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New iodometric methods for the microdetermination of arsenic in organic compounds
Tuhru, Vol 23, pp. 513-516. Pergamon Press. 1916. Printed in Great Bntain
NEW IODOMETRIC METHODS FOR THE MICRODETERMINATION OF ARSENIC IN ORGANIC COMPOUNDS Y. A. GAWARGIOUS, L. S. Bou~os National
(Received
Research
30 July 1975. Revised
Centre,
and B. N. FALTAOOS Dokki,
18 January
Cairo,
Egypt
1976. Accepted
25 January
1976)
Summary-New methods are described for the iodometric microdetermination of arsenic in organic compounds after wet digestion or oxygen flask combustion. After evaporation of the arsenic solution to dryness and dissolution of the residue in water, acetone is added and the solution is treated with iodide-iodate and the iodine liberated (by the interfering acids and the first dissociation step of arsenic acid) is reduced with thiosulphate. The KHsAsO, left is then reacted with zinc sulphate in presence of excess of KI and KIOs. Acetone is added and the liberated iodine is titrated with thiosulphate. This titration corresponds to the second and third dissociation steps of arsenic acid and is used to calculate the arsenic content of the compound. When arsenicals not containing sulphur are decomposed by the oxygen flask method, the arsenic acid solution obtained is reacted directly with zinc sulphate in presence of KI and KIO, and the iodine released is titrated with thiosulphate. In this case, the titration corresponds to all three dissociation steps of arsenic acid. The average recoveries obtained by the two methods are 99.5 and 99.9%, respectively.
Several
methods
of arsenic or oxygen
are
in organic flask
available
for
compounds
combustion.‘13
the after
Most
Reagents
determination
All reagents were of AR or MAR grade except where otherwise mentioned, and doubly distilled water was alwavs used. Sodium thiosulphate solution, 0.05 and O.OlM, standardized against potassium iodate solutions of suitable concentrations. Potassium iodide solution (O.lM), potassium iodate solution (O.O2M), zinc sulphate solution ( - 0.1 M).
wet digestion’ are
based
on
production of arsenate which is then determined gravimetrically, ’ s4 potentiometrically,5 colorimetrically,z,3 or by atomic-absorption spectrophotometry.6 Iodo- and iodimetric methods are familiar’ for arsenic though both suffer from drawbacks. The reaction of arsenic acid with iodide in acidic medium has been reported’ to be very sensitive to pH. The titration with iodine yields better results, but involves a rather lengthy and involved procedure. In the present work, new iodometric methods are described for the microdetermination of arsenic in organic compounds, similar in principle to the methods proposed for phosphorus,7s exploiting the hydrogen ions of arsenic acid. Thus, the arsenic acid, obtained after sample decomposition, is allowed to react with zinc sulphate and on addition of potassium iodide and iodate, iodine is liberated, which is titrated with standard thiosulphate.
EXPERIMENTAL Apparatus A 500-ml oxygen flask, the stopper of which is provided with a silica spiral (5 or 6 turns) made from a 7-8cm length of 2-mm diameter tubing. The spiral is sealed at one end to a rod protruding from the stopper, and tapered upwards at the lower end. The tapering end of the spiral, which replaces the conventional platinum holder, is situated in the centre of the conical flask. The spiral is a slightly modified version of that advocated by Belcher et (~1.~for submicro work.
Procedures Wet digestion method. Transfer 6-8 mg of sample carefully into the bottom of a 25-ml Kjeldahl flask, add 1 ml each of concentrated sulphuric and nitric acids, and heat cautiously till the appearance of SO9 fumes. Add a further 0.5 ml of concentrated nitric acid, heat to fuming and repeat the same step after addition of 0.5 ml of 30% hydrogen peroxide, continuing repetition until the digest becomes colourless. Continue heating till practically all the sulphuric acid has been removed. Cool and transfer the contents of the flask quantitatively into a 25-ml glass dish, with small portions of water, and evaporate to dryness on a boiling water-bath. Dissolve the residue in ca. 5-7 ml of hot water and transfer the solution into a lOO-ml conical flask. Add 10 ml of acetone and 2 ml each of O.lM KI and 0.02M KI03 solutions. Stopper the flask and after 5 min titrate the liberated iodine, first with 0.05M sodium thiosulphate to a pale yellow and then with O.OlM thiosulphate solution till colourless. Then add 1 ml of -0.lM zinc sulphate. - 100 mg of solid KI and -5Omg of KIO,. Stopper the Rask and after 5 min again add 10 ml of acetone. Titrate the liberated iodine wiih O.OlM sodium thiosulphate till colourless. Run a blank experiment with benzoic acid as sample. Since the reaction sequence corresponds to H,AsO; = I, 5 2S#-, then I ml of O.OlM Na,S,O, = 0.3746 mg of As. and the percentage of arsenic in the organic compound is As = 37.46(Y - X)M W 513
x 100%
Y. A.
514
CAWARGIOUS%
L. S. Bouros and B. N.
where Y and X are the titration volumes (ml) for the sample and the blank respectively, M is the molarity of the thiosulphate solution, and W is the weight (mg) of the sample. Oxygen jIc.& ~er~~~. For arsenicais containing no sulphur weigh enough sample to give at least 0.7 mg of arsenic. Transfer it onto a piece of ashless filter paper, and (if necessary) add 5 mg of potassium nitrate as an aid to combustion. Fold the paper twice at right angles and roll it up to fit into the quartz spiral. Burn the sample as usual in a SOO-ml oxygen-filled flask containing 3 ml of cont. nitric acid. Shake the flask occasionally during the next IOmin, then rinse the stopper and walisof the Rask with - 2 ml of cont. nitric acid. Place the flask on a steam-bath and evaporate the solution to dryness; repeat the evaporation step twice, adding 3 ml of water each time. Dissolve the residue in 5 ml of water and add 1 ml of O.lM zinc sulphate solution, followed by solid KI (- 1OOmg) and KIO, (- 50 mg). Stopper the flask and leave it aside for 5 min. Titrate the iodine with O.OlM sodium thiosulphate solution, using starch as indicator. Run a blank experiment with benzoic acid as sample. Since the reaction corresponds to 2HsAs0, = 31, E 6&O:-, 1ml of O.OlM Na&Os z 0.2497 mg of As, and the arsenic content of the sample is 24.97 (Y - X)M x loo w where Y, X, M and tV have the same meanings as before. For compounds containing sulphur as well as arsenic, proceed similarly up to the end of the evaporation steps. Then dissolve the residue in 5 ml of hot water and apply the double titration procedure described for the wet digestion method, starting with the first addition of acetone, and using the same method of calculation. RESULTS AND DISCUSSION The two methods are based on complete conversion of the arsenic into arsenic acid, the protons of which are used to produce an equivalent amount of iodine in the iodide-iodate reaction. The wet digestion method It would be desirable not to have to use an acidic digestion medium, but none was known that would decompose all the compounds tested. With the mineralization method used, the problem is to get rid of the excess of acid. This should be possible by repeated evaporations since arsenic acid dehydrates at 180-200” to As205 without loss of arsenic and AsZO, is stable up to 380”.9 Only above this temperature does decomposition to As,Os occur (2As,05 = As&, + 20,). However. it was found that a variable amount of sulphuric acid was always left, causing high or low apparent recoveries of arsenic, depending on the relative amounts of acid left in the sample and blank solutions, In contrast to sulphuric acid, other acids (e.g., halogen acids and/or nitric acid) expected to arise from digestion of samples containing acidic elements other than arsenic, do not interfere since these are completely eliminated in the repeated evaporation steps. However, to make the procedure generally applicable to any arsenic-containing compound, the prob-
FALTACXJS
lem of interferences was overcome completely by incorporating the two-step titration method advocated previously’ for the analogous determination of phosphorus. In the first step, the arsenic acid and any other acid remaining are allowed to react in aqueous acetone medium with KI and K103: 6H3As04
I- IO;
+ 51+6H,AsO,
6HX + IO;
+ 3H20 + 31z
+ SI-+6X_
+ 3H*O + 312.
The total iodine liberated, due to arsenic(V) and any other acidity, is removed by titration, first with O.OSM thiosulphate, to avoid excessive dilution, till pale yellow, and then with O.OlM ~iosulphate till colourless. This titration value is not needed, of course, in the calculation. In the second step, the dihydrogen arsenate is allowed to react with zinc sulphate whereby zinc arsenate is formed along with liberation of an equivalent amount of protons, which then react with KI and KIO,, both added this time in solid form to avoid dilution, with quantitative liberation of iodine, which is directly titrated with O.OlM thiosulphate, to a colourless end-point. The titration value obtained in this second step is used to calculate the arsenic content of the organic sample. 2H,AsO;
+- 3Zn*’ -+Zn3(As0.&
6H+ -t SI- + IO; -+ 3H,O
+ 4H’
+ 31,
As in the case of phosphorus determination,’ the acetone in the medium allows complete reaction, in the first step, of the traces of interfering acids, while making the arsenic acid function only as a monobasic acid. The presence of acetone also renders the iodine colour very bright yellow, so the iodine also acts as the indicator. To improve the end-point detection the acetone is added in two portions, one along with the iodide-iodate mixture for the first reaction, followed by titration after a -5-min waiting period, and the second only just before the second titration so that the reaction with zinc sulphate has enough time to proceed rapidly and completely. Satisfactory results (Table 1) were obtained, showing an average absolute error of f0.3% and an average recovery of 99.5%. Arsenazo [3-(2-arsonophenylazo~4,5-dihydroxy-2,7-naphthalene disulphonic acid] gave ca. 3.5% low arsenic recovery probably because of impurity and/or incomplete decomposition. The blank values did not exceed 0.06 ml of O.OlM thiosulphate solution which is considered reasonable for such a rather lengthy digestion procedure (up to 6 hr). It was the slowness of the d~omposition that led to development of the oxygen flask method. The oxygen jask
method
Preliminary tests with sample holder and saturated
the ordinary bromine-water
platinum or hydro-
Arsenic in organic compounds Table 1. Iodometric microdetermination
515
of arsenic in organic compounds by the wet digestion method
As, % Weight, mg Triphenylarsine n-Butylarsonic acid Phenylarsonic acid Benzylarsonic acid 4-Dimethylaminoazobenzene-4’-arsenic Triphenylarsine
acid
sulphide
Arsenazo
gen peroxide as absorption medium gave very low arsenic results. This was not unexpected, because arsenic has been reported” to alloy with platinum.
Thus, a modified version of the quartz spiral stopper, introduced previously,‘s3 was used, but the arsenic values found were still low, indicating incomplete decomposition and/or incomplete oxidation of As(II1) to As(V). Although it is customary to use alkaline hypobromite3 or alkaline hydrogen peroxide4 as absorption medium to obtain arsenic(Vk3 the use of alkaline medium was ruled out because the present method is based on the acidity of the arsenic acid. Trials were then made with concentrated nitric acid as absorbent. In addition to oxidizing As(II1) to As(V), the excess of nitric acid was easily and smoothly removed by heating on a boiling waterbath, leaving a white residue of arsenic acid which Table 2. Iodometric microdetermination
4.936 5.805 7.871 4.956 6.100 4.344 7.006 4.106 5.520 5.986 8.240 5.606 8.300 6.010 8.416
Recovery, % Found
CMC.
24.46 41.15 37.09 35.67 21.46 22.11 13.13
24.2 24.2 24.8 41.6 40.8 37.4 36.8 35.4 36.0 21.2 21.7 22.4 21.8 12.7 12.7
98.9 99.0 101.4 101.0 99.2 100.9 99.2 99.2 100.9 98.9 101.2 101.4 98.7 96.6 96.7
dissolved easily in hot water. Addition of zinc sulphate to this solution gave a precipitate of zinc arsenate and an equivalent amount of protons, the arsenic acid acting as a tribasic acid: 2H3As04 + 3Zn2+ + 2Zn3(As04)2 + 6H+ The protons liberated can be determined by the iodideiodate reaction. The volume should not exceed 6-7 ml for the precipitation reaction to proceed quantitatively within 5 min, and an equal period should be allowed for complete iodometric reaction. The use of concentrated nitric acid as the absorbent succeeded for all the samples analysed except thorin and 4-dimethylaminoazobenzene-4’-arsenic acid, which gave results 69% low. About 5mg of potassium nitrate, mixed with the sample before wrapping in the ashless filter paper flag, proved highly efficient
of arsenic in organic compounds by the oxygen-flask method As, % Weight, mg
Compound Triphenylarsine* n-Butylarsonic acid* Phenylarsonic acid* Benzylarsonic acid* 4-Dimethylaminoazobenzene-4 Triphenylarsine
-arsenic acid*
sulphidet
Arsenazot Thorint
* Calculated on the basis of 3-fold amplification. t Calculated on the basis of 2-fold amplification.
4.380 5.000 7.531 4.506 5.354 4.152 5.650 4.950 7.017 4.976 7.401 5.906 7.555 5.004 6.826 5.162 8.005
Recovery, % Calc.
Found
24.46
24.7 24.7 24.2 40.9 41.4 37.4 37.3 34.4 34.5 21.7 21.2 22.4 22.2 13.4 12.8 17.7 17.7
41.15 37.09 34.67 21.45 22.11 13.13 18.04
101.1 101.0 99.1 99.4 100.5 100.7 100.6 99.3 99.5 101.2 98.9 101.2 100.4 102.2 97.8 97.9 97.8
516
Y. A. GAWARGIOUS,L. S. E~XJLOSand B. N. FALTAOOS
as an auxiliary oxidant during combustion. In addition to raising the arsenic recoveries to an average of ca. 98 and 99% for these two compounds, respectively, the potassium nitrate caused no interference, and may conveniently be added to any arsenical sample. Satisfactory results (Table 2) were obtained. As expected, nitrogen and halogens did not interfere, because their products are readily expelled in the evaporation steps. The effect of halogens was tested by addition of micro-amounts of hydrochloric acid, bromine-water, and iodine either to the absorbed combustion products of triphenylarsine or in blank experiments. On the other hand, sulphur-containing arsenicals could not be successfully analysed by the simple oxygen flask procedure, owing to the incomplete removal of sulphuric acid in the evaporation step, but this problem was readily solved by applying the twostep procedure developed for the wet digestion method. Arsenazo yielded better arsenic recoveries by the oxygen flask method than by wet digestion. Thorin, however, gave 2% low recovery, possibly ascribable to impurity. The results in Table 2 show an average absolute error of +0.3% and overall average recovery of 99.9%. The blank values (0.04ml of O.OlM thiosulphate) were smaller than those for wet digestion.
Conclusions
The oxygen flask method is superior in the following respects: (i) it is simpler and less tedious than the wet digestion method; it offers a 3-fold amplification (except for the sulphur arsenicals) compared with the 2-fold amplification for the wet-digestion method: (ii) in general it gives the better results. Phosphorus, of course, interferes quantitatively, as expected from the previous work’,’ but compounds containing both arsenic and phosphorus are not very common. REFERENCES
1. G. Ingram, Methods
qf Organic Elemental Iysis, p. 297. Reinhold, New York, 1962. Acta. 1959, 640. 2. %. Merz, Mikrochim.
Microunu-
S. E. Phang and 3. R. Belcher, A. M. G. Macdonald, T. S. West, J. Chem. Sot.. 1965, 2044. 4. A. D. Wilson and D. T. Lewis, Analyst, 1963, 88, 510. Mikrochim. Acts. 1968. 5. B. GrieDink and W. Kriiesman. -_ 330, lOb3. 6. S. S. M. Hassan and M. H. Eldesouky, 2. Awl. Chem.. 1972, 259, 346.
7. Y. A. Gawargious 1971, 16, 342. 8. Y. A. Gawargious
16, 333. 9. J. W. Mellor, Atld Theoretical
and
A. B. Farag,
and A. B. Farag,
Microchem.
J.,
Idem, ibid., 1971.
A Comprehensive Treatise On Inorganic Chemistry. Vol. IX. p. 137. Longmans,
London, 1947. 10. R. Belcher, A. M. G. Macdonald Talanta, 1958, 1, 408.
and
T. S. West,
NEW IODOMETRIC METHODS FOR THE MICRODETERMINATION OF ARSENIC IN ORGANIC COMPOUNDS Y. A. GAWARGIOUS, L. S. Bou~os National
(Received
Research
30 July 1975. Revised
Centre,
and B. N. FALTAOOS Dokki,
18 January
Cairo,
Egypt
1976. Accepted
25 January
1976)
Summary-New methods are described for the iodometric microdetermination of arsenic in organic compounds after wet digestion or oxygen flask combustion. After evaporation of the arsenic solution to dryness and dissolution of the residue in water, acetone is added and the solution is treated with iodide-iodate and the iodine liberated (by the interfering acids and the first dissociation step of arsenic acid) is reduced with thiosulphate. The KHsAsO, left is then reacted with zinc sulphate in presence of excess of KI and KIOs. Acetone is added and the liberated iodine is titrated with thiosulphate. This titration corresponds to the second and third dissociation steps of arsenic acid and is used to calculate the arsenic content of the compound. When arsenicals not containing sulphur are decomposed by the oxygen flask method, the arsenic acid solution obtained is reacted directly with zinc sulphate in presence of KI and KIO, and the iodine released is titrated with thiosulphate. In this case, the titration corresponds to all three dissociation steps of arsenic acid. The average recoveries obtained by the two methods are 99.5 and 99.9%, respectively.
Several
methods
of arsenic or oxygen
are
in organic flask
available
for
compounds
combustion.‘13
the after
Most
Reagents
determination
All reagents were of AR or MAR grade except where otherwise mentioned, and doubly distilled water was alwavs used. Sodium thiosulphate solution, 0.05 and O.OlM, standardized against potassium iodate solutions of suitable concentrations. Potassium iodide solution (O.lM), potassium iodate solution (O.O2M), zinc sulphate solution ( - 0.1 M).
wet digestion’ are
based
on
production of arsenate which is then determined gravimetrically, ’ s4 potentiometrically,5 colorimetrically,z,3 or by atomic-absorption spectrophotometry.6 Iodo- and iodimetric methods are familiar’ for arsenic though both suffer from drawbacks. The reaction of arsenic acid with iodide in acidic medium has been reported’ to be very sensitive to pH. The titration with iodine yields better results, but involves a rather lengthy and involved procedure. In the present work, new iodometric methods are described for the microdetermination of arsenic in organic compounds, similar in principle to the methods proposed for phosphorus,7s exploiting the hydrogen ions of arsenic acid. Thus, the arsenic acid, obtained after sample decomposition, is allowed to react with zinc sulphate and on addition of potassium iodide and iodate, iodine is liberated, which is titrated with standard thiosulphate.
EXPERIMENTAL Apparatus A 500-ml oxygen flask, the stopper of which is provided with a silica spiral (5 or 6 turns) made from a 7-8cm length of 2-mm diameter tubing. The spiral is sealed at one end to a rod protruding from the stopper, and tapered upwards at the lower end. The tapering end of the spiral, which replaces the conventional platinum holder, is situated in the centre of the conical flask. The spiral is a slightly modified version of that advocated by Belcher et (~1.~for submicro work.
Procedures Wet digestion method. Transfer 6-8 mg of sample carefully into the bottom of a 25-ml Kjeldahl flask, add 1 ml each of concentrated sulphuric and nitric acids, and heat cautiously till the appearance of SO9 fumes. Add a further 0.5 ml of concentrated nitric acid, heat to fuming and repeat the same step after addition of 0.5 ml of 30% hydrogen peroxide, continuing repetition until the digest becomes colourless. Continue heating till practically all the sulphuric acid has been removed. Cool and transfer the contents of the flask quantitatively into a 25-ml glass dish, with small portions of water, and evaporate to dryness on a boiling water-bath. Dissolve the residue in ca. 5-7 ml of hot water and transfer the solution into a lOO-ml conical flask. Add 10 ml of acetone and 2 ml each of O.lM KI and 0.02M KI03 solutions. Stopper the flask and after 5 min titrate the liberated iodine, first with 0.05M sodium thiosulphate to a pale yellow and then with O.OlM thiosulphate solution till colourless. Then add 1 ml of -0.lM zinc sulphate. - 100 mg of solid KI and -5Omg of KIO,. Stopper the Rask and after 5 min again add 10 ml of acetone. Titrate the liberated iodine wiih O.OlM sodium thiosulphate till colourless. Run a blank experiment with benzoic acid as sample. Since the reaction sequence corresponds to H,AsO; = I, 5 2S#-, then I ml of O.OlM Na,S,O, = 0.3746 mg of As. and the percentage of arsenic in the organic compound is As = 37.46(Y - X)M W 513
x 100%
Y. A.
514
CAWARGIOUS%
L. S. Bouros and B. N.
where Y and X are the titration volumes (ml) for the sample and the blank respectively, M is the molarity of the thiosulphate solution, and W is the weight (mg) of the sample. Oxygen jIc.& ~er~~~. For arsenicais containing no sulphur weigh enough sample to give at least 0.7 mg of arsenic. Transfer it onto a piece of ashless filter paper, and (if necessary) add 5 mg of potassium nitrate as an aid to combustion. Fold the paper twice at right angles and roll it up to fit into the quartz spiral. Burn the sample as usual in a SOO-ml oxygen-filled flask containing 3 ml of cont. nitric acid. Shake the flask occasionally during the next IOmin, then rinse the stopper and walisof the Rask with - 2 ml of cont. nitric acid. Place the flask on a steam-bath and evaporate the solution to dryness; repeat the evaporation step twice, adding 3 ml of water each time. Dissolve the residue in 5 ml of water and add 1 ml of O.lM zinc sulphate solution, followed by solid KI (- 1OOmg) and KIO, (- 50 mg). Stopper the flask and leave it aside for 5 min. Titrate the iodine with O.OlM sodium thiosulphate solution, using starch as indicator. Run a blank experiment with benzoic acid as sample. Since the reaction corresponds to 2HsAs0, = 31, E 6&O:-, 1ml of O.OlM Na&Os z 0.2497 mg of As, and the arsenic content of the sample is 24.97 (Y - X)M x loo w where Y, X, M and tV have the same meanings as before. For compounds containing sulphur as well as arsenic, proceed similarly up to the end of the evaporation steps. Then dissolve the residue in 5 ml of hot water and apply the double titration procedure described for the wet digestion method, starting with the first addition of acetone, and using the same method of calculation. RESULTS AND DISCUSSION The two methods are based on complete conversion of the arsenic into arsenic acid, the protons of which are used to produce an equivalent amount of iodine in the iodide-iodate reaction. The wet digestion method It would be desirable not to have to use an acidic digestion medium, but none was known that would decompose all the compounds tested. With the mineralization method used, the problem is to get rid of the excess of acid. This should be possible by repeated evaporations since arsenic acid dehydrates at 180-200” to As205 without loss of arsenic and AsZO, is stable up to 380”.9 Only above this temperature does decomposition to As,Os occur (2As,05 = As&, + 20,). However. it was found that a variable amount of sulphuric acid was always left, causing high or low apparent recoveries of arsenic, depending on the relative amounts of acid left in the sample and blank solutions, In contrast to sulphuric acid, other acids (e.g., halogen acids and/or nitric acid) expected to arise from digestion of samples containing acidic elements other than arsenic, do not interfere since these are completely eliminated in the repeated evaporation steps. However, to make the procedure generally applicable to any arsenic-containing compound, the prob-
FALTACXJS
lem of interferences was overcome completely by incorporating the two-step titration method advocated previously’ for the analogous determination of phosphorus. In the first step, the arsenic acid and any other acid remaining are allowed to react in aqueous acetone medium with KI and K103: 6H3As04
I- IO;
+ 51+6H,AsO,
6HX + IO;
+ 3H20 + 31z
+ SI-+6X_
+ 3H*O + 312.
The total iodine liberated, due to arsenic(V) and any other acidity, is removed by titration, first with O.OSM thiosulphate, to avoid excessive dilution, till pale yellow, and then with O.OlM ~iosulphate till colourless. This titration value is not needed, of course, in the calculation. In the second step, the dihydrogen arsenate is allowed to react with zinc sulphate whereby zinc arsenate is formed along with liberation of an equivalent amount of protons, which then react with KI and KIO,, both added this time in solid form to avoid dilution, with quantitative liberation of iodine, which is directly titrated with O.OlM thiosulphate, to a colourless end-point. The titration value obtained in this second step is used to calculate the arsenic content of the organic sample. 2H,AsO;
+- 3Zn*’ -+Zn3(As0.&
6H+ -t SI- + IO; -+ 3H,O
+ 4H’
+ 31,
As in the case of phosphorus determination,’ the acetone in the medium allows complete reaction, in the first step, of the traces of interfering acids, while making the arsenic acid function only as a monobasic acid. The presence of acetone also renders the iodine colour very bright yellow, so the iodine also acts as the indicator. To improve the end-point detection the acetone is added in two portions, one along with the iodide-iodate mixture for the first reaction, followed by titration after a -5-min waiting period, and the second only just before the second titration so that the reaction with zinc sulphate has enough time to proceed rapidly and completely. Satisfactory results (Table 1) were obtained, showing an average absolute error of f0.3% and an average recovery of 99.5%. Arsenazo [3-(2-arsonophenylazo~4,5-dihydroxy-2,7-naphthalene disulphonic acid] gave ca. 3.5% low arsenic recovery probably because of impurity and/or incomplete decomposition. The blank values did not exceed 0.06 ml of O.OlM thiosulphate solution which is considered reasonable for such a rather lengthy digestion procedure (up to 6 hr). It was the slowness of the d~omposition that led to development of the oxygen flask method. The oxygen jask
method
Preliminary tests with sample holder and saturated
the ordinary bromine-water
platinum or hydro-
Arsenic in organic compounds Table 1. Iodometric microdetermination
515
of arsenic in organic compounds by the wet digestion method
As, % Weight, mg Triphenylarsine n-Butylarsonic acid Phenylarsonic acid Benzylarsonic acid 4-Dimethylaminoazobenzene-4’-arsenic Triphenylarsine
acid
sulphide
Arsenazo
gen peroxide as absorption medium gave very low arsenic results. This was not unexpected, because arsenic has been reported” to alloy with platinum.
Thus, a modified version of the quartz spiral stopper, introduced previously,‘s3 was used, but the arsenic values found were still low, indicating incomplete decomposition and/or incomplete oxidation of As(II1) to As(V). Although it is customary to use alkaline hypobromite3 or alkaline hydrogen peroxide4 as absorption medium to obtain arsenic(Vk3 the use of alkaline medium was ruled out because the present method is based on the acidity of the arsenic acid. Trials were then made with concentrated nitric acid as absorbent. In addition to oxidizing As(II1) to As(V), the excess of nitric acid was easily and smoothly removed by heating on a boiling waterbath, leaving a white residue of arsenic acid which Table 2. Iodometric microdetermination
4.936 5.805 7.871 4.956 6.100 4.344 7.006 4.106 5.520 5.986 8.240 5.606 8.300 6.010 8.416
Recovery, % Found
CMC.
24.46 41.15 37.09 35.67 21.46 22.11 13.13
24.2 24.2 24.8 41.6 40.8 37.4 36.8 35.4 36.0 21.2 21.7 22.4 21.8 12.7 12.7
98.9 99.0 101.4 101.0 99.2 100.9 99.2 99.2 100.9 98.9 101.2 101.4 98.7 96.6 96.7
dissolved easily in hot water. Addition of zinc sulphate to this solution gave a precipitate of zinc arsenate and an equivalent amount of protons, the arsenic acid acting as a tribasic acid: 2H3As04 + 3Zn2+ + 2Zn3(As04)2 + 6H+ The protons liberated can be determined by the iodideiodate reaction. The volume should not exceed 6-7 ml for the precipitation reaction to proceed quantitatively within 5 min, and an equal period should be allowed for complete iodometric reaction. The use of concentrated nitric acid as the absorbent succeeded for all the samples analysed except thorin and 4-dimethylaminoazobenzene-4’-arsenic acid, which gave results 69% low. About 5mg of potassium nitrate, mixed with the sample before wrapping in the ashless filter paper flag, proved highly efficient
of arsenic in organic compounds by the oxygen-flask method As, % Weight, mg
Compound Triphenylarsine* n-Butylarsonic acid* Phenylarsonic acid* Benzylarsonic acid* 4-Dimethylaminoazobenzene-4 Triphenylarsine
-arsenic acid*
sulphidet
Arsenazot Thorint
* Calculated on the basis of 3-fold amplification. t Calculated on the basis of 2-fold amplification.
4.380 5.000 7.531 4.506 5.354 4.152 5.650 4.950 7.017 4.976 7.401 5.906 7.555 5.004 6.826 5.162 8.005
Recovery, % Calc.
Found
24.46
24.7 24.7 24.2 40.9 41.4 37.4 37.3 34.4 34.5 21.7 21.2 22.4 22.2 13.4 12.8 17.7 17.7
41.15 37.09 34.67 21.45 22.11 13.13 18.04
101.1 101.0 99.1 99.4 100.5 100.7 100.6 99.3 99.5 101.2 98.9 101.2 100.4 102.2 97.8 97.9 97.8
516
Y. A. GAWARGIOUS,L. S. E~XJLOSand B. N. FALTAOOS
as an auxiliary oxidant during combustion. In addition to raising the arsenic recoveries to an average of ca. 98 and 99% for these two compounds, respectively, the potassium nitrate caused no interference, and may conveniently be added to any arsenical sample. Satisfactory results (Table 2) were obtained. As expected, nitrogen and halogens did not interfere, because their products are readily expelled in the evaporation steps. The effect of halogens was tested by addition of micro-amounts of hydrochloric acid, bromine-water, and iodine either to the absorbed combustion products of triphenylarsine or in blank experiments. On the other hand, sulphur-containing arsenicals could not be successfully analysed by the simple oxygen flask procedure, owing to the incomplete removal of sulphuric acid in the evaporation step, but this problem was readily solved by applying the twostep procedure developed for the wet digestion method. Arsenazo yielded better arsenic recoveries by the oxygen flask method than by wet digestion. Thorin, however, gave 2% low recovery, possibly ascribable to impurity. The results in Table 2 show an average absolute error of +0.3% and overall average recovery of 99.9%. The blank values (0.04ml of O.OlM thiosulphate) were smaller than those for wet digestion.
Conclusions
The oxygen flask method is superior in the following respects: (i) it is simpler and less tedious than the wet digestion method; it offers a 3-fold amplification (except for the sulphur arsenicals) compared with the 2-fold amplification for the wet-digestion method: (ii) in general it gives the better results. Phosphorus, of course, interferes quantitatively, as expected from the previous work’,’ but compounds containing both arsenic and phosphorus are not very common. REFERENCES
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