(6)-Xacdinh Selen DHXT-metylen Blue

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Catalytic Method for Determining Traces of Selenium

Philip W. West and T. V. RamakrishnaCoates ChemicaI Lab oratories, Institute fo r the Environmental Sciences, Louisiana S tate University, Baton Roug e,La. 70803

A sens i t ive method for the determinat ion of t raceamounts of selenium based on i ts catalyt ic effect in thereduct ion of methyl ene b lue by sodium sul f ide i s de-scr ibed . The method i s very s imple and rap id andworks sa t i s factor i ly when color com par i sons are mad evisual ly, in the rang e of 0.1 to 1.0 pg of selen iu m. Cop-per i s the only ion tes ted that in ter fered ser ious lywhen present in excess of 10 pg: how ever, hig herconcentrat ion of copper can be cleanly separatedusing an exchange procedure.

THEMOST COMMONLY USED METHOD for the determination ofselenium is based o n the m easurement of piazselenol spectro-photometrically or fluorimetrically when Se(1V) reacts witharom atic 0 -diam ines such as 3,3 -diaminobenzidineor 2,3-diamino naphthale ne. Althoug h these methods offer excellentsensitivity and specificity, the procedures are lengthy a nd oftenrequire close control of pH. Furt herm ore, the reagent solu-tions used are generally unstable and must be pre pared daily.

Based on the consideration of simplicity, sensitivity, andspecificity, catalytic methods offer an excellent alternativechoice for the determination of trace concentration of manyelements I , 2 ) . Kawashima and Tanaka 3) have recentlyproposed a very sensitive procedure fo r selenium based on thecatalytic reduction of 1,4,6,1l-tetraazanaphthacene; ut themethod is subject to interference from several ions includingtellurium. Feigl an d West 4 ) developed a highly sensitivespot test for selenium based on the catalytic effect of elementalselenium on the reductionof meth ylene blue by sodium sulfide.Got o, Hirayama, and Ikeda 5) applied the findings of Feigland West to develop a quantitative procedure for the determi-nation of selenium. Their method, however, is neither sensi-tive, because at least 2 pg of selenium should be present tomake satisfactory measurements, nor selective, because almostall cations that react w ith sulfide interfere.

The p resent paper describes an investigation on the catalyticdeterm ination of selenium. The mechanismof the reactionhas been discussed in detail by Feigl and West 4 ) . Anydeleter ious effect cause d by polysu lfide is readily overc ome bythe add ition of sodium sulfite to form thiosulfates which, likesodium sulfite, have no interfering actions on the redu ctionofmethylene blue. In the method developed it was noticed thatthe addition of formaldehyde suppresses the reducing pow erofsodium sulfide and, therefore, stabilizes the blank consider-ably. Using EDTA as a general masking agent, the deter-mination is made quite specific for selenium and by taking ad-vantage of the inducing effect of Fe(III), as little a s0.1 pg ofselenium can be measured.

No instruments are required and the method is rapid,accurate , and simple.

1) K. B. Yatsimirskii, Kine tic Methods of A nalysis, Pergamon

(2) P. W . West, ANAL.C H E M . ,3, 176 (1951).(3) T. Kawashima and M. Tanaka, Anal. Chim. Acra, 40, 137

(4) F. Feigl andP. W. West, ANAL. HEM . ,9,351 (1947).(5) H. Goto, T. Hirayama, and S. Ikeda, J . Chem. SOC. apan,

Press, Oxford,1966.

(1968).

Pure Chem. Sect. 13,652, (1952).

0.25

0 2 0

- 0.15.-E

-I O I O -

0 0 .2 0 4 0 6 0 8 1.0

pg o f S e l e n i u m

Figure 1. Calibration curve for selenium

EXPERIMENTAL

Materials. Stoppered test tubes of 30-ml capacity, a stopwatch that registers minutes and seconds, and a suitable pHmeter are necessary.

Stock selenium solution, containing0.5 mg of seleniumper milliliter, is prepared by dissolving50 mg of pure seleniummetal in a few drops (minimum required) of concentratednitric acid, boiling gently to expel brown fumes, and makingup to 100 ml with distilled water. Stan dard working solu-tions are prepared by appropriate dilution of the stock solu-tion.

Alkaline sodium sulfide solution ca. 0 . 1 M ) is prepared bydissolving 2.40 grams of sodium sulfide, an equal weightofsodium sulfite, and 4 gramsof sodium hydroxide in 100 mlof distilled water. This solution is stable for2 days.

Conditioner solution consists of 25 grams of EDTA (di-sodium salt), 0.4 of a gramof ferric chloride, and 50 ml of tri-

ethanolam ine dissolved in distilled water an d diluted t o 1 liter.Methylene blue solution is 0.05 in distilled water.Formaldehyde solution is reagent grade (assay 36-3 82).Recommended Procedure. Transfer 10 ml of sample or an

aliquot containing 0.1 to 1.0 pg of selenium into a test tubeand dilute to 10 ml with distilled water. Add5 ml of condi-tioner solution, ml of formaldehyde solution, and1 ml ofalkaline sulfide solution. Mix the contents of the tube wellafter each addition. Add 2 drops of methylene blue solutionand sh ake briefly to ensure complete mixing and start the sto pwatch to measure the time required for complete decoloriza-tion of the methylene blue. The amountof selenium presentis obtained from a calibration graph constructed by plottingT (min-1) us micrograms for amounts varying from0.1 t o1.0 pg of selenium and treated as above (Figure 1).

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0 20

0.15

0.1 t0.4 0 8 1.2 1 6 2 0

m i I N N o O H

0.1 5 1

0 0.1 0 2 0.3 0 4

M g o f i r o n

Figure 2. Effect of pH on the reac- Figure 3. Effect of iron con-

0 2 5

0 2 0

i .1 5

.

E

-i 0.10

tion rate ceitration

0 0

RESULTS AND DISCUSSION

Effect of pH. The optimum pH for the reduction to takeplace was established by adding different aliquotsof 1 Nsodium hydroxide to samples containing1 pg of seleniumwhich were then app ropriately diluted with distilled waterbe-fore the addition of m ethylene blue for th e recommended pro-cedure. The final volume remained constant in each in-stance. The pH of eachof the solutions was measured afterthe reaction was complete. Figure2 shows the effect of p Hon the reaction rate from which it is evident that the rate re-mains constant between pH 10.6 an d 11.1. Subsequent ex-periments were conducted at a pH of approximately 10.8 ob-tained when ml of 4 sodium hydroxide solution was adde dto the solution. Under these conditions the blank was stablefor at least 25 minutes which is more than sufficient for themeasurement of as little as0.05 pg of selenium.

Effect of Iron Concentration. An unexpected enhancem entin the rate of the selenium-catalyzed reaction of methyleneblue with sodium sulfide was noticed when iron was addedto the solution. It is reported that iron produces a blackprecipitate in the presenceof ED TA and excess sulfide 6) and

a cherry-red soluble complex, as was observed in the presentinvestigation, in the presence of small amounts of sulfide7).The color system was unstable and disappeared on standing.However, the reappearance of the red color on shaking sug-gested that oxygen is involved in the form ationof the red com-plex. In the method described it is probable that upon addi-tion of iron to the solution containing ED TA and sulfide, thefaintly colored cherry-red complex is formed with the resul-tant remov al of dissolved oxygen. Such action would createan ideal condition to initiate the reduction of methylene blueby selen o sulfide.

In order to establish the optimum concentrationof iron es-sential to accelerate the rate of reduction of methylene blue,experiments were performe d as in the recommended procedure

but with varying concentrations of iron. The rate of reactionincreased with increasing amounts of iron upto 250 pg (Figure3) and then remained constant through increased amounts ofiron up to 500 pg. It was decided to carry out the rea ction inthe presence of about 300 pg of iron. It may be mentionedhere that the formation of the red complex of iron-EDTA-sulfide was unaffected even in the presenceof triethanolaminewhich forms a colorless complex with iron(II1) in alkaline solu-tions. The addition of triethanolamine was found to be bene-ficial in stabilizing the blank, particularly in the presence of

6) R. ribil, Collection Czech. C hem.Comm. 16/17, 42 1951).7) T . West, Analyst ,17,630 1962).

2 0 2 5 3 0 35

Te m p e r a t u r e C

Figure 4. Effect o temperature

A: 1 p g Se; B: 0.5 p g Se; and C : 0.25 p g Se

large concentrations of iron, perhaps by maintaining thehigher oxidation state of iron.

Because the re-duction of methylene blue is caused by the sulfide ion, signif-icant change in the rate of the reaction, as anticipated, wasnoticed as the concentration of sodium sulfide was varied.Although the reaction proceeds faster at higher concentrationof sulfide solution, the blanks also became increasinglyunstable and the analytical value of the catalytic effect ofselenium was found to be of little significance. Based onthe consideration of the stability of the blank as well as thespeed of the reduction mechanism, 1 ml of a 0.1 M solution wasfound to be ideal for controlof the reaction. Because it isdifficult to prepare accurately 0.1 M solutions of sodium sulfidewithout involving considerable expendituresof time, the cali-bration curve should be checked each time before subjectingthe samples to analysis of the selenium content. However, nosignificant deviation in the day to day reproducibility wasnoticed when solutions containing constant weight of sodiumsulfide were used for analysis.

In the very early stageof the investigation it was realizedthat th e reduction of methylene blue by so dium sulfide shouldbe arrested before the analytical valueof the ca talytic effect ofselenium could be examined satisfactorily. Attemp ts werethen made to achieve stabilization of the blank, withoutjeopardizing the catalytic effect of selenium, by adding va riouswater-miscible organic solvents to the colored system. Thesolvents examined included acetone, ethanol, methanol, 1,4-dioxane, diethylene glycol, 2-propanol, methyl ethyl ketone,and formaldehyde. Only formaldehyde was found to bepromising as the o ther solvents either completely prevented thecatalytic effect of selenium or w ere found t o be ineffective.

The optimum concentration of formaldehyde required wasnext established by treating 1 pg of selenium with 2, 4 ,5 ,6 , and8 ml of formaldehyde solution as in the recommended proce-dure. Solutions were appro priately diluted with watersothat the final volume remained the same in each instance.Although higher concentrations of formaldehyde effectivelystabilized the b lank,it was found that the rate of reduc tion ofmethylene blue in the presenceof selenium was also signifi-cantly reduced. Lower concentration of formaldehyde, onthe other hand, decreases the stability of the blank and there-

Effect of Other Reagent Concentrations.

VOL. 40 NO. 6, M A Y 1968 * 967


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GroupII1111IVVVIVI1VI11

Table I. Interference Studies

Li+, Na+, K+, Cu2+,Ag+.Bet+, Mgz+, Ca2+, Sr2+, Ba2 +,Zn2+,Cd2+,Hg2 .B ~ O ~ - ,l3+, Ce4+.HCQ-, co3 2-, Zr4+,Sn2+,Pb2+.N H ~ + ,NO^-, Pods+, 0 3 - , HAs012-,Sbb+,Bia+.SO,>-,S042-,Cr3+,Te032-, M0042-,wo4'.F-, C1-, Br-, I-, Mn2+.Fe3+, Co2+ ,Ni2+.

Table 11. Precision of the Prop osed Meth odNo. of Selenium,p gdetns. Taken Found Std dev

8 1 o 1.09 0 .258 0 5 0 .49 0.028 0 . 2 0.20 0.01

fore will affect the result when trace s of selenium a re involved.Taking into acc ount the stability of the blank as well as speedof the reduction of methylene blue in the presence of selenium,5

ml of formaldehyde was foun d to be satisfactory.It was also interesting to no te the ap parent lack of effect onthe reaction when methylene blue concentrations were variedand the other parameters were held constant. Using therecommended procedure with0.5 pg of selenium, the respec-tive reaction times for the blank as well as the test reaction re-mained constant over the range of 0.2 to1.0 ml of 0.05 ofadded methylene blue. Obviously, in the presence of excesssulfide, the rate of reduction of the methylene blue is a directfunction of methylene blue concentration so that the time re-quired for conversion to the leuco form remains constant.Two or 3 drops of 0.05 solution added by meansof a drop-per 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 th e rate of reduction increases with rise in tempera-ture, a considerable decrease in the stability of the blank wasalso observed. It was, therefore, decided to perform theexperiment at room temperature.

Because of the reactivity of sulfideion toward many cations, it was decided to investigate thecatalytic effect of selenium in a medium containing EDTA.Table I lists the ions that were examined as potential inter-ferents by the recommended procedure. The concentrationof selenium in these experiments was pg and thatof interfer-ent 50 pg. With the exception of copper, silver, bismuth, andantimony, none of the ions listed interfered in the determina-tion. Antimony was found to inhibit the reaction slightly as

Effect of Diverse Ions.

in its presence the recovery w as reduced by25 . However,when the concentration of antimony was reduced to25 pg, nointerference was noticed. Cop per, silver, and bismuth, on theothe r hand , interfered by giving higher recovery of selenium.When the concentrationsof silver and bismuth were reducedto 25 pg each, and that of copper to 10 pg, no interferenceswere encountered. Higher concentration of copper wasreadily separated, however, from traces of selenium by passingthe solution through a IO-cm long column prep ared by fillinga25-ml buret with Dowex 50W-X2,50 to 100 mesh, cation ex-

change resin at a flow rate of ml per minute. The eluateand washings should be evaporated, if necessary,so that thesample volume does not exceed10 ml before subjecting toanalysis.

In order to study this effect aseries of solutions containing0.5 pg of selenium were treatedwith all the reagents except methylene blue as described in therecommended procedure. The solutions were allowed tostand different periods of time before the addition of methyl-ene blue. Blanks were also run simultaneously. N o changein the rate of reaction was noticed for solutions that stood upto 8 minutes after which the reduction of methylene blue be-came rapid both in blanks and samples. It was decided toad d methylene blue as soon as other reagents have been added .

The recovery of known amounts of seleniumare shown in Table 11. The determinations were made over aperiod of several days. The results indicate that the re-covery is satisfactory in spite of the fact that the color com-parisons were m ade visually.

Conclusions. The proposed procedure is almost com-pletely free from interference from several anions and cationsand should find wide application where traces of seleniumare to be determined. Its outstanding features are that itissimple, highly sensitive, very rapid, and no special skill isnecessary to ope rate the method as color comparisons can bemade visually as precisely as in indicator titrations. Aspectrophotometer was not used because there was somedifficulty in obtaining an accurate absorbance measurementbecause of nonuniformity in the decolorizationof the coloredsystem particularly when selenium concentrations are low.In these circumstances the accuracy of visual comparison wasunaffected because the solution can be tilted gently to com-plete the reduction. Another practical advantage is that in amatter of minutes several samples containing as low as0.01ppm of selenium can be determined simultaneously andshould, therefore, be very convenient for routine analyses.

RECEIVEDor review December 27, 1967. Accepted Februa ry28, 1968. This investigation was support ed in part by PublicHealth Service Research Grants No. AP 00117 and AP00128 from the National Center for Air Pollution Control,Bureau of State Services.

Effect of Standing Time.

Precision.

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(6)-Xacdinh Selen DHXT-metylen Blue