density and the logarithm of the sugar concentration is linear. Effect of Sodium Sulfate on Recovery of Sugars. Four known mixtures of glucose, mannose, a n d xylose were weighed out and dissolved in 200 ml. of 18% sulfuric acid (simulated conditions following hydrolysis), Aliquots of 50 nil. were withdrawn immediately, neutralized, and carried through the chromatographic procedure. All of the known mixtures were run on the same day; three chromatograms were run for each mixture. Therefore, the standard curve represented the average of 12 density readings for each concentration level of the three sugar standards. The value for each sugar in the known mixtures was obtained from a n average of nine density readings. The results (Table I)
show that neither sodium bicarbonate neutralization nor paper partition separation in the presence of sodium sulfate seriously interferes with the recovery of the sugars. Analysis of Pulp Samples. Three wood pulp samples supplied by t h e Joint ACS-ASTM-TAPPI Committee on Chromatographic Methods were analyzed by t h e present procedure. The pulps were also analyzed by J. F. Saeman. His method ( 4 ) includes: acid hydrolysis with a relatively short primary and secondary stage; neutralization with barium hydroxide; separation with butanol, pyridine, and water; elution of the sugars from the paper; and estimation of the sugars by copper reduction. The results of both methods are shown in Table 11.
ACKNOWLEDGMENT
The authors n-ish to thank J. F. Saeriian of the Forest Products Laboratory for permitting use of his analytical data. LITERATURE CITED
Durso, D. F., il:[ueller, W. A , , -4s.~~.
CHEW28, 13386 (1956). Hanes, C. S., Isiherwood. F. A , , S a Lure 164, 1107 (1949). McFarren, E. F , Brand, K., R u b komki, H. R ., ANAL. CHEM.23, 1146 (1951). Saeman, J. F., RII3ore, TT. E., Mitchell, R. L., Nillett AI. A , , T a p p i 37, 336 (1954). RECEIVED for review October 16, 1957. Accepted February 6, 1958. Division of Cellulose Chemistry, 132nd Meeting, ACS, New York. ?;. Y., September 1957
Photono met ric Determination of Va nad ium and Chromium CLARK E. BRICKER and STEPHEN S. SCHONBERG Department of Chemistry, Princeton University, Princeton, N. J.
b The number of photons or the time of exposure to a mercury vapor lamp i s measured for determining chromium(VI) and/or vanadium(V). The photochemically sensitive solution of iron(ll1) and oxalic acid containing the chromium and/or vanadium i s exposed to measured increments of radiation and the formation of chromium(lll) or vanadium(lV) is followed photometrically. Because the quantum efficiency of these reductions remains essentially constant over a rather wide range of experimental conditions, the radiation from the lamp can b e considered to generate a titrant at a constant rate. The rate a t which the titrant is formed is determined b y calibrating the lamp with known amounts of chromium or vanadium. The effect o f a number of variables on this reaction has been determined.
G
RAO and Aravamudan (6) have shown that iron(II1) is photochemically reduced in the presence of excess citric acid and can then be titrated with a standard solution of sodium vanadate. They determined oxalic acid by “photochemically oxidizing” this acid quantitatively in the presence of excess iron(II1) sulfate and then titrating the iron(I1) formed (6). Later, Rao and coworkers (8, 9) reported that uranium(V1) is reduced photochemically in the presence of OPALA
922
ANALYTICAL CHEMISTRY
excess lactic acid or ethyl alcohol and the uranium(1V) can be titrated with sodium vanadate. No effort was made to measure the amount of light required for the reduction, but the solutions were exposed to ultraviolet radiation long enough to ensure a quantitative reaction. Previous work with actinometers and with most photochemical reactions in solution has shown that the quantum efficiency of the reaction tends to decrease with increasing time of exposure. This nonlinearity is usually attributed to the internal filter action of a product of the photolysis and/or to the decrease in the concentration of the reacting materials in solution (IO). On the other hand, the total energy of x-rays or gamma rays has been measured by chemical dosimeters which show linearity in the amount of reaction over at least a limited range of energy absorbed. Dewar and Hentz (1) have shown t h a t the amount of iron(I1) oxidized, which was followed either potentiometrically or photometrically, is directly related to the time of exposure to x-rays. I n this paper, a new idea in photolytic reactions is described, in which a constant quantum yield is realized throughout the reaction by having a product from the primary photolysis react with the substance to be determined and thereby return to its
initial oxidation state. This is analogous to the idea used in coulometry a t constant current, where an excess of an ion is added to serve as a n intermediate in the transfer of electrons and thereby a uniform current efficiency is maintained even when the concentration of the substance to be determined is lorn. A solution of iron(II1) and oxalic acid undergoes a photochemical reaction and iron(I1) is produced. The amount of iron(I1) formed per unit time with constant irradiation decreases with the time of exposure. If, however, some vanadium(V) is present with a large excess of iron(II1) and oxalic acid, the iron(I1) formed reduces the vanadium(V) to vanadium(1S’) and iron(II1) is regenerated. With these conditions, the amount of vanadium(1V) produced per unit time of exposure remains constant. Consequently, vanadium can be determined by measuring the time of exposure needed to reduce the vanadium(V) quantitatively. This paper describes such a photochemical method for the determination of vanadium and chromium, separately and in mixtures. Because the number of photons needed to complete the reaction is measured, this method has been called a photonometric determination. APPARATUS AND REAGENTS
A Beckman Model B spectrophotom-
eter was modified, as slion-n schematically in Figure 1, for following these photolytic determinations. The bottom plate of the cell compartment on the spectrophotometer was replaced with a brass plate which had an elevated platform 1 5 , g inches high and 2 inches square (Figure 1, b, c). The housing of a magnetic stirrer (Arthur H. Thomas Co.) was removed; the base of the stirrer was placed under the spectrophotonieter and positioned so that the magnet rotated within the elevated section of the brass plate. To eliminate any stray magnetic effects from the niagnetic stirrer on the spectrophotometer, the stirrer was always shut off when a spectrophotometric reading was taken. Furthermore, because the absorbance changed approximately 0.008, depending on which pole of the magnet pointed toward the photocell, a marker was placed on one side of the motor-driven magnet. The magnet was alu-ays turned so that the marker faced the front of the spectrophotometer. The top of the cell Compartment of the spectrophotometer was replaced with a Bakelite cover. A hole 3 inches in diameter was cut in this cover, so that the center of the hole was perpendicular to the light path of the spectrophotometer. A brass shutter was mounted on the Bakelite cover, so that it could be rotated quickly to close or open the hole in the cover completely. A small brass bolt was attached to the Bakelite cover to make contact with the brass shutter when the hole was approximately one third or more open. This bolt served as one contact for the switch, which operated a GraLab electric microtimer calibrated to 0.001 minute (Gray Laboratory and h9anufacturing Co., Dayton, Ohio). The other contact for the electric timer was the bolt on which the brass shutter pivoted. A Hanovia general utility mercury vapor lamp, Type 30600, 115 volts, 140 watts (Hanovia Chemical and RIanufacturing Co., Newark, N. J.), was mounted directly over the hole in the Bakelite cover. This lamp was fastened to a movable block of wood that had two holes in its under side. By having these holes fit over two stationary nails, the block of wood and the lamp could be repositioned accurately. To adjust and keep the voltage to the mercury vapor lamp constant, a Sola constant voltage transformer (Type CVH-1; 1000-watt capacity; catalog No. 23-13-210-01'43, Sola Electric Co., Chicago, Ill.) and a T'ariac with a 5ampere capacity were used. The voltage to the lamp was measured with an alternating current voltmeter and maintained a t 115 =t0.2 volts. A 250-ml. beaker was used for all determinations. Because the position of the beaker in the spectrophotometer was not critical, the beaker was positioned by eye, so that the light path of the spectrophotometer through the beaker was a maximum. A 16/s-inch Kel-F coated bar was used for stirring. Because the rate of stirring was thought to be critical, the rheostat of the magnetic stirrer was kept a t a position which gave approxi-
(b)
(a)
Figure 1. apparatus a.
b , c.
Schematic diagram of
Schematic diagram 1. Beckman Model B spectrophotometer 2. Bakelite cover for cell compartment 3. Mercury vapor lamp 4. Hole in Bakelite cover 5. Brass shutter 6. Magnetic stirrer 7. Leads for electric timer Top and side view o f brass plate for bottom of cell compartment
mately 250 r.p.m. of the magnet. The magnetic stirrer was connected to the constant voltage transformer. The sodium oxalate solution (O.Oi4M) was prepared by dissolving 20.0 grams of anhydrous sodium oxalate, reagent grade, in sufficient water to make 2 liters of solution. The iron(II1) solution (0.5M) was prepared by dissolving 60.25 grams of Fe?(SO&. (NH4)2S04. 24H20 in water containing 6 ml. of 6 M sulfuric acid and diluting to 250 ml. Four to 6 hours of stirring may be required to produce a homogeneous solution. Standard chromium(V1) solutions (0.1545N and 0.2012N) were prepared by dissolving weighed amounts of dried and cooled potassium dichromate in known volumes of distilled water. A sodium vanadate solution was prepared by dissolving 16.0 grams of ammonium metavanadate (Amend Drug and Chemical Co., Inc.) with 10 grams of sodium hydroxide in approximately 600 ml. of water. This solution was boiled about 0.5 hour, until the odor of ammonia was very faint. Then, 25 ml. of 6 X sulfuric acid was added to the cooled solution and some vanadic acid precipitated. This precipitate was dissolved by adding 8 more grams of sodium hydroxide. The resulting slightly alkaline solution was filtered and diluted to 1 liter. This solution was standardized against a standard ferrous sulfate solution and found to be 0.13iiS. EXPERIMENTAL
I n initial experiments, the photolysis of a solution of iron(II1) sulfate and oxalic acid was studied. The amount of iron(I1) formed \vas followed photometrically and was observed to decrease per unit time of exposure as the irradiation of the solution was continued. To make the yield of iron(I1) linear with exposure time, 1,lO-phenanthroline was added to the solution, so
that the iron(I1) would be coniplexed as it was formed. This modification gave very little improvement in the linearity of the amount of reduction taking place as a function of exposure time. Because a constant amount of reduction of iron(II1) alnays occurred during the first interval of irradiation nith fresh solutions of iron(II1) sulfate and oxalic acid, it seemed that if some substance were present to oxidize the iron(I1) formed and thereby regenerate the initial concentration of iron(III), the 6ame amount of reduction lvould be produced with a second equal interval of exposure. I t was decided t o investigate this idea, using vanadiuni(V) as the oxidant for iron(I1). The absorption curve of a solution containing 2 ml. of sodium vanadate and the quantities of reagents given in the recommended procedure indicated that the solution was virtually opaque to wave lengths shorter than 460 m9 and almost transparent to wave lengths longer than 525 mp. After this solution was exposed to the lumination of the mercury vapor lamp for 10 minutes, the same general absorption curve was observed, except for a decided absorption band with a A,,,, at i 5 0 mp. This absorption was shown to be characteristic of vanadium(1V) ; therefore, the formation of vanadium(1V) as a function of time can be followed by observing the increase in absorbance a t 750 mp. Another solution containing the same reagents was placed in the spectrophotometer and its absorbance a t 750 mp was adjusted by slit adjustment to read zero. After the magnetic stirrer was turned on, the solution was exposed to short measured intervals of radiation from the mercury vapor lamp. After each period of exposure, the stirrer was shut off and the absorbance of the solution was observed. The results of this experiment (Figure 2) indicate that the same amount of vanadium(1V) is apparently formed for each unit of exposure time until the limiting value is reached. When the same amount of vanadium(\') in 126 ml. of dilute sulfuric acid was reduced u-ith a standard iron(I1) sulfate solution. the same absorbance mas observed as shown for the limiting value in Figure 2. Therefore, the intersection of the two straight lines in Figure 2 must indicate the time of exposure necessary for quantitative reduction of vanadium(V). K h e n the amount of vanadium(") taken initially was varied from approximately 0.025 to 0.65 meq. and the concentration of the other reagents was kept constant, the time of exposure needed to reduce vanadiumcY) quantitatively to vanadium(1V) varied linearly with the concentration of vanadium (Figure 3). Therefore, determination of vanadium can be based on VOL. 30, NO. 5, M A Y 1958
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Figure 2.
Photonometric titration curves
-Titration
of vanadiurn(V)
- - - _Titration of chromiurn(V1)
4
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0
the exposure time needed to reduce vanadium(V) photochemically in a solution containing an excess of iron(II1) and oxalic acid. Because the results with vanadium(V) looked so promising, the reduction of chromium(V1) was investigated. The absorption curves of a solution containing potassium dichromate instead of sodium vanadate showed an absorpat 575 mp after tion band with a A,, the solution was exposed to the radiation from the ultraviolet lamp. This absorption band was shown to be due to chromium(II1); therefore, the absorbance a t 575 mp can be used to follow the formation of chromium(II1) in the solution. TT’ith 0.3 meq. of potassium dichromate in 125 ml. of solution containing 75 ml. of sodium oxalate, 5 ml. of iron(II1) sulfate, and 4 ml. of 6M sulfuric acid, the absorbance of the solution a t 575 mp was adjusted to zero. Then, by the technique described the absorbance due to the formation of chromium(II1) was followed as a function of exposure time (Figure 2). Even though formation of chromium(II1) is not as uniform with exposure time as the reduction of vanadium(V), it is comparatively simple to determine the irradiation time needed to produce the limiting absorbance in a titration graph. A plot of times needed to reduce various concentrations of potassium dichromate photochemically is s h o m in Figure 3. It is apparent immediately that the two linear calibration lines do not h a r e the same slope; therefore, there must be a difference in the quantum efficiency or in the mechanism that causes reduction of chromium(V1) and vanadium(V). When the photochemical reduction of mixtures of chromium(V1) and 924
ANALYTICAL CHEMISTRY
.2
.4
.6
.8
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Figure 3.
Calibration curves
-Vanadium Chromium
---_ vanadium(V) was studied, rather UIIexpected results were obtained. The reduction of chromium(V1) can lie followed a t 575 mp with only little interference caused by the absorption of vanadium(V) or vanadium(1T’). At 750 mp the reduction of vanadium(T’) to vanadium(1V) can be folloned with essentially no interference of the chromium system. Initially, therefore, for mixtures of chromium and vanadium, the reduction of chromium(T’1) n-as followed a t 575 mp until a marked change in the slope of the absorbance as a function of exposure time was detected. Then, the monochromator on the spectrophotometer was changed t o give 750 mp and the reduction of vanadium(V) was followed. The results for chromium mere always slightly high and those for vanadium low. Becawe simultaneous reduction of vanadium(V) was suspected during detection of the reduction of chromium(T’I), it was decided to follow the reduction of these mixtures entirely a t 750 mp. At this wave length, neither chromium(V1) nor chromium(II1) absorbs: therefore. if any vanadium(1V) was being produced concurrently with the reduction of chromium(VI), it would be detected. On the other hand, if no vanadium(1V) was formed, no increase in absorbance would be observed until reduction of chromium(V1) was complete. Figure 4 shows the results of a photonometric titration of a mixture of chromium(V1) and vanadium(V) using iron(111)-oxalic acid as the photosensitive reagents. Absorbance does not rise
Figure 4. Photonometric titration curve for a mixture
-Reduction of vanodiumh‘)
- - - - Reduction of chromium(V1)
End points determined from intersection of straight lines
appreciably until after the chromium(V1) is reduced. However, apparently a small amount of vanadium(1V) is produced concurrently, because a small absorbance reading was observed immediately after closing the shutter. This absorbance drifted back to nearly zero in 1 or 2 minutes, indicating that the vanadium(1T‘) formed was being reoxidized slowly by some unreacted chromiuni(T’1). After the absorbance reading for one exposure time became constant, the second exposure was made, and so on. However, as soon as all the chroniium(V1) was reduced, the ahsorhance readings were steady
almost immediately after the shutter had been closed. RECOMMENDED PROCEDURES
Determination of Vanadium(V). T o a solution in a 250-ml. beaker containing 75 m!. of 0.074.V sodium oxalate, 5 1111. of 0.5M iron(II1) sulfate, and 4 ml. of 6 M sulfuric acid, a d d a k n o m volume of t h e solution containing vanadium(V). Dilute t h e resulting solution mith water t o 125 ml. Place the solution in the spectrophotometer and set the iyave length on the instrument to 750 mp. Use the red-sensitive photocell, d d j u s t the slit setting so that the absorbance of the solution is zero. Turn on the magnetic stirrer and open the shutter rapidly, so that the solution is exposed t o radiation from the mercury vapor lamp. ilfter an appropriate interval (6 t o 120 seconds, depending on the amount of vanadium present), close the shutter rapidly and turn off the magnetic stirrer. Record the time of exposure and measure the ahsorbance of the solution. Continue alternately t o expose the solution and measure the absorbance and time of exposurc until a limiting value of absorbance or a marked change in the slope of the plot of absorbance against time is reached. Either plot the data as in Figure 2 to determine the point of intersection of the two straight lines or determine thiq time niathematicallv. From a calihration curve, ohtained under the same expcrinic~nt:il conditions using knon-n aniounts of vanadium(V), determine the amount of wnadium in the unknon-11 solution. Determination of Chromium(V1). ‘C7s(~ thc experimental conditions as foi the determination of vanadiuni(V). h u t iiw a !!-aye length of 575 n i p t o follon- this analysis. Determination of Chromium(VI) a n d Vanadium(V) in Mixtures. I-se exactly t h e same experimental conditions as t h e determination of vanad i i i m K ) . However, after the first fcw exposures t o the radiation from the mercury vapor lamp, do not record the ahiorhance for approximately 2 minutes or iintil the needle on the nicter stops drifting. After the absorbance of the solution .tarts to increase rapidly n ith exposure time, absorbance mav be rcmrdetl a. soon as the shutter i. closed arid tlrc stirrer is turned off. hecauqe thcrc ip no further detectable drift in tlir ah~orhancereading. Plot ahsorbance rc~:iding. against time of exposure, as in Figurc 4. The points of intersection of the straight lines indicate the time required for the reduction of chroniium(T-I) alone and for both chroniiuni(T’1) anti vanadiuni(T’). Because the reduction of chromium(T’1) and vansdium(T’) in mixtures appears to proceed TT ith the same quantum efficiencjas hen present separately, the amount of chroniium(V1) and vanadium(V) in
the mixture can be calculated from the calibration curves already available. RESULTS
The results obtained over a period of 4 months for the photonometric reduction of various amounts of vanadium(V) are shown in Table I. The average deviation for these 26 results is I.t1.5%. A similar tabulation for the reduction of chromium(V1) is given in Table 11. The average deviation for 13 results is 1.3%. The reduction of mixtures of chromium(V1) and vanadium(V) is shown in Table 111. The average times of exposure for up to 1 meq. of chromium(V1) and vanadium(V) in mixtures agree extremely well with the average times for the reduction of these substances alone. Furthermore, the average deviations are only slightly greater than for the single determinations. The tables indicate that over several months chrornium(V1) and vanadium(V)present either singly or in mixtures can be determined by the recommended procedures with a n average deviation of 2 to 3%. Because the intensity of the ultraviolet lamp decreases slon-ly with age, calibration curves cannot be expected to remain tlie same indefinitely. For the greatest possihle accuracy, a known amount of vanadium or chromium should be determined a t approximately the same time as tlie unknon n. DISCUSSION
I n the development of these photonometric procedures, the effect of many variables was studied, to find optimum conditions for obtaining reproducil3le results. Obviously, the first requirement for a n analytical procedure based on a photolysis reaction is a constant and reproducihle source of radiation An old Hanovia general utility niercurv vapor lamp was used initially. The results obtained with this lamp during 6 months became erratic rvith time and a nen lamp was substituted. The radiation from this nen lanip seemed to decrease about 4% during the first n eek of use, but during the following 4 month% the iiitenqity remained essential1)- constant. The reproducibility of the results in Table I, obtained oyer a period of 4 months, is evidence that the intensity of the radiation is constant. A constant voltage niust be supplied to the lamp. K h e n the voltage to the mercury vapor lamp vas raried from 110 to 115 to 118 to 120 volts, the exposure times needed to reach the end points in the recommended procedure for the same amount of vanadiuni(V) n-ere 6.13, 5.23, 4.96, and 3.73 minute.. respectirely. K i t h a constant-voltage transformer and a T’ariac, tlie voltage
Table 1.
Reduction
of Vanadium(V)
Exposure ExPosure Time to Reach Time Vanadium(V) Present, End Point, hfeq. Meq. Min. Vanadium(V) 0.275 0.138 0 689 0 275 0 413 0 551 1 377 0.275 0,0275 0.9640 . 275 0.689 0.551 0,273 0.275 0.689 0.551 0.275 0.413 0.275 0.689 0.275 0.275 0.275 0.551 0.275
Table II.
3.38 1.67 8 62 3 38 5 08 6 82 17.78 3.32 0.335 12.363.45 8.68 6.85 3.49 3.42 8.74 6.98 3.46 5.27 3.45 8.86 3 53 3.48 3.50 6.94 3.47
12.3 12.1 12 5 12 2 12 3 12 4 12 9 12.1 12.2 12.8 12.5 12.6 12.4 12.7 12.4 12.7 12.7 12.6 12.8 12.5 12.9 12.8 12.5 12.5 12.6 12.6 Av. 12.5 zk 1 . 5 %
Reduction of Chromium(V1)
E ~ ChrornirimTime (1-1) to Reach Present, End Point. Neq. 0 618 0 618 0 300 0 773 0 155 0 l5,5 0 164 11 2111 0 604 0 604 1 006 0 402 0 102
Eunosure ~ ~ Time Me?.
Chrominm-
(VI)
31in. 3 80 3 77 1 82 4 77 0 0 94 3 00
6 6 5 6 6 6
1
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3 93 3 911 G 60 2 50 2 55
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29 10 80 17 00 06
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6.51 6 46 6 56 6 21 6 34 Av. 6 . 23 f 3mc
was maintained a t 115 =t0.2 volts a t all times. To determine the optimum concentration of iron(III), oxalic acid, and sulfuric acid for the photolysis reaction, the effect of each of these reagents was studied systematically for the reduction of both vanadium(V) and chromium(V1). K i t h 75 ml. of sodium oxalate solution, 4 ml. of 6 M sulfuric acid, a constant amount of either vanadiurn(V) or chromium(VI), and sufficient n a t e r to make 125 mi. alrrays present, the exposure time needed to reach the end point vias determined for various concentrations of iron(II1) sulfate. These results shown in Figure 5 indicate that with 5 ml. of 0.5M iron(II1) sulfate preqent, the minimum time of exposure if required for the reduction of vanadiuni(V) . Furthermore, because the VOL. 30, NO. 5, MAY 1958
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Table 111.
Chromium(V1) Present, Meq.
Vanadium(V) Present, Meq.
0.464 0 618 0 618 0.155 0.464 0.773 0.201 0.201 1 006 0 402
0.551 0.138 0.413 0.689 0.689 0.689 0.275 0.591 0.275 0.138
concentration of iron(II1) is not very critical for the reduction of either chromium(V1) or vanadium(") as long as a certain minimum is present, it was decided to use 5 ml. of the iron(II1) sulfate solution in all experiments. The effect of the amount of sodium oxalate solution used in the recommended procedure for constant amounts of vanadium(") or chromium(V1) is shown in Figure 6. Because 75 ml. is about in the middle of the more horizontal portion of these two curves, this volume of sodium oxalate is used in the recommended procedures. Using the recommended procedures except for the amount of sulfuric acid added, the effect of this component was also studied for the reduction of both vanadium and chromium (Figure 7). The photolytic process causing the reduction of either vanadium(V) or chromium(V1) is not affected significantly if between 3 and 6 ml. of 6M sulfuric acid are present; when more than 10 ml. is added to a solution containing chromium(VI), a slow b u t detectable spontaneous reduction of the chromium in the dark is observed. To allow for the basicity of the vanadium(V) solution and any other basic salts that may be present in a n unknon-n and still keep withinthe optimum acid concentration, 4 ml. of 6JI sulfuric acid is used in the recommended procedure. When the amounts of reagents given in the recommended procedures were used but the solution was diluted to only 110 ml. instead of 125 ml., the exposure time required to reduce 0.275 meq. of vanadium(V) was 670 higher. Similarly, when a solution was diluted to 140 ml., the exposure time was 6% shorter. These results cannot be explained on the basis of a variation in absorbance due to a change in depth of solution through which the radiation of the mercury vapor lamp passes. Increasing the volume of solution from 125 to 140 ml. decreases the concentration of all reactants 12%, whereas the depth of solution in the beaker is increased 10%. Thus, the absorbance 926
maintained at 125 =k 1 ml. As further proof of this explanation, a beaker containing 125 ml. of solution \\-as elevated 4 mm. in the spectrophotometer. The time required to reach the end point was 8.5% lower than when the beaker was placed in its normal position. Studies were made in nhich other organic acids were substituted for the oxalic acid in the photolysis solution. K h e n 75 ml. of O.Oi4N tartaric acid was used in place of oxalic acid, more than three times as much exposure was required to reduce the vanadium. Furthermore, the apparent reduction of vanadium was not linear i\-ith exposure time, especially in the vicinity of the end point; consequently, it \\as virtually impossible to determine the exact end point. Essentially the same result was obtained Rhen citric acid was used in place of oxalic acid. With lactic or malonic acid substituted for oxalic acid, little or no photochemical reduction of vanadium was observed. When a mixture of i 5 nil. of 0.Oi4.V sodium oxalate and 25 ml. of 0.074M tartaric acid was used in place of sodium oxalate, the time of exposure needed to reach the end point for the reduction of vanadiumil-) was reduced from 1.50 to 1.44 minutes. With a similar mixture substituting citric for tartaric acid, the end point occurred a t 1.38 minutes. A mixture of sodium oxalate and lactic acid showed no difference in the time of exposure to reach the end point from a solution containing only sodium oxalate. Therefore, oxalic acid is by far the best organic acid to use in this photolysis reaction. Furthermore, lactic acid does not interfere and there is only a small interference with tartaric or citric acid. By using the reconiniended proce-
Reduction of Mixtures of Chromium(V1) and Vanadium(V)
ANALYTICAL CHEMISTRY
Exposure Time Meq. of Meq. of chromium(VI) vanadium(V) 6.39 6.03 6.00 6.84 6.06 6.32 6.00 6.00 6.45 6.02
12.3 11.7 12.6 12.2 12.7 12.7 12.8 12.8 12.8 12.3
of the solution with 140-ml. volume should be approximately 3% less than when the volume is 125 nil. Because the exposure time needed with 140 ml. would require greater absorption, the change in absorbance of the solution by dilution is a n implausible explanation of these results. On the other hand, the lamp is approximately 100 mm. from the surface of the solution when 125 ml. of solution is in the beaker. This distance is decreased 4 mm. by increasing the volume to 140 ml. The intensity of the radiation reaching the surface of the beaker is, therefore, approximately 8% greater for the shorter distance. Similarly, rvith a volume of 110 ml., the intensity of the radiation reaching the surface of the solution is 8% less. Thus, the more logical explanation of the variation of exposure times with changes in volume of solution merely involves a change in intensity of radiation reaching the solution rather than a change in absorbance. To keep this factor as reproducible as possible, the total volume of solution subjected to photolysis was
'r
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Figure 5.
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Effect of iron(ll1) sulfate concentration -Vanadium system Chromium system
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Figure 6.
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I I I I I I I I I 20 40 60 80 100 120 Sodurn O x a l a t e , m l .
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Figure 8. Comparison of photometric and potentiometric end points in photonometric titration of a mixture
dures, the effect of rate of stirring and temperature on the rate of the photolysis reaction was determined. Usually the rheostat for the magnetic stirrer was kept a t a setting of 30, which produced about 250 revolutions of the magnet per minute and the end point for 0.2i5 meq. of vanadiuni(V) occurred a t 3.45 minutes. When the setting of the rheostat was changed to 20 and then to 40, titration of the same amount of vanadium required 3.50 and 3.44 minutes, respectively. At least over a limited range, the rate of stirring appears to have a negligible effect on the efficiency of this reaction. The significance of temperature in this photolysis reaction was investigated between 14"and 36" C. At 14" C., approximately 1.5y0 more time was required to reach the end point for the reduction of vanadium(V) than a t room temperature, which was about 24" C. Ai 36" C. the end point occurred a t 2.5y0 less time. Essentially the same results were obtained for the
i2
3 , T
-Vanadium system Chromium system
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Effect of sulfuric acid concentration
Effect of sodium oxalate concentration
1
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-Vanadium system Chromium system
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I
4
reduction of chromium(V1) when the temperature was varied. Therefore. the temperature coefficient of this reaction appears to be not greater than approximately 0.2% per degree, which is also the value reported for the teniperature coefficient of the iron oxalate actinometer ( 2 ) . The effect of addition of various substances on the efficiency of this reaction was studied. When 1 gram of ammonium sulfate, ammonium nitrate, ammonium chloride, ammonium perchlorate, sodium bromide, or boric acid or 1 ml. of either glacial acetic acid or 90% formic acid mas added to the solution in the recommended procedure, no change could be detected in the exposure time needed to reduce vanadium(V) quantitatively. Similarly, no interference was found with 0.5 gram of sodium fluoride, sodium arsenate, cobalt sulfate, manganese sulfate, or potassium chromium sulfate or with 0.1 gram of potassium iodate. On the other hand, 7 ml. of 0.2.11 di-
sodium (ethylenedinitrilo)tetraacetate, 0.5 gram of ammonium molybdate. 1 gram of ammonium persulfate, or 1 ml. of 85% phosphoric acid retarded the reaction significantly. When 0.5 gram of copper sulfate or potassium ferricyanide mas added, a turbidity developed in the solution after it was evposed to radiation from the mercury vapor lamp. Obviously, this turbidity interfered with the photometric readings. The reciprocity law of this photolysis reaction was investigated for the reduction of both vanadium(V) and chromium(V1). Changing the intervals of exposure from 0.1 to 3.0 minutes gave no detectable change in the amount of vanadium(1V) formed per unit time of exposure. I n other words. the slope of the line in Figure 2 showing the formation of vanadium(1V) as a function of time of exposure remains constant, regardless of how many increments of radiation are used. Likewise, exactly the same time was required to reach the end point for the reduction of chromium(V1) with 0.5minute exposures as ivith a 3.5-minute exposure folloived by 0.5-minute intervals. No effort mas made to determine the quantum yield of this reaction absolutely. Instead, a comparison of this reaction with the uranyl oxalate actinometer was made. The solutions recommended by Noyes and Boekelheide (4) for the uranyl oxalate actinometer mere used, but under the conditions of exposure described here. Using a value of 0.55 for the quantum yield of the uranyl oxalate actinometer, the solution was found to be absorbing 5.7 x lo1' photons per second from the mercury vapor lamp. At the time these experiments with the actinometer rrere carried out, 11.9 minutes of esposure were required to reduce 1 meq. of vanadium(V) to vanadium(1V). The quantum efficiency of the iron(II1)-oxalic acid photolysis reaction for the reduction of vanadium(V) is, therefore, about 1.5. Hatcherd and Parker ( 2 ) VOL. 30, NO. 5 , M A Y 1958
927
have recently obtained 1.21 to 1.25 for the quantum yield of the iron(II1) oxalate actinometer with various wave lengths of ultraviolet light. This apparent difference in the quantum yields will be studied to determine if the presence of vanadium(V) in the iron oxalate system makes the photochemical reaction more efficient. Potentiometric as well as photometric measurements were used to follow the photolytic reduction of some mixtures of chromium(V1) and vanadium(V). The potentiometric measurements were made with a Leeds &- Northrup studenttype potentiometer with a 1 sq. em. platinum foil indicating electrode and a Leeds Bi: Northrup No. 1199-30 calomel reference electrode. The results for a solution containing 0.275 meq. of vanadium(V) and 0.309 meq. of chromium(V1) are shown in Figure 8. Even though the potentiometric end points agree very well with those obtained from the photometric data, it is apparent, because of the linear extrapolation, that i t is more convenient and probably more accurate to determine the end points photometrically. A very curious observation mas made from the potentiometric measurements. During the photochemical reduction of chromium(V1) in an iron(II1)-oxalic acid solution, the e.m.f of the platinum electrode us. S.C.E. increased approximately 150 mv. when the solution was exposed to the mercury vapor lamp. After the shutter was closed, the potential decreased slowly until a steady reading was observed; the time required to reach this value was several times longer than that required to reach a stable absorbance reading. As soon as the chromium(V1) was reduced completely, further exposures of the solution to the mercury vapor lamp produced only decreasing values of the measured e.m.f. Exactly the same phenomenon was observed when a solution containing the same concentrations of iron(II1) sulfate, oxalic acid. sulfuric acid, chromium(VI), and vanadium(V) was subjected to coulometric reduction a t a constant current of 20 ma. T h a t is, when the generating current was turned on, the e.m.f. of the galvanic cell increased but, again, gradually decreased to an apparent equilibrium value when the generating current 17-as shut off. No such effect was observed after the chromium(V1) was reduced completely and only vanadium(V) was being reduced. However, this phenomenon was not observed during a coulometric reduction of a similar solution containing all components except the oxalic acid. Furthermore, during the coulometric reduction of chromium(V1) in an iron(II1) -oxalic acid solution, the time re-
928
ANALYTICAL CHEMISTRY
quired to reach the end point from the potentiometric data was only 591 seconds, compared to a theoretical value of 980 seconds. On the other hand, with oxalic acid absent, the time required for the coulometric reduction of the chromium(V1) was within 1% of the theoretical value. The initial e m f . measured for a solution containing iron(II1)-oxalic acid and chromium(V1) was very steady for a t least 30 minutes. Therefore, no spontaneous reduction of the chromium(V1) by the oxalic acid would appear to be occurring. The reduction of chromium(V1) either photonometrically or coulometrically in a n iron(II1)-oxalic acid solution appears to be somewhat anomalous. Only 50% as much exposure time is required to reduce 1 meq. of chromium(V1) as for 1 meq. of vanadium(V) (see Tables I, 11, and 111). I n the coulometric reduction, 60% of the theoretical number of coulombs is required to reduce the chromium in a n iron oxalate solution. Rao and coworkers ( 7 , 11) have s h o n that oxalic acid and some other organic compounds interfere with the titration of ferrous sulfate by dichromate. They attribute this interference to a n induced oxidation of oxalic acid by dichromate Iyhich is initiated by the reaction of iron(I1) and dichromate. This explanation could account for the anomalous results observed in this work for the reduction of chromium(V1). However, the results reported by Gopala Rao and coworkers, as well as those described here, can be explained by assuming that some of the chromium(V1) in the presence of oxalic acid is reduced to a n intermediate oxidation state d i i c h has a higher reduction potential than the chromium(T’1)-chroniium(II1) couple. This would account for the increase in potential of an indicating electrode during the photonometric or coulometric generation of reducing agent in the presence of chromium(V1) and oxalic acid. This intermediate oxidation state of chromium can react spontaneously with oxalic acid to give chromium(II1). Thus, in the presence of oxalic acid, less than the theoretical amount of reductant must be added to reduce chromium(V1) to chromium(II1) because some of the reduction proceeds spontaneously. The difference between the theoretical and actual amounts of reductant that must be added to chromium(V-I) remains remarkably constant for a given set of experimental conditions, but varies greatly with changes in the oxalic acid concentration and the rate and method of adding the reductant. S o t only iron(I1) but other reducing agents in the absence of iron produce the same
effect with chromium(V1) in the presence of oxalic acid. The elucidation of this reaction as well as the identification of the intermediate oxidation state of chromium is under extensive study. So far, the mechanism of this photochemical reaction has not been elucidated. However, it appears that the absorption of photons produces iron(I1) in some way from the iron(II1)-oxalic acid solution [see work of Ingram et al. (3) on the detection of labile photochemical free radicals]. The iron(II), in turn, reduces either the chromium(V1) or vanadium(V). The fact that iron(I1) is a precursor in the reduction of chromium or vanadium was s h o m by precipitation of the blue ferroferricyanide when the reaction was attempted in the presence of potassium ferricyanide. When all other reactants except iron(II1) were present and exposed t o the radiation, no ferroferricyanide was detected. The fact that ferroferricyanide precipitates even vhen a large excess of vanadium(T’) is present suggests that precipitation of iron(I1) by the ferricyanide is a faster process than the reaction between iron(I1) and vanadium(V). ACKNOWLEDGMENT
The authors wish to acknowledge the work performed by Charles llackall during the initial stage of this investigation, and to thank the Fisher Scientific Co. for providing a Beckman spectrophotometer for this work. LITERATURE CITED
(1) Delyar, 11.-4., Hentz, R. R., A7ucleonics 13, 54 (1955). (2) Hatchard, C. G., Parker, C. A, Proc. Rou. Soc. (London) A235. 518 (1956). (3) Ingram, D. J. E., Hodgson, \I7. G I Parker, C. A,, Rees, W. T., Natitre 176, 1227 (1955). (4) Soyes, IT. A., Jr., Boekelheide, Y.,
Chapt. on Photochemical Reactions in ‘(Technique of Organic ChemisJry,” Vol. 11, pp. 105-8, Interscience Publishers. Xew York. 1948.
Rao, G. G., Aravamudan, G., Anal. C h i m Acta 13, 328 (1955).
Ihid.,p. 415. Rao. G. G., Ramanianevulu. J. V. S., Current Sei. (In8ia) is, 72 (1949): Rao, G. G., Rao, V. P., Rama Rao, AI. V., Anal. Chim. Acta 15, 97 11956).
Rao, G.’G., Rao, V. P., Venkatanima, N. C., 2. anal. Cheni. 150, 178 (1956).
Smorski, T. J., J . Am. Chern. SOC.79, 3655 (1957).
Vim-anadham, C. S., Rao, G. G.,
Current Sci. (India) 12, 327 (1943); 13, 180 (1944).
RECEIVEDfor review August 31, 1957. .Iccepted December 28, 1957.
