Ultraviolet Spectrophotometric Determination of Molybdenum -
GEORGE TELEP AND D. F. BOLTZ, Wayne Univerdty, Detroit, Mich.
A rpectrophotometric rtudy of the pemxymolybdic acid complex war made uring a Beckman Model DU rpectrophotometer. The effeat of various variabler on the maximum abrorbancy war determined and optimum conditione were arcertoined. Conformity to Beer's law war found for conwntrationr from 0 to 150 p.p.m. of molybdenum uring 1.000-am. quartz w h . The main interfering ionr are iron, tungrten, vanadium, titanium, and fluorine. The recommended general procedure ir rapid, awurate, and convenient.
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HE oolorimetric determination of molybdenum utilizing the peroxymolybdate complex in an alkaline solution waa introduced by Funck in 1920 (8). This procedure had little significance, owing to the instability of the complex. Weissler utilized the peroxymolybdate complex in an acidic solution for the simultaneous spectrophotometric determination of molybdenum, titanium, and vanadium (4). Although a simultaneous determination of the three peroxide complexes wm poasible, little attention waa given to the determination of molybdenum, T h e w fore, it neemed desirable to investigate critically the absorption spectra of the peroxymolybdic acid complex in the ultraviolet region of the spectrum. This etudy waa made in order to awertain the suitabiHty of the pale yellow peroxymolybdic aoid complex for an ultraviolet epeatrophotometric determination of small amounts of molybdenum.
0 to 160 p.p.m, An intense absorption peak occurs at 330 mp, as shown in Figure 1. Acid Concentration. The effect of various ooncentratiom of perchloric acid waa determined using 100 p.p.m. of molybdenum and 1 ml. of hydrogen peroxide in a final volume of 60 ml. It was found that 1 to 16 ml. of perchloric acid had little effect upon the maximum a h r b a n c y measured at 330 mp. From thia study, 5 to 10 ml. of perchloric acid per 60 ml. of solution were deemed to be a sufRcient amount for attainment of maximum abaosbanoy. The atudy of the effect of other acids indicated that rmll oonoentrationn of phosphoric acid have little effect, but other acids should not be substituted for perchloric acid in the procedure.
APPARATUS AND SOLUT10NS
The absorbancy measurements were made with a Beckman Model DU spectrophotometer and 1.000-cm. quartz cells. The s ctrophotometer was equipped with an ultravioletsensitive pRtotube for high sensitivit in the 260 to 636 mp region of the s ectrum. The reference ces contained redistilled water for all tEe measurements. A standard molybdate solution was prepared by dissolvin 0.5oOo gram of pure sodium molybdate in 1 liter of rediatild water containing 6 ml. of concentrated sulfuric acid. This solution was standardized by an A.S.T.M. rocedure of anal sis according to which d y e r molybdate is weigfed I). One milli&er of this solution conhned 0.20 mg. of molyb enum. The perchloric acid solution used waa 72% reagent grade. The hydrogen peroxide was 37' analytical reagent grade. Other acids used were c.p. reagent pa&.
P
I
6
Oe8 0.6
0.4
COWR REACTION
0.2
The treatment of an acidic solution of molybdate ions with hydrogen peroxide reaulte in the formation of the peroxymolybdic acid complex. Thia complex haa a pale yellow hue with maximum absorbancy in the ultravialet region. In order to etudy theeffect of certain variables on the maximum absorbancy, the following procedure waa used.
A definite amount of the standard molybdate solution was transferred by means of a ipet to B 60-ml. volumetrio tlemk. The deeired amount of perchroric acid was added and the volume waa adjusted to 60 ml. with redistilled water. After addition of 1 ml..of 3% hydrogen roxlde, the contents of the flaak were thoroughly mixed and aEorbancy measurements were taken from 260 to 460 mp at Zmp intervals. The complexation is immediate, and the s stem is stable for a t least 72 hours. In the etudy of the effect of &verse ions, a definite amount of the solution containing each ion was added before dilution and complexation. EFFECT OF CONCENTRATION
Molybdenum Concentration. The absorption spectra for various concentratione of molybdenum waa determined and conformity to Beer's law WM found a t 330 mp in concentratiom from
"O
0
250
330 WAVE LENGTH, rnr
410
Figure 1. Effect of Molybdenum Concentration 1. 20 p.p.m. 2 60 .p.m. a: 10gp.p.m. 4. 140p.p.m.
Hydrogen Peroxide Concentration. The effect of various amounts of hydrogen peroxide waa etudied using 100 p.p.m. of molybdenum and 10 ml. of perchloric acid in a final volume of 60 ml. It waa found that a minimum volume of 1ml. of 3% hydrogen peroxide (per 60 ml. of solution) is necwary for attainment of maximum absorbancy. Effect of Diver60 Ion@. The effect of various diverse ions waa studied wing 100 pap.m. of molybdenum. Absorbancy readings were taken a t 330 mp in order to ascertain any changes in the maximum abeorbancy. A negligible e m r waa obtained with lo00 1030
1031
V O L U M E 22, NO. 8, A U G U S T 1 9 5 0 Table I. Interfering Diverse Ions Ion Fe + +
+
WO,--
Ti
Added 8s Fe(ClO4)r NarWOd
Ti SO,),
Amount Added P,P.M: 100 100
100 100 NILVO, 120 FNaF O In presence of citrate ions. b In presence of phosphate ione. +
+
vo, -
+
+
Error, % of Desired Constituent 60 4.5 25
43
8.0
Permissible Amount, P.P.M. 10 40”
20b 5
0
p.p.m. of the following ions: aluminum, borate, calcium, cadmium, cobalt, cupric, bismuth, citrate, acetate, dichromate, magnesium, manganous, nickelous, plumbous, oxalate, malonate, stannate, silicate, sulfate, nitrate, tartrate, zinc, and zirconyl. Table I lists the interfering ions and their effect. Fluoride Concentration. The effect of various amounts of fluoride waa studied, using 100 p.p.m. of molybdenum. A negative error was caused by the fluoride ion. This precludes the advisability of attempting to remove the iron and titanium interference by use of fluoride ions. INTERFERENCES
Tungsten Interference. The effect of various concentrations of tungsten was studied using 100 p.p.m. of molybdenum. It was found that 10 p.p.m. of tungsten gave an appreciable error. Wells and Grimaldi utilized citrate ions to lessen the tungsten interference in using the molybdenum thiocyanate colorimetric method (6). An investigat,ion of the effectiveness of citrate ions in removing the tungsten interference when absorbancy is determined at 330 mp revealed that up to 40 p.p.m. of tungsten could be tolerated in the presence of 1600 p.p.m. of citrate ions. Iron Interference. The use of phosphate and tartrate ions to remove the iron interference by complexation waa studied. A maximum concentration of 8 p.p.m. of ferric ion was succeasfully complexed using 5 ml. of phosphoric acid. A maximum conrentration of 20 p.p.m. of ferric ion was complexed by 2200
p.p.m. of tartrate ions. Citrate ions had a complexing effect similar to that of the phosphate ions. Interference of Titanium and Vanadium. Merwin studied methods of bleaching the titanium peroxide complex (3). Merwin’s study indicated that phosphoric acid and citric acid reduce the intensity of the titanium peroxide color. The effect of various concentrations of titanium was studied using 10 p.p.m. of molybdenum and an acidic mixture of 5 ml. of perchloric and 5 ml. of phosphoric acids. A maximum concentration of 20 p.p.m. of titanium was successfully complexed. A maximum concentration of 15 p.p.m. of titanium can be complexed, using 1600 p.p.m. of citrate ions and 5 ml. of perchloric acid. There was no evidence that vanadium waa romplexed by these reagents. For larger amounts of titanium and for vanadium, corrections based on the absorbancies of the pure titanium and vanadium peroxy complexes can be made according to the method outlined by Weissler (4). RECOMMENDED GENERAL PROCEDURE
Sample. Weigh, or measure by volume, a sample containing an amount of molybdenum such that the final solution contains not m v e than 0.20 mg. of molybdenum per ml. of solution. Acidify this solution with perchloric acid and dilute to a given volume. Desired Constituent. Transfer a 25-ml. aliquot of thia prepared solution to a 50-ml. volumetric flask and add 10 ml. of a 1 to 1 mixture of perchloric and phosphoric acids and sufficient water to bring the meniscus to the mark. Add 1 ml. of 3% hydrogen peroxide and mix thoroughly. Measure the absorbancy at 330 mp in 1.000-cm. quartz cells. LITERATURE CITED
SOC.Testing Materials, Philadelphia, “A.S.T.M. Methods of Chemical Analysis of Metals,” p. 25, 1946. (2) Funok, A. D., 2.anal. Chem., 68, 283 (1926). (3) Merwin, H.W., Am. J . Sei., 28,119 (1909). (4) Weissler, A,, IND.ENQ.CREM.,ANAL. ED., 17, 695-8 (1946). (6) Wells, R.C., and Grimaldi, F. S., Ibid., 15, 316-16 (1943).
( 1 ) .4m.
RECEIVED March 3, 1950. Presented before the Pittsburgh Conference on Analytical Chemiatry and Applied Spectrosropy, February 15 to 17, 1950, cosponsored by Analytical Division of Pittsbiirgh Bection, AMERICAN CAKMICAL SOCIETY, and Speotroacopic Society of Pittsburgh.
Determination of levulose in Fruit Polarographic Method K. T. WILLIAMS, ELIZABETH A. MCCOMB, AND E. F. POTTER Western Regional Research Laboratory, Albany, Calif.
A polarographic procedure for the determination of levulose in fruit is described. The sugars are extracted and prepared for measurement by conventional methods. The dextrose and sucrose present do not interfere and concentration8 of levulose varying from 0.05 to 2.0 mg. per ml. are suitable for measurement. Ion exchange resins are used to control pH.
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H E chemical methods for the determination of levulose are, in general, involved and time-consuming. I n many of the procedures now in use it is necessary to determine the dextrose and levulose together as well aa the levulose alone. An adjustment must then be made in the levulose value by the use of a factor and a series of approximations to compensate for the reducing power of the dextrose present. In some methods of analysis sucrose interferes and it must also be determined and a correction made. Because a direct determination of levulose, unhampered by dextrose or sucrose interference, is desirable, it seemed feasible to investigate the polarographic determination of this sugar.
Heyrovskf and Smolef ( 4 )found that although aldehydes are reduced a t the dropping mercury electrode, the aldoses (glucose, rhamnose, arabinose, mannose, galactose, lyxose) and disaccharides (maltose, lactose, sucrose) are not reduced. However, they found that ketoses (levulose and sorbose) give well defined polarographic waves. Koffnek and BabiEka (‘7‘) made analytical use of this reduction to follow the inversion of sucrose by the action of different bacteria. Heyrovsky, Smolef, and SttLstnp (6) determined levulose in wine. Heyrovskp (3)described its determination in the presence of sucrose and glucose in honey. Vavruch and Rubel (11) determined levulose in candy. Wiesner ( l a ) reported a polarographic investigation of the electroreduc-
