A Catalytic Method for Determining Traces of Selenium Philip W. West and T. V. Ramakrishna Coates ChemicaI Laboratories, Institute for the Environmental Sciences, Louisiana State University, Baton Rouge, La. 70803
A sensitive method for the determination of trace amounts of selenium based on its catalytic effect in the reduction of methylene blue by sodium sulfide is described. The method is very simple and rapid and works satisfactorily when color comparisons are made visually, in the range of 0.1 to 1.0 pg of selenium. Copper is the only ion tested that interfered seriously when present in excess of 10 pg: however, higher concentration of copper can be cleanly separated using an exchange procedure. THEMOST COMMONLY USED METHOD for the determination of selenium is based on the measurement of piazselenol spectrophotometrically or fluorimetrically when Se(1V) reacts with aromatic 0-diamines such as 3,3 ’-diaminobenzidine or 2,3diaminonaphthalene. Although these methods offer excellent sensitivity and specificity, the procedures are lengthy and often require close control of pH. Furthermore, the reagent solutions used are generally unstable and must be prepared daily. Based on the consideration of simplicity, sensitivity, and specificity, catalytic methods offer an excellent alternative choice for the determination of trace concentration of many elements ( I , 2). Kawashima and Tanaka (3) have recently proposed a very sensitive procedure for selenium based on the catalytic reduction of 1,4,6,1l-tetraazanaphthacene;but the method is subject to interference from several ions including tellurium. Feigl and West (4) developed a highly sensitive spot test for selenium based on the catalytic effect of elemental selenium on the reduction of methylene blue by sodium sulfide. Goto, Hirayama, and Ikeda (5) applied the findings of Feigl and West to develop a quantitative procedure for the determination of selenium. Their method, however, is neither sensitive, because at least 2 pg of selenium should be present to make satisfactory measurements, nor selective, because almost all cations that react with sulfide interfere. The present paper describes an investigation on the catalytic determination of selenium. The mechanism of the reaction has been discussed in detail by Feigl and West (4). Any deleterious effect caused by polysulfide is readily overcome by the addition of sodium sulfite to form thiosulfates which, like sodium sulfite, have no interfering actions on the reduction of methylene blue. In the method developed it was noticed that the addition of formaldehyde suppresses the reducing power of sodium sulfide and, therefore, stabilizes the blank considerably. Using EDTA as a general masking agent, the determination is made quite specific for selenium and by taking advantage of the inducing effect of Fe(III), as little as 0.1 pg of selenium can be measured. No instruments are required and the method is rapid, accurate, and simple. (1) K. B. Yatsimirskii, “Kinetic Methods of Analysis,” Pergamon Press, Oxford, 1966. (2) P. W. West, ANAL.CHEM., 23, 176 (1951). (3) T. Kawashima and M. Tanaka, Anal. Chim. Acra, 40, 137 (1968). (4) F. Feigl and P. W. West, ANAL.CHEM., 19,351 (1947). (5) H. Goto, T. Hirayama, and S. Ikeda, J. Chem. SOC.Japan, Pure Chem. Sect., 13,652, (1952).
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ANALYTICAL CHEMISTRY
0.25
-
‘-
.-
-
0.20
-
0.15
-
E
v
I-
OIO-
0
0.2
0.4
pg
0.6
0.8
1.0
of Selenium
Figure 1. Calibration curve for selenium
EXPERIMENTAL
Materials. Stoppered test tubes of 30-ml capacity, a stop watch that registers minutes and seconds, and a suitable pH meter are necessary. Stock selenium solution, containing 0.5 mg of selenium per milliliter, is prepared by dissolving 50 mg of pure selenium metal in a few drops (minimum required) of concentrated nitric acid, boiling gently to expel brown fumes, and making up to 100 ml with distilled water. Standard working solutions are prepared by appropriate dilution of the stock solution. Alkaline sodium sulfide solution (ca. 0.1M) is prepared by dissolving 2.40 grams of sodium sulfide, an equal weight of sodium sulfite, and 4 grams of sodium hydroxide in 100 ml of distilled water. This solution is stable for 2 days. Conditioner solution consists of 25 grams of EDTA (disodium salt), 0.4 of a gram of ferric chloride, and 50 ml of triethanolamine dissolved in distilled water and diluted to 1 liter. Methylene blue solution is 0.05% in distilled water. Formaldehyde solution is reagent grade (assay 36-382). Recommended Procedure. Transfer 10 ml of sample or an aliquot containing 0.1 to 1.0 pg of selenium into a test tube and dilute to 10 ml with distilled water. Add 5 ml of conditioner solution, 5 ml of formaldehyde solution, and 1 ml of alkaline sulfide solution. Mix the contents of the tube well after each addition. Add 2 drops of methylene blue solution and shake briefly to ensure complete mixing and start the stop watch to measure the time required for complete decolorization of the methylene blue. The amount of selenium present is obtained from a calibration graph constructed by plotting T-’ (min-1) us. micrograms for amounts varying from 0.1 to 1.0 pg of selenium and treated as above (Figure 1).
0.20
0.1 5
1
J
0.25
0.15
0.20
0.1
t i- 0.15
0.4
0.8
mi
1.2
1.6
0
2.0
0.1
I N NoOH
Figure 2. Effect of pH on the reaction rate
0.2 0.3 0.4
M g of iron
Figure 3. Effect of iron conceitration
.E C
-i-
0.10
0.0f
RESULTS AND DISCUSSION
Effect of pH. The optimum pH for the reduction to take place was established by adding different aliquots of 1 N sodium hydroxide to samples containing 1 pg of selenium which were then appropriately diluted with distilled water before the addition of methylene blue for the recommended procedure. The final volume remained constant in each instance. The pH of each of the solutions was measured after the reaction was complete. Figure 2 shows the effect of pH on the reaction rate from which it is evident that the rate remains constant between pH 10.6 and 11.1. Subsequent experiments were conducted at a pH of approximately 10.8 obtained when 1 ml of 4 sodium hydroxide solution was added to the solution. Under these conditions the blank was stable for at least 25 minutes which is more than sufficient for the measurement of as little as 0.05 pg of selenium. Effect of Iron Concentration. An unexpected enhancement in the rate of the selenium-catalyzed reaction of methylene blue with sodium sulfide was noticed when iron was added to the solution. It is reported that iron produces a black precipitate in the presence of EDTA and excess sulfide (6) and a cherry-red soluble complex, as was observed in the present investigation, in the presence of small amounts of sulfide (7). The color system was unstable and disappeared on standing. However, the reappearance of the red color on shaking suggested that oxygen is involved in the formation of the red complex. In the method described it is probable that upon addition of iron to the solution containing EDTA and sulfide, the faintly colored cherry-red complex is formed with the resultant removal of dissolved oxygen. Such action would create an ideal condition to initiate the reduction of methylene blue by seleno sulfide. In order to establish the optimum concentration of iron essential to accelerate the rate of reduction of methylene blue, experiments were performed as in the recommended procedure but with varying concentrations of iron. The rate of reaction increased with increasing amounts of iron up to 250 pg (Figure 3) and then remained constant through increased amounts of iron up to 500 pg. It was decided to carry out the reaction in the presence of about 300 pg of iron. It may be mentioned here that the formation of the red complex of iron-EDTAsulfide was unaffected even in the presence of triethanolamine which forms a colorless complex with iron(II1) in alkaline solutions. The addition of triethanolamine was found to be beneficial in stabilizing the blank, particularly in the presence of (6) R.Pribil, Collection Czech. Chem. Comm., 16/17,542 (1951). (7) T.S.West, Analyst, 17,630 (1962).
20
25
35
30
T e m p e r a t u r e 'C
Figure 4. Effect of temperature A: 1 pg Se;
B: 0.5
pg
Se; and C: 0.25
pg
Se
large concentrations of iron, perhaps by maintaining the higher oxidation state of iron. Effect of Other Reagent Concentrations. Because the reduction of methylene blue is caused by the sulfide ion, significant change in the rate of the reaction, as anticipated, was noticed as the concentration of sodium sulfide was varied. Although the reaction proceeds faster at higher concentration of sulfide solution, the blanks also became increasingly unstable and the analytical value of the catalytic effect of selenium was found to be of little significance. Based on the consideration of the stability of the blank as well as the speed of the reduction mechanism, 1 ml of a 0.1M solution was found to be ideal for control of the reaction. Because it is difficult to prepare accurately 0.1M solutions of sodium sulfide without involving considerable expenditures of time, the calibration curve should be checked each time before subjecting the samples to analysis of the selenium content. However, no significant deviation in the day to day reproducibility was noticed when solutions containing constant weight of sodium sulfide were used for analysis. In the very early stage of the investigation it was realized that the reduction of methylene blue by sodium sulfide should be arrested before the analytical value of the catalytic effect of selenium could be examined satisfactorily. Attempts were then made to achieve stabilization of the blank, without jeopardizing the catalytic effect of selenium, by adding various water-miscible organic solvents to the colored system. The solvents examined included acetone, ethanol, methanol, 1,4dioxane, diethylene glycol, 2-propanol, methyl ethyl ketone, and formaldehyde. Only formaldehyde was found to be promising as the other solvents either completely prevented the catalytic effect of selenium or were found to be ineffective. The optimum concentration of formaldehyde required was next established by treating 1 pg of selenium with 2,4,5,6, and 8 ml of formaldehyde solution as in the recommended procedure. Solutions were appropriately diluted with water so that the final volume remained the same in each instance. Although higher concentrations of formaldehyde effectively stabilized the blank, it was found that the rate of reduction of methylene blue in the presence of selenium was also significantly reduced. Lower concentration of formaldehyde, on the other hand, decreases the stability of the blank and thereVOL. 40, NO. 6, M A Y 1968
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Table I. Interference Studies Group I I1 111 IV V VI VI1 VI11
Li+, Na+, K+, Cu2+,Ag+.
Bet+, Mgz+, Ca2+, Sr2+, Ba2+,Zn2+,Cd2+,Hg2+. B ~ O ~Al3+, - , Ce4+.
HCQ-, co32-, Zr4+, Sn2+,Pb2+. NH~+,NO^-, Pods+,V03-, HAs012-, Sbb+,Bia+. SO,>-, S042-,Cr3+,Te032-, M0042-, wo4'. F-, C1-, Br-, I-, Mn2+. Fe3+, Co2+,Ni2+.
Table 11. Precision of the Proposed Method No. of Selenium, p g detns. Taken Found Std dev 8 8 8
1 .o 0.5
0.2
1.09 0.49 0.20
0.25 0.02 0.01
fore will affect the result when traces of selenium are involved. Taking into account the stability of the blank as well as speed of the reduction of methylene blue in the presence of selenium, 5 ml of formaldehyde was found to be satisfactory. It was also interesting to note the apparent lack of effect on the reaction when methylene blue concentrations were varied and the other parameters were held constant. Using the recommended procedure with 0.5 pg of selenium, the respective reaction times for the blank as well as the test reaction remained constant over the range of 0.2 to 1.0 ml of 0.05% of added methylene blue. Obviously, in the presence of excess sulfide, the rate of reduction of the methylene blue is a direct function of methylene blue concentration so that the time required for conversion to the leuco form remains constant. Two or 3 drops of 0.05 % solution added by means of a dropper gave sufficient color for measurement of reaction time. Effect of Temperature. Figure 4 illustrates the effect of temperature on the catalytic reduction of methylene blue. Although the rate of reduction increases with rise in temperature, a considerable decrease in the stability of the blank was also observed. It was, therefore, decided to perform the experiment at room temperature. Effect of Diverse Ions. Because of the reactivity of sulfide ion toward many cations, it was decided to investigate the catalytic effect of selenium in a medium containing EDTA. Table I lists the ions that were examined as potential interferents by the recommended procedure. The concentration of selenium in these experiments was 1 pg and that of interferent 50 pg. With the exception of copper, silver, bismuth, and antimony, none of the ions listed interfered in the determination. Antimony was found to inhibit the reaction slightly as
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ANALYTICAL CHEMISTRY
in its presence the recovery was reduced by 25 %. However, when the concentration of antimony was reduced to 25 pg, no interference was noticed. Copper, silver, and bismuth, on the other hand, interfered by giving higher recovery of selenium. When the concentrations of silver and bismuth were reduced to 25 pg each, and that of copper to 10 pg, no interferences were encountered. Higher concentration of copper was readily separated, however, from traces of selenium by passing the solution through a IO-cm long column prepared by filling a 25-ml buret with Dowex 50W-X2, 50 to 100 mesh, cation exchange resin at a flow rate of ml per minute. The eluate and washings should be evaporated, if necessary, so that the sample volume does not exceed 10 ml before subjecting to analysis. Effect of Standing Time. In order to study this effect a series of solutions containing 0.5 pg of selenium were treated with all the reagents except methylene blue as described in the recommended procedure. The solutions were allowed to stand different periods of time before the addition of methylene blue. Blanks were also run simultaneously. No change in the rate of reaction was noticed for solutions that stood up to 8 minutes after which the reduction of methylene blue became rapid both in blanks and samples. It was decided to add methylene blue as soon as other reagents have been added. Precision. The recovery of known amounts of selenium are shown in Table 11. The determinations were made over a period of several days. The results indicate that the recovery is satisfactory in spite of the fact that the color comparisons were made visually. Conclusions. The proposed procedure is almost completely free from interference from several anions and cations and should find wide application where traces of selenium are to be determined. Its outstanding features are that it is simple, highly sensitive, very rapid, and no special skill is necessary to operate the method as color comparisons can be made visually as precisely as in indicator titrations. A spectrophotometer was not used because there was some difficulty in obtaining an accurate absorbance measurement because of nonuniformity in the decolorization of the colored system particularly when selenium concentrations are low. In these circumstances the accuracy of visual comparison was unaffected because the solution can be tilted gently to complete the reduction. Another practical advantage is that in a matter of minutes several samples containing as low as 0.01 ppm of selenium can be determined simultaneously and should, therefore, be very convenient for routine analyses. RECEIVED for review December 27, 1967. Accepted February 28, 1968. This investigation was supported in part by Public Health Service Research Grants No. AP 00117 and AP 00128 from the National Center for Air Pollution Control, Bureau of State Services.
