Anal. Chem. 1980, 52, 1540-1541
1540
Microdetermination of Selenium with Ascorbic Acid Fawzy F. El-Enany,* Kariman Mahmoud, and M. M. Varma' Office of Occupational Health and Industrial Hygiene, Ministty of Public Health, P.O. Box 5, Kuwait, Arabian Gulf
Selenium is a toxic metal; in humans i t concentrates in the liver, kidneys, spleen, pancreas, heart, and lungs. It is excreted largely in the urine; however, significant amounts are also found in the feces. It is a potential carcinogen-mutagen. One mechanism by which selenium exerts toxic effect is apparently through its competition with sulfur for the sites, for which sulfur plays a vital role in the metabolism. Possibly, it also substitutes sulfur in the enzymatic system, thereby dysfunctioning t h e SH group. The iodometric titration for quantification of selenium and its compounds with sodium thiosulfate is the most popular technique (1). T h e lower limit for measurement is in the neighborhood of 50 mg of selenium; otherwise, in dilute aqueous solution, the margin of error increases. Standard Methods ( 2 ) recommends the diaminobenzide colorimetric technique with or without distillation; both are long and time consuming techniques. T h e proposed method involves iodometric titration of selenium and its compounds with standard L-ascorbic acid; the sensitivity is up to 50 pg of selenium. Furthermore, the methodology can be applied for quantification of selenium in urine and blood.
Table I.
-
+
HO-C
H-F
il I 0
I
'I
'7
HO-C-H
I
CHzOH
+
mg
%
25.18 22.97 20.20 17.86 15.20 12.72
9.94 9.07 7.98 7.05 6.00 5.02 4.00 2.99 2.00 1.oo
0.60 0.77 0.37 0.7 1
10.14 7.58 5.06 2.54
" 1 mL of 0.02 N L-ascorbic acid selenium.
error,
_._
0.8
_-_
0.33
___ ___
= 0.3948
mg of
Table 11. Recovery of Selenium by the Proposed Method titer of origin a1 selenium content, Clg
0.04 N
L-ascorbic selenium acid soln," recovered, mL
1000
12.64 11.47 10.12 8.90 7.66 6.36 5.06 3.80 2.50 1.26 0.63
900 800 700 600 500 400 300 200
+ 3H20 (1)
100 50 a
error, %
0.19 0.63 0.12 0.40 0.81 0.44
99 6 906 799 703 60 5 50 2 400 300 197
___
-__ 0.13
-__
100
___
50
1m L of 0.004 N L-ascorbic acid = 78.96 pg of
selenium.
o=c
il1
1mmol(O.l76%) L-ascorbic acid solution in the presence of starch. A t the end point, blue color disappeared. (at. wt. Se) Of = (v) (mol. wt.) (L-ascorbic acid) (2)
H-C
'_I
II
2HI
selenium recovered,
0
II
12
L -ascorbic acid soh," mL
10 9 8 7 6 5 4 3 2 1
T h e liberated iodine is titrated with L-ascorbic acid in the presence of starch (Equation 2). 0
titer of 0.002 N
original selenium content, mg
THEORY The theory governing the proposed method is based on the fact that compounds of selenium such as sodium selenite, in the presence of hydrochloric acid, are readily reduced with excess potassium iodide, to selenium and liberate iodine. The reaction is stoichiometric at room temperature (Equation 1).
Na2SeOs + 6HC1 + 4 K I Se + 212 + KC1 + 2NaC1
Recovery of Selenium by the Proposed hlethod
o=c
HO-C
I
o
(2)
78.96 = (v) = (v)(C) ( 0 . 2 3 )
m
I
I
CH20H
L-Ascorbic acid is a n excellent reducing agent ( 3 ) ,and, kinetically, the reaction proceeds to the right. Since L-ascorbic acid has lower molecular weight than sodium thiosulfate, this probably also increases the sensitivity of the proposed method.
EXPERIMENTAL Five milliliters of 1 mmol solution (0.1729 g/100 mL) was transferred to a stoppered Erlenmeyer flask. Then 10 mL of dilute hydrochloric acid and 5 mL of 10% potassium iodide was added, the flask was well shaken for 2 min. Next, 10 mL of distilled water and 2 mL of chloroform were added to prevent the absorption of liberated iodine on selenium. The mixture was titrated with Dr. Varma is consultant of the Ministry of Public Health, Kuwait. 0003-2700/80/0352-1540$01 .OO/O
where V = volume of standard L-ascorbic acid solution and C = concentration of L-ascorbic acid, mg or pg/mL. Recovery of Selenium. Before applying the proposed method for the determination of selenium, it was decided to verify its quantitative recovery by following the procedure outlined earlier. The reaction was stoichiometric at room temperature. The results were: mL of sodium selenite solution ( 1 mL mo1/100 mL) 10
5
4
3
2
1
m L of titer, L-ascorbic acid solution (1mmo1/100 mL) 20.02
10
8
6.02
3.98
2
m L of titer, L-ascorbic acid solution ( 2 mmo1/100 m L ) 9.98 5 4.02 2.98 2 1.02
RESULTS A N D DISCUSSION T h e common interfering elements appear to be iron and copper. Iron can be masked by sodium fluoride and copper 0 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
Table 111. Comparative Analysis of Selenium by the Proposed and the Sodium Thiosulfate Method proposed method sodium thiosulfate method original 0.1 N sodium titer of sodium 0.1 N sodium selenite sodium L -ascorbic selenite selenite acid s o h found, thiosulfate, found, content, mg used,u mL mg error, % mL mg error, 50 40 30 20 10 9 8 7 6 5 4 3 2 1
49.99 40.08 29.92 19.98 10.03 8.99 8.00 7.00 5.97 5.02 3.98 3.03 1.99 0.99
11.55 9.27 6.92 4.62 2.32 2.08 1.85 1.62 1.38 1.16 0.92 0.70 0.46 0.23
___
--_ 0.5 0.4 0.5 1.0 0.5 1.0
a 1 mL of 0.1 N L-ascorbic acid = 4.324 mg of sodium selenite. sodium selenite.
200 170 140 110 80 50
2.30 1.94 1.64 1.26 0.91 0.59
vol. of
error, 7%
199 168 142 109 79 51
a 1 mL of 0.002 N L-ascorbic acid = 86.47 selenite.
%
1.40 1.60 1.63 1.80 2.30 2.90 2.60 2.60 2.66 3.20 2.50 5 5 5
1 mL of 0.1 N sodium thiosulfate = 4.325 mg of
vol. of
2 1.7 1.4 1.1 0.8 0.5
49.3 39.36 30.49 19.64 9.78 8.74 7.79 6.75 5.84 4.84 3.89 2.66 1.90 0.95
Table V. Determination of Selenite in Urine
Table IV. Determination of Selenite in Blood sodium titer of sodium blood selenite 0.002 N selenite filtrate, content, L-ascorbic found, mL pg acid, mLa pg
11.40 9.10 7.05 4.54 2.26 2.02 1.80 1.56 1.35 1.12 0.90 0.66 0.44 0.22
0.02 0.2 0.21 0.1 0.3 0.1
1541
0.56 1.17 1.42 0.91 1.25 2 pg
urine taken, mL 10 9 8 7 6 5
of sodium
by diphenylamine. T h e proposed method was compared with the sodium thiosulfate technique (I) for determining selenium in varying quantities. An aqueous stock solution of 0.2192 g/100 mL of sodium selenite containing 0.1 g/100 mL of selenium was prepared. A series of progressive dilutions were made. These were checked (as if unknown) by the two methods. The titrations for sodium selenite ranging from 50 to 1 mg were performed with 0.1 N L-ascorbic acid and 0.1 N sodium thiosulfate; for lower dilutions 0.004 N L-ascorbic acid was used. The results (mean of 5 tests) are presented in Tables 1-111. The proposed method was checked for its application in urine and blood, as described below. A stock solution of 0.5 g of reagent grade sodium selenite was made in distilled water. T o this was added animal blood t o bring the total volume to 100 mL to yield 500 pg sodium selenite/mL of blood. T o 10 mL of this solution (500 pg sodium selenite/mL of blood) was added 40 mL of 20% (w/v) trichloroacetic acid. T h e mixture was stirred for 10 min and filtered through Whatman 40. T h e filtrate was shaken with 1 g of activated carbon for 5 min and refiltered (1 mL of the filtrate had 100 fig of sodium selenite). Various known volumes of filtrate were titrated with 0.002 N L-ascorbic acid (Table IV). For determining selenium in urine, 0.1 g of sodium selenite was dissolved in 10 mL of normal urine. A set of serial solutions was made and checked for selenium (Table V). For lower concentration of selenium, a 500-mL urine sample containing a known concentration of sodium selenite was acidified with 10 mL of 1 N HC1 and evaporated a t 100 "C
original titer of sodium sodium selenite 0.002 N selenite content, L-ascorbic found, pg acid, mL pg 100 90 80 70 60 50
1.15 1.03 0.91 0.80 0.68 0.57
99 89 79 69 59 49
error, %
1.00 1.11 1.25 1.42 1.66 2.00
Table VI. Determination of Selenium in Urine after Concentration with Evaporation volume of urine taken, mL 500 500 500 500 500
original sodium titer of selenite 0.002 N content, L-ascorbic pg acid, mL 500 400 300 200 100
5.82 4.53 3.50 2.28 1.17
sodium selenite found, PJg
error, %
503.26 391.71 302.65 197.15 101.17
0.033 0.083 0.027 0.03 1.17
on a hot plate until the volume was approximately 50 mL. The analytical results by the proposed method are presented in Table VI.
CONCLUSIONS The experimental error of the proposed method does not exceed fl% in the concentration of selenium ranging from 10 mg to 50 pg (Tables I and 11);the error in biological fluids, blood and urine, increased to A270 (Tables IV and V) whereas the error in the iodometric test was higher ( 5 % ) when the lowest concentration was 1 mg, which means a t micro level the error will be greater. LITERATURE CITED (1) Vogel, A. I."A Text Book of Quantitative lnorganic Analysis. Inorganic Analysis"; Longmans, Green, London/New York. 1968; p 303. (2) American Public Health Association, "Standard Methods for the Examination of Water and Wastewater", 13th ed., 1970; pp 296, 298. (3) Barakat, M. Z.,et al. Microchem. J. 1964, 8, 131.
RECEIVED for review July 7 , 1978. Accepted April 14, 1980.
