Determination of selenium in technical sulphuric acid by flame atomic absorption spectrometry after electrochemical preconcentration

Chimica Acta, 131(1981) 37-43 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands Analytica

DETERMINATION OF SELENIUM IN TECHNICAL SULPHURIC ACID BY FLAME ATOMIC ABSORPTION SPECTROMETRY AFTER ELECTROCHEMICAL PRECONCENTRATION

BBRGE

HOLEN,

Department

RAGNAR

of Chemistry,

BYE and WALTER

University

LUND*

of Oslo. Box 1033, Blindern,

Oslo 3 (Norwcy)

(Received 20th July 1981)

SUMMARY Electrochemical preconcentration on a platinum filament, followed by atomization in an argon-hydrogen flame with simultaneous electrothermal heating of the filament, is used for the determination of selenium in technical sulphuric acid. The effect of experimental parameters such as acid co?lcentration, deposition potential and temperature are described, and the speciation of selenium in sulphuric acid is discussed. The technical acids were found to contain selenium in the range 30+50 pg 1-l.

Sulphuric acid is one of the most widely used industrial chemicals, and the trace element content of this acid is therefore of interest in many different contexts. The concentration of selenium in sulphuric acid is of special interest because the acid is used in the production of fertilizers. From an agricultural point of view, both high and low concentrations of selenium should be avoided, as the former may have toxic effects and the latter may cause deficiency diseases. There are few reports on the determination of selenium in sulphuric acid. Barcza and Sommer [l] have suggested the use of spectrophotometry with 3,3’diaminobenzidine in this analysis, after distillation of selenium tetrabromide in a microdistillation apparatus and extraction of the formed piazselenol into toluene. Shimoishi and Tdei [ 21 have used gas chromatography with electroncapture detection, after treatment with a redox buffer and subsequent extraction of 5-nitropiazselenol into toluene. Selenium in concentrated sulphuric acid is not easily determined by atomic absorption spectrometry. The concentration of this element is normally below the detection limit of flame atomization, and the high concentration of sulphate prevents the use of graphite-furnace atomization in this analysis [ 31. The latter fact was also confirmed in this work. The use of the hydride generation technique was also attempted, but serious interferences were observed [4] . In this work, electrochemical preconcentration on a platinum filament, followed by atomization in an argon-hydrogen flame with simultaneous electrothermal heating of the filament, is used for the determination of OOOS-2670/81/0000-0000/$02.50

0 1981 Elsevier Scientific Publishing Company

38

selenium in technical sulphuric acid. The technique, which is described in detail in a previous paper [5], has a very low detection limit for selenium, and in addition most chemical interferences are eliminated. Furthermore, the technique does not require the addition of chemical reagents. EXPERLMENTAL

Apparatus A Perkin-Elmer 303 atomic absorption spectrometer was used, with a Perkin-Elmer 359 recorder, an electrodeless discharge lamp and a three-slot burner head. For atomization, an argon-hydrogen flame was used, with flow rates of 11 end 2.4 1 min-’ of argon and hydrogen, respectively. A quartz tube with inner diameter 12 mm, length 100 mm and a circular hole midway along the wall, was placed in the optical path, with the central axis 27.5 mm above the burner head. The tube rested on nickel supports. The filament electrode was made from a spiral-wound platinum wire: details of the design are given elsewhere [5]. It was mounted in a holder similar to that used for the Delves Cup technique. The filament was heated electrically; a transformer and a Variac were connected to the filament elec&rode via a microswitch, which allowed the current to pass only when the filament. was correctly positioned in the flame just below the hole in the quartz tube [ 51. Selenium was deposited on the filament by controlled-potential electrolysis using a home-made potentiostat. The filament served as the working electrode, an Ag/AgCl/sat. KC1 electrode (Metrohm EA427) with an additional saltbridge filled with 0.09 M I12S04 was used as the reference electrode, and the counter electrode was a platinum coil. The electrolysis cell (Metrohm EA880, 20 mi) WZG equipped with a thermostatted jacket, through which water at 50°C circulated. The solution was stirred with a constant-rate (500 r-pm) magnetic stirrer during the electrolysis. When the electrolysis current exceeded the maximum output of the potentiostat, a twoelectrode circuit was used, which incorporated the filament electrode, the counter electrode and a d-c. power supply with adjustable output. The potential of the filament electrode was measured against an Ag/AgCI reference electrode, with a high-impedance voltmeter. Samples and solutions The technical sulphuric acid was obtained from the Norwegian manufacturer, Borregaard Industries Limited. Sulphuric acids of analytical-grade were obtained from Merck, Baker and Fluka, whereas acid of Suprapur quality was obtained from Merck. Standard solutions of selenium(IV) were prepared from analytical-grade selenous acid. A solution containing the Sea’+ ion was prepared by dissolving 25 mg of elemental selenium (Merck) in 1 1 of concentrated sulphuric acid [ 6 1_ The solution was heated to 5OGO”C, to speed up the dissolution process.

39

Further dilutions with concentrated acid were carried out to obtain solution with 25 pg Se l-‘. The water used was purified by a Millipore Milli-Q system.

a final

Procedure Dilute the sulphuric acid with water in the proportion 1 + 4. Transfer a 25-ml aliquot to the thermostatted cell and allow the temperature to reach 50°C before electrolysing with efficient stirring for 5 min at -0.8 V vs. Ag/AgCl. Remove the filament electrode from the cell, and rinse it with water and acetone before disconnecting the electrical circuit. Adjust the flow rates of the flame gases to 11 1 min-’ of argon and 2.4 1 min-’ of hydrogen, and ignite the flame. Let the filament dry for 30 s above the flame, ca. 10 cm above the burner (temperature 70-80°C). Ylace the filament electrode in the holder, and connect it to the transformer. Move the filament into its preadjusted position in the flame, directly below the hole in the absorption tube, and record the atomic absorption signal at 196.1 nm, as the microswitch triggers the electrothermal heating of the filament. The heating power to the filament should be as high as possible, without risking that the filament melts. In this work the power used was 41.2 W (4.05 V, 10.18 A). To new aliquots, add different volumes (lo-50 ~1) of a standard solution of selenium(IV) and finally calculate the concentration of selenium by the method of standard additions. RESULTS

AND

DISCUSSION

Effect of acid concentration F’reliminary experiments showed that the sulphuric acids contained very low concentrations of selenium. With a 1:lOO dilution the concentration of selenium was found to be below the detection limit of the method, unless an unreasonably long electrolysis time was used for preconcentrating the element. However, no deposition of selenium occurred in concentrated sulphuric acid, hence some dilution of the acid was needed. The deposition of selenium proved to be a function of the concentration of the sulphuric acid. The effect is illustrated in Table 1, where the peak height of the atomic absorption signai is shown for different concentrations of the acid. All solutions were spiked with selenium(N) (15 pg 1-l). The acid used was of Suprapur quality to avoid variations in the blank values of selenium. From Table 1 it can be seen that no atomic absorption signal is obtained in the concentrated acid, and that eventually the signal reaches a maximum value for the 1.8 M acid (1 + 9 dilution)_ For the analysis of technical sulphuric acid, a five-fold dilution (3.6 M acid) was normally preferred. A greater dilution was avoided, as this would result in correspondingly low selenium concentrations. Apart from the dilution, no reagents were added to the sample prior to the analysis. The variation in peak height with acid concentration reflects a change in the rate of deposition of selenium during the preconcentration. The fact that

40

TABLE

1 in peak

Variation

height

with

acid concentration

for a fied

concentration

(15 rg I-‘) of

selenium

Acid (mol

Dilution H,SO, * Hz0 acid

Cont. 4+1 3+2 2+3

cont. I-‘)

17.8 14.2 10.7 7.1

Absorbance

Dilution H,SO, + H,O

Acid cont. (mol I-‘)

Absorbance

0 0.029 0.053 0.076

1+4 1+9 1 + 24

3.6 1.6 0.7

0.126 0.136 0.089

selenium is not deposited on the filament electrode in concentrated sulphuric acid is probably explained by an inhibition of the charge-transfer reaction at the platinum electrode. A similar effect is apparently not observed at a mercury electrode [ 7]_ The change in the deposition rate may to a certain extent also depend on the rate of mass transport to the electrode surface. Here two experimental parameters are of particular interest, namely the viscosity of the solution and the evolution of hydrogen gas at the filament surface. The concentrated acid has a very high viscosity, which limits mass transport in this medium. The evolution of hydrogen gas, which increases the mass transport rate because of the “stirring” effect, was insignificant in the concentrated acid, but reached a maximum for the 3.6 M acid. Effect of deposition potential and time The variation in the atomic absorption signal with the deposition potential is shown in Fig. 1 for two concentrations of the acid. As can be seen from the curves, the deposition of selenium starts at +0.6 V in 0.09 M acid, and at -2 V in 3.6 M acid. The difference in reduction potential between the

OIZ-

007:

6p,.p

Y 004 g

OOLI

z 0°’ t/ 002

id-

06

OL

02

_-_.-i. 0

POTENTIAL

-02

-0L

_.i -06

““‘I 1 -0.6

I V vs. AglAg

-10

-12

in peak height

with

20

, 10

‘ LO

50

TEMPERATURE

Cl)

Fig. 1. Variation in peak height with deposition I-‘;(B) 3.6 M H,SO,, 6.8 rg Se(W) i-‘. Fig. 2.Variation

-’

temperature

potential:

for 6.8 rg

60

70

60

(CT

(A) 0.09 M H, SO,, 20 pg Se(IV)

Se(IV)

1-I in 3.6 M H,SO,.

two curves is thus 0.8 V, which again indicates that the reduction of selenium is inhibited at a platinum electrode in strong sulphuric acid. A similarly marked shift in the reduction potential with the acid concentration was not observed for the polarographic reduction of selenium(N) 181, which illustrates that the reaction depends strongly on the electrode material. In this work selenium was normally deposited at -0.8 V vs. Ag/AgCl/sat. KCI. A vigorous evolution of hydrogen gas occurred at this potential, and the corresponding high electrolysis current (above 2 A) almost exceeded the capability of the potentiostat. Therefore the use of a simple twoelectrode circuit for the preconcentration step was investigated. A d-c. power supply with adjustable output was found to serve the purpose well. The power supply was regulated so that the potential of the filament was -0.8 V. This potential was measured against an Ag/AgCl reference electrode by a high-impedance voltmeter. The potential between the filament and the counter electrode was then ca. 4.0 V. The accuracy and precision obtained with the two-electrode circuit, which could tolerate high currents, were the same as those obtained with the potentiostat. The current remained nearly constant during the electrolysis. A 5-min electrolysis time was mostly used in this work. Within this time, the depletion of the solution was insignificant, hence the separation was The amount deposited and consequently the virtually nondestructive. sensitivity of the method increased linearly with the deposition time. Effect of temperature on deposition rate The deposition rate was found to increase markedly with temperature, particularly in the lower temperature region, from 20 to 40°C. as illustrated in Fig. 2. The slope of the curve at 20°C was five times that of the linear region above 40°C. The temperature coefficient at 20°C was 12% deg-‘. The value, which was calculated from the expression ( ~OO~A/A)/LJT, where A is absorbance and T is temperature, is significantly higher than that normally found for simple inorganic ions in aqueous solution. There seems to be no simple explanation for the marked increase in the deposition rate at lower temperatures. However, the improvement in sensitivity was turned to account by employing a temperature of 50°C for the electrolytic preconcentration step. Higher temperatures were avoided for practical reasons. Speck&ion of selenium Normally, selenium exists in the tetravalent oxidation state in aqueous solutions. However, Barr et al. [6] found that when e!emental selenium is dissolved in concentrated sulphuric acid the polyatomic cation Sea2+, with a green colour, is formed. The ion may be further oxidized to See2*, which has a yellow colour. A solution containing the Sen2+ ion was prepared by dissolving selenium metal in the concentrated acid. The atomic absorption signal obtained for this solution (after dilution to 3.6 M acid) was then compared with the signal found for a corresponding solution where selenium

42

had been added as selenium(IV) ions. Identical peak heights were obtained for the two solutions, indicating that the method described in this work will give correct results whether selenium is present as selenium(IV) or in a lower oxidation state. However, if selenium were present as selenium(VI), it would not be deposited in the preconcentration step. Although the presence of selenium(VI) in sulphuric acid is unlikely, an aliquot of the acid was treated with hydrochloric acid, which would reduce any Se(V1) to Se(IV), and the atomic absorption signal was then recorded in the usual way. No increase in signal was obtained after this treatment, indicating the absence of selenium(V1) in the acid. Analysis

of different

acids

Acids of both technical and analytical grade were analyzed for selenium_ The results, which are given in Table 2, were obtained by using the method of standard additions_ For converting the results from ~g 1-l to pg kg-‘, the value 1.83 kg 1-l was used for the acid density. The samples were analyzed in triplicate both before and after each standard addition. Separate aliquots were taken for each determination. A linear relationship between concentration and atomic absorption signal was ascertained by adding increasing amounts of standard. The relative standard deviation of 20 measurements, incorporating four different concentrations of the standard, was found to be 5-4s. As can be seen from Table 2, all acids contained very low concentrations of selenium. The reagent-grade acids contained less than 2.5 pg I-‘; this value represents the detection limit of the method, when a five-fold dilution and a 5-min electrolysis are used. The results are significantly lower than those of Shimoishi and T&i [ 21, which were within the range 13-94 pg kg-‘. The values for the technical acids are also much lower than expected, a fact which is probably explained by a low content of selenium in the pyrite used in the production process. Attempts were made to obtain results for the technical acids by other techniques. Unfortunately, neutron activation analysis could not be used because of the risk involved in irradiating corrosive acids, and the atomic absorption procedure with hydride generation also proved unsuccessful, because of interference effects [4] _ l

TABLE 2 Results ‘@pe

for selenium ----.

in different - - -.--

of acid

sulphuric

Diluted (rg

Technical Technical

quality. quality, Analytical-gradeb

PBorregaard

acids

batch batch

Industries

1’ 2a

Concentrated

acid (1 + 4)

(rg

I-‘)

6.82 11.4

34.1 57.0 C2.5


Sarpsborg.

Norway.

I-‘)

bMerck,

Baker

and Fluke.

acid

(rg 18.6 31.1 < 1A

kg-‘)

43 REFERENCES 1 2 3 4 5 6 7

L. Y. G. R. B. J. E. B, 8 G.

Barcza and L. Sommer, Fresenius 2. Anal. Chem., 192 (1963) 304. Shimoishi and K. Taei, Talanta, 17 (1970) 165. C. Kunsebnau and E. A. Huff, At. Absorpt. Newsl., 15 (1976) 29. Bye, B. Holen and W. Lund, in preparation. Holen, R. Bye and W. Lund, Anal. Chim. Acta, 130 (1981) 257. Barr, R. J. Gillespie, R. Kapoor and K. C. Malhotra. Can. J. Chem., 46 (1968) 149. I. Patsauskas, I. V. Janitskij and E. P. Rinkjavitsjene, Tr. Akad. Nauk. Bit. SSR, Ser. 3 (1966) 55. D. Christian, E. C. Knoblock and W. C. Purdy, Anal. Chem., 35 (1963) 1128.