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Suggested procedure for microdetermination of arsenic in arsenical animal feed
MICROCHEMICAL
JOURNAL
15,
649-652
(1970)
Suggested Procedure For Microdetermination of Arsenic In Arsenical Animal Feed K. N. JOHRI, HARISH C. MEHRA. AND N. K. KAUSHIK Dcpnrtrnent
of Ckoni.str.v,
Urlivc’txir?
of Drlhi,
Delhi-7,
India.
Kccc,il,c>d No\~c~v~hcr 3, 1969 INTKODUCTION
Due to the highly poisonous nature of arsenic, much attention has been given by analytical chemists to the detection and determination of even trace amounts of it, especially when suspected to be present in foodstuffs. Consequently, from time to time various analytical methods for its identification and estimation have been proposed. The earliest though sensitive but inconvenient method is by Gutzeit (I). Lately, numerous calorimetric (1 ) and paper chromatographic (2) methods have been reported. More recently, a review (2) and a method by Wilson and Lewis (3) discussing the estimation of arsenic have been reported. It is gathered that the methods rcportcd so far are either qualitative in scope, or if quantitative, are inconvenient and not so sensitive as sometime desired. Present studies relate to the problem which the authors had received from the United States Department of Agriculture, Agriculture Research, Animal Health Division. Beltsville, Maryland. This problem requires a sensitive and convenient analytical method of determining as low an amount as 0.2% arscnitc (as AslO .) present in arsenical animal feed. For this study. the startin g material required is the muriatic acid extract of the cattle feed. The presently proposed method for the dctection and determination of tract amounts of arsenite is based on the ring oven technique using potassium thiocarbonate (PTC) reagent. PTC (4-6) has already been reported as a good source of thiocarbonate and sulfide ions for cfficicnt precipitation of various cations and as well as for complexation of those cations which yield soluble thiocarbonate products which are characteristically colored. Weisz (7) ring oven technique is a versatile and elegant method for increasing the sensitivity of spot test on microscalc, and for the development of semiquantitative and quantitative procedure at the submicro level. By this technique, a single drop of a test solution is applied to the center of filter paper placed on an electrically heated metallic ring. One or more of the components of a test sample arc fixed (precipitated) 649
650
JOHRI,
MEHRA,
KAUSHIK
with one or more suitable reagents, others are washed out to the ring zone. For the semiquantitative analysis the rings after development with specific reagents are compared with the rings forming the standard scale. The standard scale in a particular case is prepared by developing known amounts of metal ions in the ring with the same reagents. Standard scale (7) procedure Colorimetry
of Semiquantitative
Analysis
by Ring
The standard scale rings are prepared with 1, 3, 5, 7, and 9 drops using self-filling pipette and marked as I, III, V, VII, and IX. During the application of several drops great care is taken to avoid enlargement of the spots. This is done by drying the previously spotted drop before applying the new ones. Generally, the standard scale is prepared with not more than 9 to 10 drops of the standard solution. This is, because with large number of drops the intensity of the color is too high to be distinguished. A single standard solution is sufficient for preparing one standard scale. EXPERIMENTAL
METHOD
Instrument. Weisz Ring Oven used for this study was obtained from National Appliance Co., Portland, Oregon. Potassium thiocarbonate (PTC) aqueous solution, prepared fresh by suitable dilution of stock solution (1 M) with distilled water. Standard Metal Solution. 0.1% solution of sodium arsenite (A.R.) was prepared as stock solution and was used after suitable dilution to work out the standard scale. Silver nitrate. 1% aqueous solution of AnalaR grade material was used. Hydrochloric acid. 1 N prepared from B.D.H., AnalaR sample. PROCEDURE
A circle of filter paper (Whatman No. 41) of 55-mm diameter was placed on the oven set at 100°C. A drop (1.5 J) of diluted test solution containing microgram quantities was spotted at the marked center of the filter paper circle using self-filling capillary pipette and was washed out into the ring with distilled water, taking 10 ~1 of it at a time. The ring was developed by applying PTC solution 2 to 3 times with the help of capillary pipette at the center and washing the same into the ring. After drying, HCl was applied to fix As& and to remove any excess of PTC from the ring. The filter paper was finally washed with excess of distilled water.
MICRODETERMINATION
lkterminntion
OF AS
651
by Sili>er Sudr (7) Method
Arscnite being present in micro amounts and the color of As,& being light ycilow, cflicicl?t comparison of the same for direct evaluation is not possible. Dztcrmination of arsenic, thcreforc. is made possible by transforming As,S, into quivaicnt amount of silver sulfide. For this. rings L comprising the standard scale as ~\cll :IS the unknown (after repeatedly washing with ciistillcd water) wcrc Anally developed by dipping into a bath of 1% silver nitrate solution. Each ring was dried and compared with standard scale rings. prepared similarly. Reprcsentcd rcsuits of :I fc\s expcrirncnts arc given in Table 1.
Detection and determination of arsenic in rnuriatic acid extract of animal feed is possible down to the (I.(?1 :~g/iJ prcscnt. The prescncr of present in animal feed does calcium, iron, and other metal ions usually not intcrferc in this cstimatioa of arsenic. Computation of results as based on tf#e silver scale method nllo\~s the determination of arsenic within 64% error.
The ring oven technique ha\ been proposed for the microdetermination of arsenic present 85 arseaite in ai-hecical animal feed u\i-p pota\\itlm thiocarhonate (PfC) reagent as the principal reagxt. It i\ possible to estimate a\ low as 0.04 ,,gipl of arsenic within 6-X’; error. ACKN0WLELKMEN-f One ot the authora (H.C.RI.1 is grateful to the Hcnd. Department of Chemistry for providing r,eceaary facilities to cat-ry out thae studies and also thankful to Dr. R. S. Saxena for hi\ keen inter& in I~C ~+ork.
652
JOHRI, MEHRA, KAUSHIK REFERENCES
1. Sandell, E. B., “Calorimetric Metal Analysis.” Wiley (Interscience), New York, 1959. 2. Krutitskaya, M. N. and Ivanova, B. S., Determination of traces of arsenic. Zuvodsk. Lab. 30, 1173 (1964). 3. Wilson, A. D., and Lewis, D. T., “Radiometric determination of microgram amounts of arsenic.” Analyst 92, 260 (1967). 4. Johri, K. N., “Macro and Semi-micro Chemical Analysis without H,S, using potassium thiocarbonate,” 2nd ed. Asia Publishing House, Bombay, 1968. 5. Johri, K. N., “The PTC scheme of systematic separation of cation-groups including the less common metals.” Indian J. Appl. Chern. 26, 4 (1963). 6. Johri, K. N., and Singh, K., “Potassium thiocarbonate (PTC) as a masking agent and as a metal indicator in EDTA titrations.” Bull. Chenz. Sot. Jup. 40, 990 (1967). 7. Weisz, H., “Micro analysis by Ring Oven Technique, Pergamon, London/New York, 1961.
JOURNAL
15,
649-652
(1970)
Suggested Procedure For Microdetermination of Arsenic In Arsenical Animal Feed K. N. JOHRI, HARISH C. MEHRA. AND N. K. KAUSHIK Dcpnrtrnent
of Ckoni.str.v,
Urlivc’txir?
of Drlhi,
Delhi-7,
India.
Kccc,il,c>d No\~c~v~hcr 3, 1969 INTKODUCTION
Due to the highly poisonous nature of arsenic, much attention has been given by analytical chemists to the detection and determination of even trace amounts of it, especially when suspected to be present in foodstuffs. Consequently, from time to time various analytical methods for its identification and estimation have been proposed. The earliest though sensitive but inconvenient method is by Gutzeit (I). Lately, numerous calorimetric (1 ) and paper chromatographic (2) methods have been reported. More recently, a review (2) and a method by Wilson and Lewis (3) discussing the estimation of arsenic have been reported. It is gathered that the methods rcportcd so far are either qualitative in scope, or if quantitative, are inconvenient and not so sensitive as sometime desired. Present studies relate to the problem which the authors had received from the United States Department of Agriculture, Agriculture Research, Animal Health Division. Beltsville, Maryland. This problem requires a sensitive and convenient analytical method of determining as low an amount as 0.2% arscnitc (as AslO .) present in arsenical animal feed. For this study. the startin g material required is the muriatic acid extract of the cattle feed. The presently proposed method for the dctection and determination of tract amounts of arsenite is based on the ring oven technique using potassium thiocarbonate (PTC) reagent. PTC (4-6) has already been reported as a good source of thiocarbonate and sulfide ions for cfficicnt precipitation of various cations and as well as for complexation of those cations which yield soluble thiocarbonate products which are characteristically colored. Weisz (7) ring oven technique is a versatile and elegant method for increasing the sensitivity of spot test on microscalc, and for the development of semiquantitative and quantitative procedure at the submicro level. By this technique, a single drop of a test solution is applied to the center of filter paper placed on an electrically heated metallic ring. One or more of the components of a test sample arc fixed (precipitated) 649
650
JOHRI,
MEHRA,
KAUSHIK
with one or more suitable reagents, others are washed out to the ring zone. For the semiquantitative analysis the rings after development with specific reagents are compared with the rings forming the standard scale. The standard scale in a particular case is prepared by developing known amounts of metal ions in the ring with the same reagents. Standard scale (7) procedure Colorimetry
of Semiquantitative
Analysis
by Ring
The standard scale rings are prepared with 1, 3, 5, 7, and 9 drops using self-filling pipette and marked as I, III, V, VII, and IX. During the application of several drops great care is taken to avoid enlargement of the spots. This is done by drying the previously spotted drop before applying the new ones. Generally, the standard scale is prepared with not more than 9 to 10 drops of the standard solution. This is, because with large number of drops the intensity of the color is too high to be distinguished. A single standard solution is sufficient for preparing one standard scale. EXPERIMENTAL
METHOD
Instrument. Weisz Ring Oven used for this study was obtained from National Appliance Co., Portland, Oregon. Potassium thiocarbonate (PTC) aqueous solution, prepared fresh by suitable dilution of stock solution (1 M) with distilled water. Standard Metal Solution. 0.1% solution of sodium arsenite (A.R.) was prepared as stock solution and was used after suitable dilution to work out the standard scale. Silver nitrate. 1% aqueous solution of AnalaR grade material was used. Hydrochloric acid. 1 N prepared from B.D.H., AnalaR sample. PROCEDURE
A circle of filter paper (Whatman No. 41) of 55-mm diameter was placed on the oven set at 100°C. A drop (1.5 J) of diluted test solution containing microgram quantities was spotted at the marked center of the filter paper circle using self-filling capillary pipette and was washed out into the ring with distilled water, taking 10 ~1 of it at a time. The ring was developed by applying PTC solution 2 to 3 times with the help of capillary pipette at the center and washing the same into the ring. After drying, HCl was applied to fix As& and to remove any excess of PTC from the ring. The filter paper was finally washed with excess of distilled water.
MICRODETERMINATION
lkterminntion
OF AS
651
by Sili>er Sudr (7) Method
Arscnite being present in micro amounts and the color of As,& being light ycilow, cflicicl?t comparison of the same for direct evaluation is not possible. Dztcrmination of arsenic, thcreforc. is made possible by transforming As,S, into quivaicnt amount of silver sulfide. For this. rings L comprising the standard scale as ~\cll :IS the unknown (after repeatedly washing with ciistillcd water) wcrc Anally developed by dipping into a bath of 1% silver nitrate solution. Each ring was dried and compared with standard scale rings. prepared similarly. Reprcsentcd rcsuits of :I fc\s expcrirncnts arc given in Table 1.
Detection and determination of arsenic in rnuriatic acid extract of animal feed is possible down to the (I.(?1 :~g/iJ prcscnt. The prescncr of present in animal feed does calcium, iron, and other metal ions usually not intcrferc in this cstimatioa of arsenic. Computation of results as based on tf#e silver scale method nllo\~s the determination of arsenic within 64% error.
The ring oven technique ha\ been proposed for the microdetermination of arsenic present 85 arseaite in ai-hecical animal feed u\i-p pota\\itlm thiocarhonate (PfC) reagent as the principal reagxt. It i\ possible to estimate a\ low as 0.04 ,,gipl of arsenic within 6-X’; error. ACKN0WLELKMEN-f One ot the authora (H.C.RI.1 is grateful to the Hcnd. Department of Chemistry for providing r,eceaary facilities to cat-ry out thae studies and also thankful to Dr. R. S. Saxena for hi\ keen inter& in I~C ~+ork.
652
JOHRI, MEHRA, KAUSHIK REFERENCES
1. Sandell, E. B., “Calorimetric Metal Analysis.” Wiley (Interscience), New York, 1959. 2. Krutitskaya, M. N. and Ivanova, B. S., Determination of traces of arsenic. Zuvodsk. Lab. 30, 1173 (1964). 3. Wilson, A. D., and Lewis, D. T., “Radiometric determination of microgram amounts of arsenic.” Analyst 92, 260 (1967). 4. Johri, K. N., “Macro and Semi-micro Chemical Analysis without H,S, using potassium thiocarbonate,” 2nd ed. Asia Publishing House, Bombay, 1968. 5. Johri, K. N., “The PTC scheme of systematic separation of cation-groups including the less common metals.” Indian J. Appl. Chern. 26, 4 (1963). 6. Johri, K. N., and Singh, K., “Potassium thiocarbonate (PTC) as a masking agent and as a metal indicator in EDTA titrations.” Bull. Chenz. Sot. Jup. 40, 990 (1967). 7. Weisz, H., “Micro analysis by Ring Oven Technique, Pergamon, London/New York, 1961.