of chromium in vanadic oxide products. The titrimetric method is inaccurate, and Foster's perchromic acid extraction procedure gave erratic results. Study of the variables influencing the extraction led
A
CCURACY in the titrimetric determination of chromium in the presence of vanadium (9) is difficult to attain when vanadium preponderates. The usual colorimetric determination of chromium ( 5 ) as chromate or with s-diphenylcarbazide necessitates a preliminary separation from vanadium. Foster (3) employed ethyl acetate as an immiscible solvent to extract perchromic acid and thereby effected the separation and concentration of small amounts of chromium in vanadium products. Attempts in the laboratory of the U. S. Bureau of Mines to applv the separation to determination of chromium in red-cake vanadic oxide led to erratic results and indicated the need for a systematic study of the factors influencing the separation. Since 1847, when Barreswil (I) first reported the formation of blue perchromic acid by the action of hydrogen peroxide on dichromate, the reaction has been used a s a sensitive qualitative test for chromium. The object of later research ( 2 , 4,7, 8) was the study, in a homogeneous system, of catalytic decomposition of hydrogen peroxide by dichromate. The reaction of oxidized compounds with hydrogen peroside, replacing oxygen atoms with
to the following optimum conditions: pH at equilibrium 1.7 r!c 0.2, concentration of hydrogen peroxide 0.02 mole per liter, temperature 20" C. or less, and number of extractions 3. Adherence to these conditions, with final estimation of chromium with s-diphenylcarbazide, yields a more sensitive and reliable method than an) heretofore described.
peroxide groups, generally leads to unstable products that lose oxygen very easily. Studies by Spitalsky ( 7 , 8)'and Bobtelsk'y (2) confirm the transitory nature of perchromic acid as an intermediate in the catalytic reduction of hydrogen peroside with dichromate. S o attempt was made to stabilize the intermediate perchromic acid, although the free acid was knonn to influence the reduction, and the presence of an organic solvent in the homogeneous system was shown to have a stabilizing effect ( 2 ) . REAGENTS AYD APPAR4TUS
Chemically pure reagents were used throughout this work. Absolute ethyl acetate was used and was recovered and purified by distillation after each esperiment. The hydrogen peroxide solution was prepared and standardized daily from Baker's C . P . 30% hydrogen peroxide solution. The standard chromium solutions were prepared from accurately weighed amounts of Kational Bureau of Standards potassium dichromate No. 136. C.P. ammonium metavanadate was used for preparing the standard vanadium solution. All p H measurements were made with the Beckman Model .1I pH meter.
V O L U M E 2 3 , NO. 8, A U G U S T 1 9 5 1
1111
The Beckman Model DU spectrophotometer was used. PROCEDURE FOR CHROMIUM DETERMINATION 111 theqe studies the chromiuin contents of the ethyl acetate qolutions were found by the spectrophotometric diphenylcarbazitie method described by Sandell ( 5 ) .
ethyl aretate was converted to chromate by the addition of potassium hydroxide. The chromate was extracted with water and determined spectrophotometrically with s-diphenj lcarbazide.
90
.I maximum chromium recovery of 93 to 96% was ohtained :tt pH 1.7 =I= 0.2. The data are plotted in Figure 1. Effect of Sexivalent Chromium Concentration. The above btudy was repeated for a series of initial weights of chromium varying from 0.4 to 4 mg., a concentration range of 0.000154 to 0.00154 mole per liter. As indicated by Table I, the initial concentration of sexivalent chromium shou ed no significant effert (in its recoverv.
8L
Table I.
lo0
Volume of
Effect of Sexivalent Chromium Concentration on Recovery aipieoiis layer a t ec~uilibriuni = .50 ml. I.” of aqueous layer a t equilihriuni = 1.7 i 0.2
$ 70
Cr”’ Taken, .\IC.
2 !
cry1
Foiind, Mg.
8 60 h
%
93 99
2 2
CrV’ Recovery,
94
95 91
5c
L
36 2c I
IO
I 2 3 4 pH OF AQUEOUS SOLUTION Figure 1. Effect of pH on Recovery of Chromium
5
Single extraction
One milliliter of 10% potassium hydroxide solution was added to the blue ethyl acetate solution of perchromic acid. This returned the chromium to the aqueous layer and liberated oxygen. The yellow chromate was extracted with water and the solution was boiled t o remove peroxide, cooled, and diluted to 50 nil. in a volunietric flask. A suitable aliquot, depending upon the approximate chromium content, was transferred to a 100-nil. volumetric flask, and 50 to 60 ml. of water were added, follonrd t)y 1 ml. of 0.257’( alcoholic s-diphenylcarbazide solution. The whole solution was diluted to 100 ml. arid mixed n-ell; after 5 to 8 minutes the absorbancy was measured relative to distilled water at a wave length of 510 mp and a t a slit width of 0.04 mm. The concentration of sexivalent chromium was obtained from a standard linear curve and the per cent recovery of chromium was calculated. This method is reproducible and far niore sensitive than tli? method h s e d upon the yellow color of chromate ion. EFFECT OF REACTION V,iRI.4BLES
Effect of pH of Aqueous Solution. The effect of pH over the range 0.5 to 4.5 was studied first. The aqueous solution containing 2.8 nig. of chromium as dichromate, and the desired amount of hydrochloric acid-potassium c-hloride buffer (6) were placed in a 250-ml. separator) funnrl and diluted to 50 ml.; 100 ml. of ethyl acetate were added, folloived by approximately 1 ml. of 6y0 hydrogen peroxide solution. The mixture was shaken vigorously for 0.5 minute, the blue perchromic acid passing into the ethyl acetate phase. The aqueous layer was drawn off and its pH measured. Because of the instability of perchromic acid only one extraction was made. After the aqueous lager \\-as drawn off, the chromium i n the
Effect of Hydrogen Peroxide Concentration. For this study a fresh solution of hydrogen peroxide was prepared from 30’% ,solution and standardized against potassium permanganate. .I known volume of this solution x a s added to the mixture of ethyl acetate and d i c h >mate,buffered to a pH of 1.7. The perchromic acid was extracted once and chromium(V1) was determined spectrophotometrically as before. As t,he volume of st,andcud hydrogen peroxide added and the volume of the aqueous solut,ion were known, the hydrogen peroxide concentration was calculated. These data, listed in Tahle 11, indicate that maximum recovery of sexivalent chromium is obtained when the concentration of hydrogen peroxide in the aqueous solution a t the time of extraction is 0.02 mole per liter. .Ibove this concent,ration the recovery decreases slowly; b ~ l o wthis concentration the decrease in recovery is rapid.
Table 11.
Effect of Hydrogen Peroxide Concentration on Sexivalent Chromium Recovery = 30 nil. pFI of aqueous aoliition a t eqiiilibririin = 1.7 + 0.2 Hydrogen CrY1 Cy”‘ Peroxide I‘orind, Rerwered, Mole/Liter Concn.. ‘ 7 LIE. /c 80 o.nn 0.786 0.813 113 o oni 0.848 8A 0 01.2 0.966
Volriiiie of aqueous layer a t eqiiilibriiiin
Cr\-I Taken, Ilp. 0 981 0 985 0 983 0 . 98.5 0 98.5 2.00 0,983 0.98.5 1: !IS> 0 98.1
Effect of Successive Extractions. It was expected that repeated extractims would improve the chromium recovery, if the chromium loss X Q S due to partition between the solvents and not to decomposition of t,he perchromic acid. Three successive extractions \?-eremade using 100 nil. of et>hylacetate for the first and 15 ml. for the second and third extractions. The chromium recovery vias deterniincd for each extraction separately (Tahle 111). Essentially quantitative recovery of the chromium may thus be attained. Effect of Temperature. The recovery of sexivalent chromium would be expected to be tlrpc+~lrlentup:)ii the temperature of the
ANALYTICAL CHEMISTRY
1112 reaction mixture, as perchromic acid is known to be unstable. The influence of temperature was determined by forming the perchromic arid and extracting a t several different temperatures. For low temperatures the separatory funnel containing the aqueous solution of buffer and dichromate, together with 70 ml. of ethyl acetate, was cooled in the freezing compartment of a refrigerator for 0.5 hour. The temperature of the mixture was measured just before the hydrogen peroxide was added. The aqueous solution was extracted three times, the extracts being combined and the chromium determined in the usual manner. One milliliter of 1 M standard hydrogen peroxide solution was sufficient to give the optimum concentration and yet sufficiently small not to effect the temperature of tht. extraction mixture. For higher temperatures a thermostatically controlled bath was used. The results of this study showed that at 10" C. or below the blue perchromic acid is very stable. The recovery of sexivalent chromium was consistently near 100%. Above 10' C., however, the results became erratic and the recovery decreased rapidly (Table IV). Table 111.
Effect of Successive Extractions on Sexivalent Chromium Recovery
Table V.
titanium, nickel, molybdenum. Blanks containing about 50 mg. of each were carried through the procedure and none was estracted by ethyl acetate in any significant amount. The small amount of iron extracted produced an absorbancy in the final solution equivalent to only 0.01 microgram of chromium per niilliliter.
Volume of aqueous solution a t equilibrium = 50 ml. Concentration of hydrogen peroxide = 0.02 mole per liter. p H of aqueous solution a t equilibrium = 1.7 & 0.2 C rv' Taken,
nIg. 0 983 200 0 985
1st extract 0 966 1 94 0 942
Table IV.
Cr" Recovered, Mg 2nd 3rd extract extract 0 023 0 004 0 010 0 004 0 006 0 020
Cr Recovered, Total 0 993 1 954 0 968
% 101 98 98
Effect of Temperature on Sexivalent Chrominm Recovery
Concentration of hydrogen peroxide = 0.02 mole per liter. Volume of aqueous solution = 50 ml. p H of aqiieou8 solntion = 1.7 i 0.2. Number of extractions = 3 Cr CrVI CrV' Recovery, Temp., Found, Taken, c. Mg. % Mg. 4.0 98 0.985 0,968 6.0 99 0.972 0 985 9.0 98 1.95 2.00 19.9 9; 0.936 0.985 25.0 96 0,950 0 983 29 3 60 0,587 0 985 74 35.4 0.732 0.985
Analysis of Synthetic Samples
Volume of aqueous layer a t equilibrium = 50 1111. p H of aqueous layer a t equilibrium = 1.7 =t0.2. Concentration of hydrogen peroxide = 0.02 inole per liter. Number of extractions = 3. Teinperature of mixture = 100 C. vzos Cr Cr 70 Chromium Taken, Taken, Found, nIg. Mg. hfg. Found Actual 100 0.100 0.096 0 096 0.10 loa 0 19 0.191 0.200 0.20 100 0 30 0.301 0.300 0.30 100 0 374 0 37 0.400 0.40 100 0 50 0.500 0 300 0.50 100 0,800 0.766 0 76 0.79 100 0.985 1.00 0 99 0.98 100 1 2B 1.40 1.28 1.38 100 1 67 1 64 1.80 1.77 1 8i 100 2.00 1 91 1.96 2 84 3.00 2.91 2 93 100
PROCEDURE FOR ANALYSIS OF SAMPLES
If the sample is an ore, the usual fusion with sodium peroxide or sodium carbonate and sodium prroside is recommended. In t,he case of vanadic oxide products it is sufficient to dissolve the sample in sodium hydroxide and boil the solution vith a lit,tle sodium peroxide to ensure complete oxidat'ion. I n either case, filter the alkaline solution, containing 0.1 gram of sample, to remove small amounts of iron. Seutralize the filtrate containing chromium(V1) and vanadium(V) with sulfuric acid and evaporate the solution to 15 to 20 ml. If much iron is present, acidify the sample solution \vith sulfuric acid and osidize chromium with ammonium persulfate, if necessary. This \vi11 prevent loss of chromium during the precipitation of iron. Cool the solution and carefully buffer to a pH of 1.7, using a pH meter.
I
ETHYL ACETATE
STABILITY OF PERCHROMIC ACID
The stability of the blue perchromic acid was studied in both aqueous and ethyl acetate solutions. -4 series of aqueous solutions containing the same amount of chromium was adjusted to the p H and temperature specified above. The perchromic acid was formed by the addition of hydrogen peroxide and the aqueous solutions were allowed to stand a t 10' C. for periods of from 0 to 15 minutes. The perchromic acid was then extracted with ethyl acetate and the per cent recover) o chromium(V1) was determined. From Figure 2 it will be seen that blue perchromic acid decomposes rapidly in aqueous solution and that its immediate extraction nith ethyl acetate is imperative. The blue substance is very stable in ethyl acetate solution for as long as 30 minutes. Summary. As a result of the above studies it is possible to establish the following set of optimum conditions.
*
pH at equilibrium, 1.7 0.2 Concentration of hydrogen peroxide, 0.02 mole per liter Temperature, 10" C. or less Number of extractions, 3 Siniultaneous extraction with ethyl acetate
.50 ci CI
$40 c,
$30
-
20 -
Twenty-five milliliters of ethvl acetate folloned by two 15-ml. portions \\ere adequate for the eytraction of 1 mg. of chromium. I
fo
IATERFEREUCES
There is no interference from the following elements which I.night be expected to cause trouble : iron, mercury, vanadium,
I
I
I
I
20
30
40
50
TIME / N MINUTES
Figure 2.
Effect of Solvent on Stability of Perchromic Acid
V O L U M E 23, NO. 8, A U G U S T 1 9 5 1 Transfer this buffered solution to a 250-ml. separatory funnel, dilute to 50 ml., and add 75 ml. of ethyl acetate. Cool the mixture by placing in a refrigerator for 0.5 hour, or if this is impossible, in cold running water. When cool, add 1 ml. of 1 M ( 3.8Yc)hydrogen peroxide. After shaking the funnel vigorously for 0.5 minute, allow the layers to separate, and draw off the aqueous solution. Repeat the extraction of the aqueous layer a t least twice, using 15 ml. of ethyl acetate each time. Combine the ethyl acetate fractions. Add 1 ml. of lOY0 potassium hydroxide solution to the blue solution of perchromic acid, and shake until the blue color is replaced by yellow. Extract the \-ellow chromate with water, and boil the solution for 10 mink e s . Dilute to 50 ml. and determine the chromium with sdiphenylcarbazide. Analysis of Synthetic Samples. For the analysis of synthetic samples, 100 mg. of vanadic acid as ammonium metavanadate and known weights of sexivalent chromium from 0.1 to 3 nig. were used. The samples were prepared by mixing the proper amounts of standard solutions of vanadate and dichromate. Eleven such samples wercx analyzed (Table V). In the range from 0.1 to several per cent, the results are in good agreement n-ith the known values. Analysis of Standard Sample. Sational Bureau of Standards icarrovanadiuni, sample 61-1 con t,aining 0.6S70 chromiuni, was
1113 analyzed by this procedure. Parallel analyses were made using 0.100- and 0.250-gram samples and a value of 0.6.?17~chromium was obtained. LITERATURE CITED
Barreswil, L. C. A , , Ann. chini. p h y s . , (3) 20, 264 (1847). (2) Bobtelsky, RI.,Glasner, A , and Bobtelsky-Chaikin, L., J . Am. Chem. Soc., 67, 966-75 (1945). (3) Foster, 11.D., U. S. Geol. Survey, Bull. 950, 16-18 (1946). (4) JIellor, J. IT.,“Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. XI, pp. 353-61, London, Longmans, Green B. Co., 1931. (1)
( 5 ) Sandell, E. B., “Colorimetric Determination of Traces of Metals.” pp. 191-5, New Yo1 k, Interscience Publishers, 1936. (6) Snell, F. D., and Sriell, C. T., “Colorimetric Methods of Analysis,” DD. 671-5, New York. D. Van Nostrand Co.. 1936. (7) Spjtksky, E., 2. anorg. Ciiem., 56, 72-108 (1907). (8) Ihid., 69, 179-208 (1910). (9) JI-illaid and Diehl, “Advanced Quantitative Analysis,” p. 234, New Yolk, I).Van Nostrand Co., 1943. RECEIVEDDeoernber 20, 1950. I’iiblished by permission of the Director, Bureau of Mines, U. S. Deliartlnent of Interior.
Estimation of Ascorbic Acid in Pharmaceuticals With Particular Reference to Interfering Substances D. G. CHAPMAN, ODETTE ROCHON, AND J. A . C 4 l I P B E L L Food and Drug Divisions, Department of Sational Health and Welfare, Ottawa, Canada Several methods have been proposed for the estimation of ascorbic acid in the presence of interfering materials such as iron and copper salts, but few data exist on the comparative reliability and limitations of these procedures as applied to pharmaceuticals which may contain these and other interfering substances. Of the eight methods investigated, the procedure of Roe e t al. was found to be the most reliable under all conditions. The Brown and Adam method was the most satisfactory for routine analysis except in the presence of large amounts of copper. Iron does not interfere in the methods of Gawron and Berg and Frosst, but ascorbic acid may not be stable in the extracting agents used in these methods. The precision of all methods in the absence of interfering substances was found to be satisfactory. Great care must be exercised in the selection of a method for the estimation of ascorbic acid in certain complex pharmaceuticals containing minerals.
A‘
IIOSG
the many methods for the determination of ascorbic acid, those using metaphosphoric acid as the extracting as the indicator appear agent and ~2,6-dichlorophenolindophenol to be the most widely used. .-\scorhic acid is stable in metaphosphoric acid ( 1 3 ) and a t the same time its oxidation by copper or enzymes is reduced (8). A disadvantage of metaphosphoric acid is that ferrous salts, which are present in many pharmacseutical products, are oxidized quantitatively by indophenol dye. To overcome this objection to nietaphosphoric acid, two possibilities are open to the investigat,or. A reagent may be added to the metaphosphoric acid which will remove the interference caused by iron, or an extracting agent may be sought in which ferrous iron does not react with the dye. Most of the common acids have been studied (13) as possible extracting agents, but ascorbic acid is relatively unstable in all except metaphosphoric and oxalic acids. However, ferrous iron
interferes with the estimat.ion of ascorbic acid in both of these, and both metaphosphoric acid ( 2 ) and oxalic acid (6) are themselves relatively unstable. T o eliminate the interference caused by ferrous iron in metaphosphoric acid extracts, Lugg ( 1 2 ) has suggested the use of formaldehyde, which at a p H of 3.5 condenses with ascorbic acid but not appreciably with ferrous salts. Because of the large number of titrations v hich are required for a single determination, this method did not appear to be adaptable to routine analysis and therefore was not studied by the authors. The addition of hydrogen peroxide \vas first used by Levy (9) to eliminate interference by sulfur dioxide. Huelin and Stephens ( 7 ) , however, have reported that for extrack containing ferrous iron, the use of hydrogen peroxide converts a positive error into a negative error of less magnitude. Hydrogen peroxide also promotes a slow reoxidation of the reduced dye and the titration must be completed as soon as possible after adding this reagent. Robinson and Stotz ( 1 4 ) have proposed a spectrophotometric method in which certain reducing materials are eliminated by the use of hydrogen peroxide or formaldehyde. Gawron and Berg ( 5 ) have suggest,ed using 8% acetic acid to extract ascorbic acid when ferrous iron is present. Brown and Adam (3)have reported that, a sodium acetate-hydrochloric acid mixture buffered to a pH of 0.65 overcomes the interference due to ferrous iron. Roe e l ai.( 1 6 ) have published a method for the determination of ascorbic acid based on the reaction between dehydroascorbic acid and 2,i-dinitrophenylhydrazine. Roe and Kuether ( 1 5 )liave stated that ferrous ion8 do not interfere in this procedure. It thus becomes apparent that a variety of methods has been proposed for eliminating or reducing the ferrous iron interference in ascorbic acid assays in foods. However, there is little published inforniation on the relative merits of these methods. Hence this study way undertaken for the purpose of comparing t,he reliability of eight methods in estimating ascorliic acid, in the presence of both ferrous iron and other substances commonly found in multivitamin pharmaceutical products.
