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Determination of nitrate and nitrite with ascorbic acid
Talanta, Vol. 27, pp. 593 to 594
0039-9140/80/0701-0593S02.00/0
© Pergamon Press Ltd 1980. Printed in Great Britain
DETERMINATION OF NITRATE AND NITRITE WITH ASCORBIC ACID N. RUKMINI, V. S. N. P. KAVITHA and K. RAMA RAO Department of Chemistry, Andhra University, Waltair-530003, India (Received 30 Au#ust 1979. Revised 29 November 1979. Accepted 31 December 1979) Summary--A direct titrimetric procedure has been developed for the determination of nitrate and nitrite with ascorbic acid in 9-12M phosphoric acid medium. Ferroin, N-phenylanthranilic acid, barium diphenylamine sulphonate and diphenylbenzidene can be used as the indicator. The method has also been applied for the assay of nitrate in fertilizers.
A literature survey reveals no determination of nitrate with ascorbic acid, and although nitrite has been titrated potentiometrically I and indirectly 2 with ascorbic acid, no titration with a visual end-point has been reported. We have therefore undertaken the study of the oxidation of ascorbic acid with nitrate and nitrate and the present paper describes a direct titrimetric determination of nitrate and nitrite with ascorbic acid, with a visual end-point.
Determination of nitrate in fertilizers Dissolve 1-2 g of sample (accurately weighed) in distilled water and dilute to give a nitrate concentration of 3-4 mg/ml in a known volume of solution, and take a 5-ml aliquot. Alternatively, dissolve a suitable weight of sample in the minimum quantity of distilled water. Add 1 ml of the manganese(II) solution and enough orthophosphoric acid to give a concentration of 10M at the end-point, followed by 0.04 ml of ferroin or 0.2 ml of any of the other indicators, and titrate as already described.
RESULTS AND DISCUSSION
EXPERIMENTAL Reaoents Sodium nitrate solution, 0.1N. Prepared from analytical grade reagent and standardized, a Sodium nitrite solution, 0.1N. Prepared from analytical grade reagent and standardized. 4 Ascorbic acid solution, 0.1N. Prepared from guaranteed grade reagent, stabilized with formic acid and EDTA, and standardized, s Manganese(II) sulphate solution, 5%. Solutions of ferroin (0.025M), N-phenylanthranilic acid (NPA) (0.1~), barium diphenylamine sulphonate (BDAS) (0.1~o) and diphenylbenzidine (DB) (0.1~o) were prepared in the usual way. Analytical grade reagents were used whenever possible. Procedures Determination of nitrate. A sample of nitrate solution is transferred to a titration vessel and treated with enough orthophosphoric acid to give 10M concentration of the acid at the end-point. To this solution are added 1 ml of manganous sulphate solution and 0.04 ml of ferroin or 0.2 ml of any of the other indicators, followed by suitable dilution with distilled water. The mixture is titrated with a standard solution of ascorbic acid to a sharp colour change from pale blue to orange red (ferroin), reddish pink to yellow (NPA), or bluish violet to light yellow (BDAS or DB). Determination of nitrite. A known volume of standard ascorbic acid solution is taken in a titration vessel and treated with enough orthophosphoric acid to give 10M concentration of the acid at the end-point. To this is added 0.04 ml of ferroin or 0.2 ml of BDAS, followed by suitable dilution with distilled water. The mixture is then titrated with the nitrite solution, with the tip of the burette under the surface of the well-stirred solution, to a sharp colour change from orange red to pale blue (ferroin) or from yellow to brown (BDAS).
In preliminary experiments it was observed that the titration of nitrate or nitrite with ascorbic acid in hydrochloric or sulphuric acid media to a visual endpoint is not feasible because the reduction with ascorbic acid is slow, and the indicator reaction is also slow (or even non-existent). The titration is found to be possible in 9-12M phosphoric acid medium but in the case of nitrate a catalyst is needed. The catalysts commonly used for redox reactions are molybdate, osmic acid, manganese(II) etc. We found that manganese(II) at a minimum final concentration of 0.4 x 10-3M (and added before the indicator) is suitable. However, it is advisable to add the first 3 or 4 drops of ascorbic acid slowly (during 10-15 see). The mechanism of the catalysis may be that Mn(II) is oxidized to Mn(III) by nitrate, followed by reduction back to Mn(II) by ascorbic acid. A large excess of manganese(lI) has no adverse effect on the course of the reaction. The reduction of ferroin by ascorbic acid is fairly fast whereas that of the oxidized forms of NPA, BDAS or DB is relatively slow. With the last three indicators quantitative results are obtained if the titrant is added slowly when the evolution of nitric oxide begins. Below 9M phosphoric acid concentration, manganese(II) is no longer effective as a catalyst. If nitrite is titrated with ascorbic acid, low values are obtained because of loss of gaseous products from the acidified nitrite. 4 Hence a reverse titration is used, with the tip of the burette kept under the surface of
593
594
SHORT COMMUNICATIONS
Table 1. Determination of nitrate and nitrite with ascorbic acid
Nitrite
Taken, m9
Ferroin
9.57
9.57 (O.Ol) 23.97 (0.02) 62.23 (0.04) 4.10 (0.006) 10.27 (0.02) 26.66 (0.03)
23.92 62.13 Nitrate
4.097 10.26 26.64
Mean found, m0 NPA BDAS ----4.11 (0.008) 10.27 (0.03) 26.67 (0.02)
9.57 (O.Ol) 23.90 (0.03) 62.2~ (0.05) 4.11 (0.008) 10.27 (0.03) 26.67 (0.03)
DB --
4.11 (0.009) 10.27 (0.03) 26.67 (0.03)
Values in parantheses are standard deviations (5 variates). the solution. Variation of the phosphoric acid concentration shows that a concentration below 9M is not suitable, because oxidation of the indicators is slow, although the oxidation of ascorbic acid is fast even at 7M phosphoric acid concentration. In 9-12M phosphoric acid medium nitrite oxidizes the indicators instantaneously. Ferroin and BDAS function satisfactorily, but not DB. N P A is destroyed and gives no colour change at all. In both titrations the reduction product is nitric oxide and the oxidation product is dehydroascrobic acid. The indicator colour changes are sharp and stable. With nitrite the indicator correction is negligible and with nitrate it is zero. Results are presented in Table 1. Results for nitrate in fertilizer were the same ( + 0.19/o) as those obtained by Leithe's method?
Interferences Ce(W), V(V), Cr(VI) and higher oxidation states of
manganese interfere. Fe(III), U(VI), Mo(VI), Ai(III), fluoride, sulphate and chloride do not interfere.
Acknowledgement--We are grateful to the authorities of C.S.I.R. (India) and U.G.C. (India) for the award of Junior Research Fellowships to V.S.N.P.K. and K.R.R. respectively.
REFERENCES
1. A. Berka, J. Vulterin and J. Z~ka, Newer Redox Titrants, p. 161. Pergamon, Oxford, 1965. 2. L. Erdey, I. Buz~is and K. Vigh, Periodica Polytech., 1959, 3, 1. 3. W. Leithe, Mikrochemie, 1947, 48, 33. 4. I. M. Kolthoff, V. A. Stenger, R. Belcher and G. Matsuyama, Volumetric Analysis, Vol. III, pp. 69-70. Interscience, New York, 1957. 5. L. Erdey and E. Bodor, Anal. Chem., 1952, 24, 418.
0039-9140/80/0701-0593S02.00/0
© Pergamon Press Ltd 1980. Printed in Great Britain
DETERMINATION OF NITRATE AND NITRITE WITH ASCORBIC ACID N. RUKMINI, V. S. N. P. KAVITHA and K. RAMA RAO Department of Chemistry, Andhra University, Waltair-530003, India (Received 30 Au#ust 1979. Revised 29 November 1979. Accepted 31 December 1979) Summary--A direct titrimetric procedure has been developed for the determination of nitrate and nitrite with ascorbic acid in 9-12M phosphoric acid medium. Ferroin, N-phenylanthranilic acid, barium diphenylamine sulphonate and diphenylbenzidene can be used as the indicator. The method has also been applied for the assay of nitrate in fertilizers.
A literature survey reveals no determination of nitrate with ascorbic acid, and although nitrite has been titrated potentiometrically I and indirectly 2 with ascorbic acid, no titration with a visual end-point has been reported. We have therefore undertaken the study of the oxidation of ascorbic acid with nitrate and nitrate and the present paper describes a direct titrimetric determination of nitrate and nitrite with ascorbic acid, with a visual end-point.
Determination of nitrate in fertilizers Dissolve 1-2 g of sample (accurately weighed) in distilled water and dilute to give a nitrate concentration of 3-4 mg/ml in a known volume of solution, and take a 5-ml aliquot. Alternatively, dissolve a suitable weight of sample in the minimum quantity of distilled water. Add 1 ml of the manganese(II) solution and enough orthophosphoric acid to give a concentration of 10M at the end-point, followed by 0.04 ml of ferroin or 0.2 ml of any of the other indicators, and titrate as already described.
RESULTS AND DISCUSSION
EXPERIMENTAL Reaoents Sodium nitrate solution, 0.1N. Prepared from analytical grade reagent and standardized, a Sodium nitrite solution, 0.1N. Prepared from analytical grade reagent and standardized. 4 Ascorbic acid solution, 0.1N. Prepared from guaranteed grade reagent, stabilized with formic acid and EDTA, and standardized, s Manganese(II) sulphate solution, 5%. Solutions of ferroin (0.025M), N-phenylanthranilic acid (NPA) (0.1~), barium diphenylamine sulphonate (BDAS) (0.1~o) and diphenylbenzidine (DB) (0.1~o) were prepared in the usual way. Analytical grade reagents were used whenever possible. Procedures Determination of nitrate. A sample of nitrate solution is transferred to a titration vessel and treated with enough orthophosphoric acid to give 10M concentration of the acid at the end-point. To this solution are added 1 ml of manganous sulphate solution and 0.04 ml of ferroin or 0.2 ml of any of the other indicators, followed by suitable dilution with distilled water. The mixture is titrated with a standard solution of ascorbic acid to a sharp colour change from pale blue to orange red (ferroin), reddish pink to yellow (NPA), or bluish violet to light yellow (BDAS or DB). Determination of nitrite. A known volume of standard ascorbic acid solution is taken in a titration vessel and treated with enough orthophosphoric acid to give 10M concentration of the acid at the end-point. To this is added 0.04 ml of ferroin or 0.2 ml of BDAS, followed by suitable dilution with distilled water. The mixture is then titrated with the nitrite solution, with the tip of the burette under the surface of the well-stirred solution, to a sharp colour change from orange red to pale blue (ferroin) or from yellow to brown (BDAS).
In preliminary experiments it was observed that the titration of nitrate or nitrite with ascorbic acid in hydrochloric or sulphuric acid media to a visual endpoint is not feasible because the reduction with ascorbic acid is slow, and the indicator reaction is also slow (or even non-existent). The titration is found to be possible in 9-12M phosphoric acid medium but in the case of nitrate a catalyst is needed. The catalysts commonly used for redox reactions are molybdate, osmic acid, manganese(II) etc. We found that manganese(II) at a minimum final concentration of 0.4 x 10-3M (and added before the indicator) is suitable. However, it is advisable to add the first 3 or 4 drops of ascorbic acid slowly (during 10-15 see). The mechanism of the catalysis may be that Mn(II) is oxidized to Mn(III) by nitrate, followed by reduction back to Mn(II) by ascorbic acid. A large excess of manganese(lI) has no adverse effect on the course of the reaction. The reduction of ferroin by ascorbic acid is fairly fast whereas that of the oxidized forms of NPA, BDAS or DB is relatively slow. With the last three indicators quantitative results are obtained if the titrant is added slowly when the evolution of nitric oxide begins. Below 9M phosphoric acid concentration, manganese(II) is no longer effective as a catalyst. If nitrite is titrated with ascorbic acid, low values are obtained because of loss of gaseous products from the acidified nitrite. 4 Hence a reverse titration is used, with the tip of the burette kept under the surface of
593
594
SHORT COMMUNICATIONS
Table 1. Determination of nitrate and nitrite with ascorbic acid
Nitrite
Taken, m9
Ferroin
9.57
9.57 (O.Ol) 23.97 (0.02) 62.23 (0.04) 4.10 (0.006) 10.27 (0.02) 26.66 (0.03)
23.92 62.13 Nitrate
4.097 10.26 26.64
Mean found, m0 NPA BDAS ----4.11 (0.008) 10.27 (0.03) 26.67 (0.02)
9.57 (O.Ol) 23.90 (0.03) 62.2~ (0.05) 4.11 (0.008) 10.27 (0.03) 26.67 (0.03)
DB --
4.11 (0.009) 10.27 (0.03) 26.67 (0.03)
Values in parantheses are standard deviations (5 variates). the solution. Variation of the phosphoric acid concentration shows that a concentration below 9M is not suitable, because oxidation of the indicators is slow, although the oxidation of ascorbic acid is fast even at 7M phosphoric acid concentration. In 9-12M phosphoric acid medium nitrite oxidizes the indicators instantaneously. Ferroin and BDAS function satisfactorily, but not DB. N P A is destroyed and gives no colour change at all. In both titrations the reduction product is nitric oxide and the oxidation product is dehydroascrobic acid. The indicator colour changes are sharp and stable. With nitrite the indicator correction is negligible and with nitrate it is zero. Results are presented in Table 1. Results for nitrate in fertilizer were the same ( + 0.19/o) as those obtained by Leithe's method?
Interferences Ce(W), V(V), Cr(VI) and higher oxidation states of
manganese interfere. Fe(III), U(VI), Mo(VI), Ai(III), fluoride, sulphate and chloride do not interfere.
Acknowledgement--We are grateful to the authorities of C.S.I.R. (India) and U.G.C. (India) for the award of Junior Research Fellowships to V.S.N.P.K. and K.R.R. respectively.
REFERENCES
1. A. Berka, J. Vulterin and J. Z~ka, Newer Redox Titrants, p. 161. Pergamon, Oxford, 1965. 2. L. Erdey, I. Buz~is and K. Vigh, Periodica Polytech., 1959, 3, 1. 3. W. Leithe, Mikrochemie, 1947, 48, 33. 4. I. M. Kolthoff, V. A. Stenger, R. Belcher and G. Matsuyama, Volumetric Analysis, Vol. III, pp. 69-70. Interscience, New York, 1957. 5. L. Erdey and E. Bodor, Anal. Chem., 1952, 24, 418.