The Determination of Traces of Arsenic: A Review - ACS Publications

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T h e Determination of Traces of Arsenic: A Review*

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 31, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0007.ch002

YAIR TALMI and CYRUS FELDMAN Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830**

This review is organized in terms of the main steps and topics characterizing a l l trace analysis: sample pretreatment and dissolution, solution stability, preconcentration, isolation and determination. The options now existing in each step have been presented, but no attempt is made to mention every investigation in which each has been used. It is hoped that this treatment will make i t easier for the analyst to assemble a procedure suited to the needs of a particular case. Pretreatment and Dissolution of the Sample If total arsenic is to be determined, the sample must be mineralized at least to the degree necessary to convert a l l arsenic present to inorganic forms. The treatment chosen should be the mildest treatment which will accomplish this conversion, so that losses and contamination will be minimized. Several approaches are possible:*** 1.Wet Ashing. According to Portmann and Riley1, prolonged digestion with nitric acid, followed by evaporation to dryness (hot-plate surface temperature ~180 ) will give no losses of arsenic, even i f several mg of chloride were originally present. This type of attack could be used for materials such as vegetation * Research supported by the National Science Foundation—RANN (Environmental Aspects of Trace Contaminants) Program under NSF interagency agreement No.389 with the U.S. Atomic Energy Commission. **Operated for the U.S. Atomic Energy Commission by Union Carbide Corporation under Contract No. W-7405-Cong.-26. *** A general and more detailed treatment of this subject is given in T.T. Gorsuch's book "The Destruction of Organic Matter", Pergamon, NYC (1970). º

13 Woolson; Arsenical Pesticides ACS Symposium Series; American Chemical Society: Washington, DC, 1975.


14

ARSENICAL PESTICIDES

and animal muscle. Stronger treatment i s u s u a l l y advocated f o r other organic m a t e r i a l s , e s p e c i a l l y those high i n l i p i d s ; however, Kingsley and S c h a f f e r t ^ s t a t e that a l l of the a r s e n i c present i n l i v e r , kidney o r muscle can be recovered merely by d i g e s t i n g the b l e n d e r i z e d t i s s u e f o r 30 minutes i n ^4N HC1. The d i g e s t i o n must be open to the atmosphere, so that s u l f i d e s and mercaptans (which may i n t e r f e r e w i t h subsequent determination of As) w i l l be e l i m i nated. S i m i l a r l y , R.F. Abernethy and F.H. Gibson** s t a t e that e s s e n t i a l l y 100% o f the a r s e n i c a l species i n c o a l can be e x t r a c t e d by b o i l i n g the powdered c o a l gently i n a 1:7 mixture of HNO3 and H 0. The HNO3 may then be e l i m i n a t e d , i f d e s i r e d , by adding 1:1 H2SO4 and evaporating the s o l u t i o n to fumes. According to R.B. B a i r d , S. Pourian and S.M. G a b r i e l i a n , raw sewage and primary and secondary e f f l u e n t s can be m i n e r a l i z e d by r e f l u x i n g with 4% HNO3 + 1% H2O2, followed by evaporation and f u r t h e r a d d i t i o n s o f HNO3. Cacodylates sprayed on sandy s o i l can be recovered by 5 minutes of shaking w i t h a mixture o f strong H2SO4 and HC1 p l u s d e c o l o r i z i n g carbon. Hamme, Young and Hunter-* s t a t e that recovery by t h i s r a p i d method averages 93%. Other treatments have a l s o been advocated which do not necess a r i l y m i n e r a l i z e b i o l o g i c a l samples completely, but l i b e r a t e their metals to a degree s u f f i c i e n t f o r some types of a n a l y s i s . W. J . A d r i a n , f o l l o w i n g G.W. Gordon, l e f t the sample overnight, w i t h HNO3 and HCIO4, i n a t i g h t l y sealed polyethylene b o t t l e . The b o t t l e was then placed i n hot running water f o r 2-3 hours, cooled and opened. Rapid d i s s o l u t i o n of t i s s u e s was a l s o achieved by A. Bouc h a r d using coned. H2SO4, Cr03 and red fuming HNO3 ( p o s s i b l e contamination from reagents must be considered i n t h i s case). Many t i s s u e s can a l s o be transformed i n t o c l e a r s o l u t i o n s with the a i d of tetramethylammonium hydroxide, e i t h e r i n s o l i d o r aqueous s o l u t i o n form** or as a s o l u t i o n i n toluene^. Heating a t 60°C may be r e q u i r e d . Many o f these milder procedures do not a t t a c k f a t t y and h i g h - l i p i d t i s s u e s or bone and tooth specimens. When stronger treatment i s needed, some combination of n i t r i c , s u l f u r i c and p e r c h l o r i c acids i s u s u a l l y used. In a t y p i c a l p r o cedure of t h i s type, Chu, Barron, and Baumgarner^ a l t e r n a t e l y heat the sample i n a mixture of s u l f u r i c and n i t r i c acids u n t i l the s o l u t i o n darkens, then add more HNO3, and repeat the process u n t i l darkening no longer occurs; they complete the o x i d a t i o n by heating with HCIO4. S a n d e l l recommends the use of r e f l u x i n g with such procedures i n order to prevent the l o s s of AS2O3 and/or ASCI3. I f the a r s e n i c i s e v e n t u a l l y to be reduced to A s 3 f o r determination, the excess HNO3 remaining a f t e r wet-ashing can be destroyed by t r e a t i n g the mixture with a few ml o f saturated ammonium oxalate and warming. G.I. S p i e l h o l t z , G.C. T o r a l b a l l a , and R.J. S t e i n b e r g - ^ r a p i d l y (and s a f e l y ) m i n e r a l i z e d powdered c o a l by r e f l u x i n g i t w i t h a mixture of 68-70% HCIO4 and para-HI04. P. Schramel-^*-^ showed that reagent blanks i n the d i s s o l u t i o n of 150 mg t i s s u e samples can be minimized by warming the samp l e with 100 y l coned. H S0^ and adding 50% H 02 s o l u t i o n dropwise. 2

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Woolson; Arsenical Pesticides ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

2. TALMI AND FELDMAN

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Traces of Arsenic

1

T h i s i s a refinement of W. Migault's "Caro's a c i d " procedure -*. * Dry Ashing. Various procedures i n v o l v i n g MgO/Mg(1*03)2 a d d i t i o n have been recommended f o r dry-ashing vegetable and animal t i s s u e s . One r e c e n t , widely a p p l i c a b l e procedure suggested by George, Frahn and McDonald ** i n v o l v e s p r e l i m i n a r y b l e n d e r i z i n g o f the t i s s u e , treatment with MgO and c e l l u l o s e powder and an i n i t i a l cautious c h a r r i n g i n a p o r c e l a i n c r u c i b l e . When c o o l , the crucible i s t r e a t e d w i t h Mg(N03)2 • 6H2O and placed i n a c o l d muffle f u r nace. The temperature o f the furnace i s slowly r a i s e d t o 555°, and kept there f o r 2 hours. Good recovery was obtained f o r 1-2 ppm spikes added to samples. R.F. Abernethy and F.H. Gibson** obtained q u a n t i t a t i v e recovery o f As from c o a l by i g n i t i n g i t w i t h MgO a t 650°C; the residue was d i s s o l v e d i n 7N R^SO,. 3. Oxygen Combustion. The Schoniger combustion method * (Ignition o f a sample i n a c l o s e d f l a s k c o n t a i n i n g O2 and an absorbing s o l u t i o n ) was used on d r i e d t i s s u e by Schwedt and R u s s e l ^ . They obtained q u a n t i t a t i v e recovery o f As on 0.5 g d r i e d specimens with a 750 ml f l a s k c o n t a i n i n g 0 and 5 ml o f 3N HC1. I g n i t i o n i n a metal bomb c o n t a i n i n g Oo was a l s o found s a t i s f a c t o r y by H.S. S a t e r l e e and G. B l o d g e t t * . According to C.E. G l e i t and W.D. H o l l a n d , b i o l o g i c a l t i s s u e s , as w e l l as other substances can be m i n e r a l i z e d a t a low temperature by using e l e c t r i c a l l y e x c i t e d oxygen. Complete recovery o f As i s obtained from blood t r e a t e d with HAs0 . 4. Fusion. Minerals are u s u a l l y fused with NaOH i n a s i l v e r or n i c k e l c r u c i b l e ; e.g., by H. O n i s h i and E.B. S a n d e l l . These authors s t a t e that l o s s e s of As are 0.5%; e s s e n t i a l l y a l l of the As i s recovered i n the l e a c h liquid*«even when a r e s i d u e i s p r e sent. J.A. James and D.H. Richards have a l s o a p p l i e d t h i s f u s i o n to the determination of As i n elemental S i . 2


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S t a b i l i t y o f Sample S o l u t i o n s During

Storage

T h i s d i s c u s s i o n p e r t a i n s mainly to n a t u r a l water samples and standards, s i n c e the sample s o l u t i o n s produced by any o f the above methods are normally analyzed soon a f t e r p r e p a r a t i o n . To t e s t the s t a b i l i t y of As a t low concentrations i n sea water, J.E. Portmann and J.P. R i l e y f i l t e r e d each sample through a 0.5u M i l l i p o r e f i l t e r , and t r e a t e d the f i l t r a t e with 5 C i o f c a r r i e r - f r e e Âs. Samples i n soda-glass b o t t l e s l e v e l l e d o f f a t 16% l o s s a f t e r 16 days; samples i n polyethylene and b o r o s i l i c a t e g l a s s l e v e l l e d o f f a t 6% l o s s a f t e r 10 days. These authors advise storage o f such samples i n frozen form i n polyethylene c o n t a i n e r s . G.C. Whitnack and R.G. Brophy ^ made small a d d i t i o n s of Na3As03 s o l u t i o n t o well-water samples, and s t o r e d them i n 25 ml p o l y s t y rene v i a l s with polyethylene caps. No l o s s of A s ( I I I ) from these s o l u t i o n s was d e t e c t a b l e a f t e r one week. A.S. A l - S i b b a i and A.G. F o g g ^ measured the s t a b i l i t y o f s o l u t i o n s o f both As (III) and As (V) i n v a r i o u s containers a t the 4-20 ug/ml l e v e l . Samples of AS2O3 were d i s s o l v e d i n NaOH s o l u t i o n and the s o l u t i o n n e u t r a l i z e d . T h i s s o l u t i o n kept i t s f u l l t i t e r f o r 56 days i n b o r o s i l i c a t e glass, soda g l a s s and polyethylene c o n t a i n e r s , i n both l i g h t and dark 1

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storage areas. S o l u t i o n s of NaHAsC>4 • 7H2O i n water d i d the same for 100 days under the same c o n d i t i o n s . R.S. Braman notes that very low concentrations of As species tend to disappear r a p i d l y from n a t u r a l water samples. In our l a b o r a t o r y , however, no l o s s e s of Âs(V) were experienced i n d i s t i l l e d water (polyethylene and s o f t g l a s s containers) 15% HNO3 or 5% HCIO4 s o l u t i o n (soft g l a s s containers) over a p e r i o d of 3 weeks. 26

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Preconcentration and I s o l a t i o n of As Species The methods described i n t h i s s e c t i o n are o r i e n t e d toward the eventual determination of t o t a l (inorganic) As, r e g a r d l e s s of the o r i g i n a l molecular form i n which the As occurs. The p r i n c i p a l methods which have been used to preconcentrate As species are cop r e c i p i t a t i o n and adsorption, v o l a t i l i z a t i o n and l i q u i d - l i q u i d extraction. 1. C o p r e c i p i t a t i o n and Adsorption. Fe(0H)3 has been known for some time to be an e f f i c i e n t c o l l e c t o r of arsenate i o n . For example, R. P i e r u c c i n i c o l l e c t e d 100 yg of As(V) from 24 l i t e r s of water (= 4 ng/ml) by using 150 mg o f Fe(0H)3 (the As was then determined s p e c t r o g r a p h i c a l l y ) . J.E. Portmann and J.P. R i l e y , p r e c i p i t a t i n g Fe(0H)~ at pH 7, recovered 99% o f the 2 ug of As(V) present i n a l i t e r or water (2 ng/ml). T h i s procedure has a l s o been used to c o l l e c t As f o r determination by the G u t z e i t ^ and xanthate-^0 e x t r a c t i o n procedures. V . I . P l o t n i k o v and L.P.Usatova31 c a r r i e d out numerous c o p r e c i p i t a t i o n experiments i n 50 ml of s o l u t i o n . They found that at pH 7, As(V) i s q u a n t i t a t i v e l y c a r r i e d down by the hydroxides of Ce, Z r , In, Fe, T i and A l . The y i e l d decreases, e s p e c i a l l y f o r A l a t pH >8. In a l l cases, c o p r e c i p i t a t i o n was more e f f i c i e n t than the a d d i t i o n to the As(V) s o l u t i o n of a prepared hydroxide s l u r r y . For A s ( I I I ) , e f f i c i e n t (-95%) coprec i p i t a t i o n was obtained only with In and Zr hydroxides at pH 8.5. W. R e i c h e l and B. G. B l e a k l e y obtained complete recovery of 0.23.0 mg of As(V) (as w e l l as s i m i l a r amounts of Se, Te, Sb, Sn, B i , Pb and Fe) from 20g o f Cu by c o p r e c i p i t a t i n g with La(OH)3 a t pH 910. P.M. S a n t o l i q u i d o removed c a r r i e r - f r e e Âs(V) q u a n t i t a t i v e l y from a 7N HNO3 s o l u t i o n by passing i t through a 7 x 40 mm c o l umn of hydrated Mn02» The c a p a c i t y of t h i s column f o r As(V) i s greater than 272 yg As(V)/g MnOo. Z.G. H a n n a used Mg(0H) as a c o p r e c i p i t a n t : 0.3 yg As(V) i n 10 ml was treated with MgC^ + NB^Cl, and then with NH4OH. The p r e c i p i t a t e obtained was d r i e d , and i g n i t e d a t 600°C. No As was l o s t . 2 8

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T h i o n a l i d e i s s o l u b l e i n acetone, but i n s o l u b l e i n water. J.E. Portmann and J.P. R i l e y , making use of t h i s f a c t , added an acetone s o l u t i o n of t h i o n a l i d e to a 0.5N H2SO4 s o l u t i o n of 50 ng A s ( I I I ) + t r a c e r i n 1 l i t e r of sea water. The s o l u t i o n was s t i r r e d , b o i l e d 30 min. to remove acetone, and allowed to stand overnight. The p r e c i p i t a t e , c o n s i s t i n g almost e n t i r e l y of t h i o n a l i d e i t s e l f , was wet-ashed with concentrated HNO3. Recovery of As was 95%. Talmi, et a l . found that the high s a l t content of the sample sol u t i o n seems to be necessary to insure complete p r e c i p i t a t i o n ; 1

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TALMI AND FELDMAN

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Traces of Arsenic

however, overnight storage i s unnecessary i f the p r e c i p i t a t i o n i s performed a t M)°C. Small amounts of A s ( I I I ) (reduced from As(V) i f necessary with KI) were c o l l e c t e d by a s u l f i d e procedure by V.V. Sergeeva, I.S. L e v i n , L . I . Tishchenko and V.S. Dankova ^. I n t e r f e r i n g elements were f i r s t removed by cupferron p r e c i p i t a t i o n and f i l t r a t i o n . Thioacetamide was added to the f i l t r a t e , and h y d r o l i z e d by heating the s o l u t i o n . The r e s u l t i n g AS2S3 p r e c i p i t a t e was c o l l e c t e d by c e n t r i f u g a t i o n ; the y i e l d was 85%. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 31, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0007.ch002

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2. L i q u i d - L i q u i d E x t r a c t i o n . A.K. K l e i n and F.A. V o r k e s were among the f i r s t to use xanthates to e x t r a c t A s ( I I I ) from food and biochemical t i s s u e s . In t h i s case, the f i r s t c o l l e c t i o n was done w i t h F e ^ H ) ^ ; the f i n a l determination, with arsenomolybdate. P.F. Wyatt 8,39 j diethylammonium d i e t h y l d i t h i o c a r b a m a t e i n CHCI3 t o i s o l a t e As. Since t h i s reagent does not e x t r a c t As(V), s e v e r a l p o t e n t i a l l y i n t e r f e r i n g metals were removed i n i t i a l l y by f i r s t performing t h i s e x t r a c t i o n with As i n the pentavalent s t a t e . As was then reduced to A s ( I I I ) , and e x t r a c t e d from 1-10N ^ S O ^ s o lutions. Some p o t e n t i a l i n t e r f e r e n c e s not e l i m i n a t e d i n t h i s way [Cu, B i , S b ( I I I ) ] can be removed by a p r e l i m i n a r y e x t r a c t i o n w i t h cupferron. According to T.J. V e l e k e r ^ , t h i s reagent can be used to e x t r a c t A s ( I I I ) away from Ge, Sb and B i i n 6N HC1. H. M a l i s s a and E. Schoffmanual found that A s ( I I I ) could be p r e c i p i t a t e d by ammonium p y r r o l i d i n e dithiocarbamate (APDC), as w e l l as by other dithiocarbamates a t pH 2-6. C.E. M u l f o r d used a methyl i s o b u t y l ketone s o l u t i o n o f APDC to e x t r a c t A s ( I I I ) f o r determination by atomic absorption. V.V. Sergeeva, I.S. L e v i n , L . I . Tishchenko and V.S. D a n k o v a , using an 0.5N s o l u t i o n of d i - 2 - e t h y l h e x y l d i t h i o phosphoric a c i d i n decane and organic/aqueous volume r a t i o s of 1:1 to 1:30, e x t r a c t e d t r a c e s o f A s ( I I I ) from 0.5N a c i d aqueous s o l u t i o n s . The As was recovered from the organic phase by shaking with bromine water. A . I . Busev and M.I. I v a n i u t i n found that a s i m i l a r reagent, d i e t h y l d i t h i o p h o s p h o r i c a c i d , used s i m i l a r l y , e x t r a c ted A s ( I I I ) from e i t h e r weakly or s t r o n g l y a c i d i c aqueous solutions. Perhaps the simplest of the procedures f o r segregating As i s E. G a g l i a r d i and H.P. W o s s ' s ^ e x t r a c t i o n of A s C l from 6-7N HC1 i n t o a mixture of 2 volumes of CCI4 w i t h 3 volumes of 2-butanone, 2-pentanone or 2-heptanone. As with other e x t r a c t a n t s , p o t e n t i a l l y i n t e r f e r i n g elements can be e l i m i n a t e d by conducting a p r e l i m i n a r y e x t r a c t i o n w i t h the As i n the pentavalent s t a t e . According to A.R. B y r n e ^ , ASI3 can be e x t r a c t e d with toluene from a s o l u t i o n 12N i n H2SO4 and 0.05 M i n KI. I t can be s t r i p p e d from the toluene with 6N H S 0 + 0 . 0 5 M KI. 3

u s e (

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3. V o l a t i l i z a t i o n . The p r i n c i p a l forms i n which As i s v o l a t i l i z e d f o r a n a l y t i c a l purposes are as a t r i h a l i d e and as a simple or s u b s t i t u t e d t r i h y d r i d e ( a r s i n e ) . A f t e r b i o l o g i c a l m a t e r i a l was decomposed with H S0^ + HNO3, E.B. S a n d e l l d i s t i l l e d AsBr^, using a special s t i l l . Some P and Sn were a l s o d i s t i l l e d . G.R. Kings ley and R.R. S c h a f f e r t noted that a r s e n i c compounds can be leached from t i s s u e homogenates w i t h 1+2 HC1, but that the operation must 1

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be performed i n the open, r a t h e r than under t o t a l r e f l u x i f arseni c i s then to be d i s t i l l e d o f f as A s C l ^ . This p r e c a u t i o n i s taken i n order to expel R^S and mercaptans which would otherwise tend to prevent the d i s t i l l a t i o n of ASCI3. Recent years have seen a r a p i d growth i n the use of a r s i n e e v o l u t i o n as a method of separ a t i n g As from i t s o r i g i n a l matrix. Most authors who have used Zn° i n a c i d to generate AsH^ (e.g. R.E. Madsen , F . J . Fernandez and D.C. M a n n i n g ) have f i r s t reduced the As to A s ( I I I ) w i t h Sn&2 and/or KI. E.N. P o l l o c k and S.J. W e s t used T i C l 3 f o r the p r e l i m i n a r y r e d u c t i o n and Mg° i n a c i d f o r generating AsH^. F.E. Lichte and R.K. Skogerboe ^, however, i n j e c t a small volume of a c i d samp l e s o l u t i o n i n t o a columm of granular Zn°; the sample i s forced through the column by a stream of argon. The AsH^ thus generated passes w i t h the argon i n t o the d e t e c t i o n system. R.S. Braman, L.L. Justen and C.C. F o r e b a c k ^ used NaBH^ to reduce As compounds to ASH3. They f i n d that they can d i s c r i m i n a t e between As(V) and A s ( I I I ) by c o n t r o l l i n g the pH of the sample s o l u t i o n : As(V) i s r e duced only i f pH ^1.5. The NaBH^ can be i n j e c t e d as a s o l u t i o n , or by dropping a p e l l e t of the s o l i d reagent i n t o the sample s o l u t i o n by means of an e x t e r n a l l y operated hopper as suggested by F.J. Fernandez . R.N. S a n d e l l , as w e l l as R.S. Braman, L.L. Justen and C.C. Foreback ^ caution that e a s i l y reduced ions such as Cu(II) may i n t e r f e r e w i t h the production of ASH3. A f t e r generation, the ASH3 (b.p.-55°C) may e i t h e r be passed d i r e c t l y i n t o the d e t e c t i o n device or accumulated i n a l i q u i d n i t r o g e n - c o o l e d trap and released to the d e t e c t o r over a very short p e r i o d of time by warming the trap (see below). 47


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Methods of Determination 1. Molecular Absorption—Spectrophotometry. There are a few reagents which produce i n t e n s e - c o l o r d e r i v a t i v e s w i t h a r s e n i c . However, two of them, s i l v e r - d i e t h y l d i t h i o c a r b a m a t e (Ag-DDC) and ammonium molybdate are u n i v e r s a l l y accepted as most s u i t a b l e f o r spectrophotometric measurements. The Ag-DDC reagent i s u s u a l l y used i n c o n j u n c t i o n with the a r s i n e generation m e t h o d * . A r s i n e i s passed through 0.5% Ag-DDC s o l u t i o n i n p y r i d i n e and the i n t e n s i t y of the red c o l o r i s measured at 533 nm. Beer's law i s obeyed over the 1-20 yg As range and the l i m i t of d e t e c t i o n i s below 0.1 ppm. The arseno-molybdate complex i s considered more s u i t a b l e by many because of i t s s e n s i t i v i t y , r e l i a b i l i t y and general freedom from i n t e r f e r e n c e s . Arseno-molybdic a c i d , formed by the reaction of arsenate with a c i d i f i e d molybdate, i s reduced to the blue comp l e x , the a b s o r p t i o n of which i s measured s p e c t r o p h o t o m e t r i c a l l y . SnCl2 i s used by a few workers f o r r e d u c t i o n ^ " , but produces an unstable c o l o r . Others use hydrazine sulfate 9»60 but r e d u c t i o n i s slow. Portmann and R i l e y found that a s o l u t i o n 0.4N i n H2SO4 and 0.12% i n ammonium molybdate w i l l produce a very s t a b l e complex at room temperature w i t h i n 30 minutes. Absorption i s measured at 866 nm. No i n t e r f e r e n c e s were observed i n the a n a l y s i s of sea water, s i l i c a t e rocks or marine organisms. The a r s e n i c can be separated from the matrix v i a a r s i n e g e n e r a t i o n ^ > ^ , solvent ex52

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TALMI AND FELDMAN

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Traces of Arsenic

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traction63,28,64,65 or c o p r e c i p i t a t i o n with f e r r i c h y d r o x i d e or t h i o n a l i d e ^ . Stara and Stary^b e x t r a c t A s ( I I I ) from s u l f u r i c a c i d s o l u t i o n s f o l l o w i n g conversion to A S I 3 . The compound develops an intense yellow c o l o r when treated w i t h 8-mercaptoquinoline. Procedures were developed to prevent i n t e r f e r e n c e s from the o x i d a t i o n of I to I or from the formation of i n s o l u b l e i o d i d e s . Mankova and Maksiraenko described a method based upon the r e d u c t i o n of AgNO-j to Ag° by AsH . The Ag° exerts a c a t a l y t i c e f f e c t on the f u r t h e r r e d u c t i o n (by F e ) of AgN0 . There i s a r e l a t i o n s h i p between the r e a c t i o n r a t e , measured p h o t o m e t r i c a l l y , and the concent r a t i o n of the c a t a l y s t and thus the c o n c e n t r a t i o n of a r s e n i c . The d e t e c t i o n l i m i t of the r e a c t i o n i s 20 pg/ml. A few workers have developed methods f o r the separate d e t e r mination of a r s e n i t e and arsenate species** »68,62 Spectrophotomeric methods have been a p p l i e d to a l a r g e v a r i e t y of samples i n c l u d i n g u r i n e * ^ , blood and b i o l o g i c a l m a t e r i a l * * , y e a s t s , s o i l * * , and atmospheric d u s t . At the present time, spectrophotometry i s s t i l l the most widespread technique f o r the determination of arseni c , mainly because of i t s inherent methodical and t e c h n i c a l simp l i c i t y and i t s low c o s t . 2

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Radiochemical

Techniques.

A. Among the v a r i o u s radiochemical techniques, neutron a c t i v a t i o n a n a l y s i s (NAA) i s unique i n i t s widespread a p p l i c a b i l i t y to the determination of a r s e n i c . Although, i n p r i n c i p l e , NAA i s a non-destructive a n a l y t i c a l technique, radiochemical s e p a r a t i o n schemes are almost always r e q u i r e d to avoid overlapping of v a r i o u s photo-peaks. Various such schemes "" are based on a combination of two or more s e p a r a t i o n techniques, such as d i s t i l l a t i o n , p r e c i p i t a t i o n , s o l v e n t e x t r a c t i o n or i o n exchange. With the advent of high r e s o l u t i o n s o l i d s t a t e d e t e c t o r s , the d i r e c t instrumental NAA approach has been attempted. Unfortunately, the high a c t i v i t y of N a induced i n many environmentally based samples prevents the determination of a r s e n i c at concentrations below a few ppm, s i n c e the y - a c t i v i t y of these samples w i l l have to decay f o r 4-5 days before measurement. The induced y - a c t i v i t y of i s measured by monit o r i n g the 559 KeV photopeak. T h i s peak w i l l appear, i n many samp l e s , as the middle peak of a t r i p l e t composed of B r ( t / = 35.3h), A s ( t y = 26.5h) and S b ( t , / = 67.2h) r e q u i r i n g the r e s o l u t i o n performance obtained by Ge(Li) d e t e c t o r s . A l s o , s i n c e A s has the s h o r t e s t h a l f - l i f e i n the t r i p l e t , the counting should be done as soon as p o s s i b l e a f t e r i r r a d i a t i o n . NAA i s one of the most s e n s i t i v e techniques with a d e t e c t i o n l i m i t of 0.1 ng using a thermal neutron f l u x of 1 0 ~ neut.-cnT-^-sec" . The method i s u s e f u l at the sub-ppm concentration l e v e l with sample s i z e s s u b s t a n t i a l l y smaller than those r e q u i r e d by the c o l o r i m e t r i c methods. Through the years, NAA has been s u c c e s s f u l l y a p p l i e d to a l a r g e v a r i e t y of samples; b i o l o g i c a l and marine samples' *" , p l a n t t i s s u e s * ^ , water s a m p l e s ^ ^ l ^ i - b o r n e p a r t i c u l a t e m a t t e r ^ , p e s t i c i d e d i s t r i b u t i o n ^ , heavy o i l s p i l l s ^ , g e o l o g i c a l samples >95,96 oal a s h , and many others. P r e c i s i o n and accuracy f o r these samples 73

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The Determination of Traces of Arsenic: A Review - ACS Publications

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