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Solvent extraction and photometric determination of molybdenum (VI) with 2-aminobenzenethiol
736 Tdumo.
SHORT
Vol. 23. pp 736-738
Pergamon
CQhiMUNICATlONS
Press. 1976 Prmted m Great Bntain.
SOLVENT EXTRACTION AND PHOTOMETRIC DETERMINATION OF MOLYBDENUM (VI) WITH 2-AMINOBENZENETHIOL* ANIL K. CtUmr~~n and SASWATIP. BAG@ Department of Chemistry, Jadavpur University, Calcutta-700032, India (Received 14 May 1975. Revised 16 March 1976. Accepted 15 April 1976) In our search for sensttive reagents contaming sulphur as a donor atom for molybdenum, 2-aminobenzenethiol’ was found to be a good colour-forming reagent. There are only a few chelating agents with the -C=c-
I
I
SH NH2 grouping that form stable complexes with metal oxocations such as VO:. WOf’, TiO’* and MOO?+.” Molybdenum usually- forms- stable oxo-species, ;.g., MoOi+. MOO*+. MoO(OH13+. in combination with var& ‘chelating agents cn t&e acidic pH range. Except for instances where ligands such as dithiocarbamates reduce Mo(VI) to MO(V), the stable chelates of oxo-molyb denum ions are reported to be formed from Mo(VI). The present investigation shows that with 2-aminobenzenethiol, molybdenum forms a green complex which is quantitatively extractable into chloroform at pH 1.4-2.8. fie colour iI stable for 2 hr at room tempera&e and the molar absormivitv is 7.08 x IO* l.mole- .cm-‘. The reagent has b&n abplied to the estimation of MO in highspeed alloy steel containing Cr. MO, W and V. EXPERIMENTAL
Reagents
Ammonium heptamolybdate o&hydrate was dissolved in water containing a few drops of ammonia and the solution standardized gravimetrically by the 8-hydroxyquinoline method.3 Working solutions were prepared by appro_. priate dilution of the stock solution. _ _ Chloroform was freed from alcohol by washing with dilute sulphuric acid, dilute ammonia and then tw&e-distilled water. Anhydrous sodium sulphate was used to free the organic phase from water. Purification of 2-aminobenzenethiol. The reagent was fractionally distilled under reduced pressure. Because the compound is sensitive to atmospheric oxidation the distillation system was saturated with nitrogen. The product distilling at 88-91”/1 mmHg was kept frozen under nitrogen. The pure liquid is almost colourless at room temperature. A 2”/, w/v solution of reagent was freshly prepared in pure chloroform before use. Procedure
Adjust the pH of the aqueous molybdenum solution (10 ml) to 2.0. Add 5 ml of 2% reagent solution and 1ml of pure chloroform, shake the mixture thoroughly for 5 min m a 50-ml separatory funnel, collect the chloroform layer m a 50-ml beaker and dry it over anhydrous sodium sulphate. Extract the aqueous phase with further 5-ml portions of chloroform. Transfer the combined extracts quantitatively to a 25-ml volumetric flask and dilute to volume with chloroform. Measure the absorbance at 7OOnm in 1-cm cells. Construct a calibration curve similarly. *Presented at the Symposium on Solvent Extraction, Convention of Chemists. I.I.S., Bombay, 1971, Session VIII.
Procedure for steel analysis
Dissolve an appropriate weight of sample (1 g for 0.2% of MO. 0.1 g for 6% MO) In 50 ml of 3M sulohuric acid in a 150-mi Erle&eyer flask and oxidize with concentrated nitric acid. Place a small funnel in the neck of the flask. Boil to reduce the volume to about 20 ml; cool. add 30ml of water and boil for a few minutes. Cool, filter if necessary, add 0.5 g of ferrous sulphate to reduce vanadium(V) and chromium(VI), transfer to a lOO-ml volumetric flask and dilute to volume. Take 5 ml of this solution, add I ml of saturated ascorbic acid solution and determine the molybdenum content by the procedure described above. Table 2 summarizes the results for two BAS standard steels. RESULTS AND
DISCUSSION
Characteristics of the complex The absorption spectrum of the molybdenum-thiol complex in chloroform, extracted at pH2.0 according to the general procedure, is reproducible for different concentrations of the metal, and has its absorbance maximum at 700nm. The reagent does not absorb at wavelengths longer than 500 nm at the concentrations used. The system obeys Beer’s law from 0.25 to 10 ppm molybdenum and a Ringbom plot4 shows the optimum range to be 0.5-4.5 ppm. The relative error per 1% absolute photometric error for this range is 2.5%. The molar absorptivity and Sandell sen&vity of ihe complex were fobnd -to be 7.08 x lO*I.mole-‘.cm-’ and 0.0075 &cm2 resmctivelv. _. Sufficient colour intensity is observed betwe& pH 016 and4.2, but the optimum range for the quantitative extraction of the chelate is pH 1.4-2.8. The amount of reagent necessary is at least 3 ml of 2% w/v solution for 4ppm MO, and 5 ml is recommended The optimum extraction time for 99.99; extraction IS 5 min. If shaking is continued for more than 15 min the colour intensity decreases. The extracted chelate is stable for at least 2 hr at room temperature, but changes rapidly to a brown complex in presence of oxidizing agents. E&cct of other ions The effect of diverse ions on the molybdenum determination was studied by adding 200 ppm of the ion in question to a solution containing 4 ppm of molybdenum and applying the recommended p&cedure. Ba, Ca, Mg, Sry -Al. TilIV). Sn(IV) and SWIII) do not interfere. Tolerance limits fo; ofher ioni are given’ in Table 1. The interference due to V(V) and Fe(II1) can be eliminated by addition of hydroxylamine hydrochloride and ascorbic acid respectively. Bi(III), Sn(II), and Re(V) obtained by reduction with stannous chloride, interfere seriqusly. Composition of the complex The composition of the complex was determined by Job’s method of continuous variations and was reproducible at different molybdenum concentrations. The Job plot (Fig. 1) showed some unusual features. At the mole ratio corresponding to the MR, species there
(a)
Table 1. Tolerance for foreign ions. (Molybdenum concentratlon: 1 ml of 1.042 x IO-‘M s 4ppm in final volume of 25 ml) Tolerance* limit, ppm
Ion added
loo@’ lOO@’ 100
Zn’+ CU2+ Cd’+ co2+ Ni2+ Mn2+ Fe3 + Cr3 + VS’ W6+
Tolerance* limit, ppm
Ion added
I$., :o,, $,c+J
loo 100 50 50 100 100 100 100 100
uo:+ Pd2+ pt*+ OS6+ Oxalate Tartrate Citrate EDTA Fluoride
100
* Amount causing < 2% error. (a) Masking with 3 ml of 0.2% EDTA. (b) Prior removal by extraction with ~m~by~iyoxime. (c) No interference in presence of ascorbic acid. (d) Using 2% NH,OH .HCI solution. Table
2. Determination of MO in (Mn-Ni-Mo-Cr-V)
high-speed
steel
Mote raTto of reawnt
Fig. 2. Mole-ratio plots: (a) [M] = [R] = 2.084 x 10m2M; (b) [M] = 1.402 x 10-‘&f, [R] = 7.01 x 10-‘&f.
MO. % Weight range studied mg
Certified
Found
Durgapur Steel (HSL, India) (0.49% Mn, 0.34% MO, 1.36% Cr, o.soo/, v BAS No. 60B
500-1000
0.34
0.338 0.337 0.337
5WlOOO
0.43
BAS No. 64B
100-200
4.95
0.427 0.427 0.429 0.429 4.94 4.94 4.94 4.94
Sample
0.6 8 i
was a ~imum in the absorbance and a sbarp ~nirn~ at tbe ratio corresponding to MR3. A mole-ratio plot (Fig. 2) gave a non-linear increase in absorbance up to an R:M ratio of about 3, then a steeper and linear increase followed by a maximum and constant absorbana when an R:M ratio of at least 500 was reached. This formation can be interpreted as indicating first an MR2 complex of low molar absorptivity (2.0 x IO* i.mole-‘.cm-‘) followed by formation of an MRs complex of low stability but high molar absorptivity (found by the Bag and Chakrabarti method5 to be 7.08 x lti I.mok-l.cm-l). The stability constant of the MR, complex was evaluated as 2.0 x 10’ at 25 k 1” by Yatsimirskii’s metbod.6 The reaction can be interpreted as MO O2 L2 + HL 2 MO O(OH) Ls where HL = 2-aminobenzenethiol. The MRs complex was isolated for verification of its composition. Elemental analysis supported the formula MoO(OH)L,. The complex gave negative tests for chloride. The infrared spectrum of the isolated complex showed a sharp absorption peak in the 3100 cm-’ region, suggesting the presence of an O-H stretching band The compound a&o showed an M& absorption band at 92Ocm-’ (s). This again supports the existence of the MoO(OH)3c species. The complexes Mo02L, in presence of a stoichiometric amount of l&and and MoO(OH)Ls in presence of a large excess of ligand are both extractable into chloroform.
0.6
*
REFERENCES
s 04
:: 3.
o-04 M
M+L * MO(PI) 10-Z M
Fig, 1. Job plot. [M] = [R] = 1.042 x lo-‘M.
4. 5. 6.
S. M. Kbopkar. J. Sci. Ind. Res. (India), 1972, 31, 233. R. G. Ch+rles and H. Freiser, J. Am. Chem. Sot., 1952, 74, 1885. A. I. Vogel, Quantitative Inorganic Analysis. 3rd Ed., Longmans, London. 1964. A. Ringbom, Z. Anal. Cttem., 1938, 115, 332. S. P. Bag and A. K. Cbakrabarti, J. Indian C&m. Sot., 1974, 51, 335. K. B. Yatsimirskii and V. P. Vasilev, Instability Constants of Complex Compounds, Consultants Bureau, New York, 1960.
738
SHORT
COMMUNlCATIONS
Summary-A new extractive photometric method IS described for estimation of molybdenum with 2-aminobenzenethiol. The green complex in chloroform has its absorbance maximum at 700nm and IS stable for 2 hr when extracted from a solution of optimum pH range 1.4-2.8. The extraction is quantitative. The sensitivity is 0.0075 &cmz. Beer’s law is obeyed over the range 0.25-10 ppm with optimum range 0.54.5 ppm. The molar absorptivity is 7.08 x 10’ 1.mole- I. cm- I. The overall stahlity constant is 2.0 x 10’ at 25 f 0.1”.
Tdanta.
Vol. 23. pp. 73X-740
Pergamon
Press. 1976 Rntcd
m Great Bntam.
ION-SELECTIVE ELECTRODES IN ORGANIC FUNCTIONAL GROUP ANALYSIS MICRODETERMINATION OF NITRATES AND NITRAMINES WITH USE OF THE IODIDE ELECTRODE Research Microanalytical
&AD s. hf. -AN Laboratory, Department of Chemistry, Faculty of Science Ain Shams University, Cairo, Egypt
(Received 20 February 1976. Accepted 12 April 1976) Organic nitrates and nitramines have been determined by titration with various reductants.’ However, these methods suffer from the defect that many nitrogenous and nonnitrogenous compounds interfere and the titrants need special precautions during preparation, storage and use.’ Methods based on spectrophotomet& and gravimetric* procedures are usually time-consuming and unreliable when used on a routine basis. Gasometric reactions using inorganic5-9 and organic’“,” reagents have also been suggested. Recently, the development of the nitrate-responsive electrodes’2-14 has made possible substantial improvement in the analysis of inorganic nitrates. Reduction of the nitrate ion followed by measurement of the liberated ammonia by means of the ammonium-responsive electrode has also been reported. I’ The use of both electrodes for the analysis of the nitrates by direct potential measurements requires careful adjustment of many variables and the resulting precision is not better than + 2%.“-” Potentiometric titration is more accurate provided that the titrant forms either a stable complex or a precipitate, but unfortunately not many such titrants are available for nitrate or ammonia. However, diphenylthallium(II1) sulphate has been applied for the titration of the nitrate ion, on the semi-micro scale only, with use of the nitrate electrode.16 On the other hand, the nitrate electrode is inapplicable to the determination of organic nitrates, and prior conversion of the organic nitrate or nitramine into inorganic nitrate by acid or alkaline hydrolysis is not quantitative.1’.‘8 The organic moiety of these compounds partially reduces the nitrate to various products such as ammonia and nitrogen oxides. The present work describes a new finish to the determination of organic nitrates and nitramines by reaction with mercury-sulphunc acid mixture, the mercurous ions released being titrated with iodide. and an iodide electrode used to detect the end-point. Several compounds used as high explosives, industrial intermediates and vasodilators have been analysed and the results obtained are accurate. EXPERIMENTAL
Reaqmts and materials
All reagents were analytical grade except where stated. Doublv-distilled water was used throughout. The nitrate and &amine samples used were of p&ty not less than 99”, as confirmed by the gasometric method.5
Apparatus
A Pye Unicam 292 MK2 pH-meter. an Orion 94-53 solid-state iodide-selective electrode and an Orion 90-02 double-junction reference electrode were used. Procedure
Weigh accurately 2-5mg of the ground dried nitrate, nitrite or nitramine sample and transfer it to a test-tube (10 x 21 cm). For smaller samples, transfer to the tube a portion of solution containing 0.1-1.0 mg of the sample and evaporate to complete dryness. Add 2-3 ml of 96% sulphuric acid and displace the air in the tube with pure nitrogen. Add 3 drops of mercury and shake the tube for 5-7min at room temperature, with a continuous flow of nitrogen. Transfer the contents of the tube to a 250-ml beaker, rinsing with co. SOml of doubly-distilled water, and stir. Insert the iodide and reference electrodes, titrate with 0.02M potassium iodide for sample sizes above 2 mg and with 0.002M solution for sample sizes below 2 mg and monitor the e.m.f. As the end-point is approached add the titrant in O.Ol-ml increments. For sample sizes above 5 mg, the titration has to be conducted slowly, with efficient stirnng from the beginning of the titration, since the equilibrium is reached slowly. Run a blank in the same manner. RESULTS
AND
DIBCUSSION
Nature of the reaction
Mercury in presence of concentrated sulphuric acid quantitatively reduces nitrates to nitric oxide,s-9 and is itself converted into mercurous and/or mercuric ions. It is found experimentally that three moles of potassium
Table 1. Effect of temperature on the reaction of mercury with 96% sulphuric acid (reaction time 15 min) Temperature, 20 30 40 60 80 100
‘C
Dissolved mercury, peel 0.4 0.6 1.6 3.6 6.4 90.0
SHORT
Vol. 23. pp 736-738
Pergamon
CQhiMUNICATlONS
Press. 1976 Prmted m Great Bntain.
SOLVENT EXTRACTION AND PHOTOMETRIC DETERMINATION OF MOLYBDENUM (VI) WITH 2-AMINOBENZENETHIOL* ANIL K. CtUmr~~n and SASWATIP. BAG@ Department of Chemistry, Jadavpur University, Calcutta-700032, India (Received 14 May 1975. Revised 16 March 1976. Accepted 15 April 1976) In our search for sensttive reagents contaming sulphur as a donor atom for molybdenum, 2-aminobenzenethiol’ was found to be a good colour-forming reagent. There are only a few chelating agents with the -C=c-
I
I
SH NH2 grouping that form stable complexes with metal oxocations such as VO:. WOf’, TiO’* and MOO?+.” Molybdenum usually- forms- stable oxo-species, ;.g., MoOi+. MOO*+. MoO(OH13+. in combination with var& ‘chelating agents cn t&e acidic pH range. Except for instances where ligands such as dithiocarbamates reduce Mo(VI) to MO(V), the stable chelates of oxo-molyb denum ions are reported to be formed from Mo(VI). The present investigation shows that with 2-aminobenzenethiol, molybdenum forms a green complex which is quantitatively extractable into chloroform at pH 1.4-2.8. fie colour iI stable for 2 hr at room tempera&e and the molar absormivitv is 7.08 x IO* l.mole- .cm-‘. The reagent has b&n abplied to the estimation of MO in highspeed alloy steel containing Cr. MO, W and V. EXPERIMENTAL
Reagents
Ammonium heptamolybdate o&hydrate was dissolved in water containing a few drops of ammonia and the solution standardized gravimetrically by the 8-hydroxyquinoline method.3 Working solutions were prepared by appro_. priate dilution of the stock solution. _ _ Chloroform was freed from alcohol by washing with dilute sulphuric acid, dilute ammonia and then tw&e-distilled water. Anhydrous sodium sulphate was used to free the organic phase from water. Purification of 2-aminobenzenethiol. The reagent was fractionally distilled under reduced pressure. Because the compound is sensitive to atmospheric oxidation the distillation system was saturated with nitrogen. The product distilling at 88-91”/1 mmHg was kept frozen under nitrogen. The pure liquid is almost colourless at room temperature. A 2”/, w/v solution of reagent was freshly prepared in pure chloroform before use. Procedure
Adjust the pH of the aqueous molybdenum solution (10 ml) to 2.0. Add 5 ml of 2% reagent solution and 1ml of pure chloroform, shake the mixture thoroughly for 5 min m a 50-ml separatory funnel, collect the chloroform layer m a 50-ml beaker and dry it over anhydrous sodium sulphate. Extract the aqueous phase with further 5-ml portions of chloroform. Transfer the combined extracts quantitatively to a 25-ml volumetric flask and dilute to volume with chloroform. Measure the absorbance at 7OOnm in 1-cm cells. Construct a calibration curve similarly. *Presented at the Symposium on Solvent Extraction, Convention of Chemists. I.I.S., Bombay, 1971, Session VIII.
Procedure for steel analysis
Dissolve an appropriate weight of sample (1 g for 0.2% of MO. 0.1 g for 6% MO) In 50 ml of 3M sulohuric acid in a 150-mi Erle&eyer flask and oxidize with concentrated nitric acid. Place a small funnel in the neck of the flask. Boil to reduce the volume to about 20 ml; cool. add 30ml of water and boil for a few minutes. Cool, filter if necessary, add 0.5 g of ferrous sulphate to reduce vanadium(V) and chromium(VI), transfer to a lOO-ml volumetric flask and dilute to volume. Take 5 ml of this solution, add I ml of saturated ascorbic acid solution and determine the molybdenum content by the procedure described above. Table 2 summarizes the results for two BAS standard steels. RESULTS AND
DISCUSSION
Characteristics of the complex The absorption spectrum of the molybdenum-thiol complex in chloroform, extracted at pH2.0 according to the general procedure, is reproducible for different concentrations of the metal, and has its absorbance maximum at 700nm. The reagent does not absorb at wavelengths longer than 500 nm at the concentrations used. The system obeys Beer’s law from 0.25 to 10 ppm molybdenum and a Ringbom plot4 shows the optimum range to be 0.5-4.5 ppm. The relative error per 1% absolute photometric error for this range is 2.5%. The molar absorptivity and Sandell sen&vity of ihe complex were fobnd -to be 7.08 x lO*I.mole-‘.cm-’ and 0.0075 &cm2 resmctivelv. _. Sufficient colour intensity is observed betwe& pH 016 and4.2, but the optimum range for the quantitative extraction of the chelate is pH 1.4-2.8. The amount of reagent necessary is at least 3 ml of 2% w/v solution for 4ppm MO, and 5 ml is recommended The optimum extraction time for 99.99; extraction IS 5 min. If shaking is continued for more than 15 min the colour intensity decreases. The extracted chelate is stable for at least 2 hr at room temperature, but changes rapidly to a brown complex in presence of oxidizing agents. E&cct of other ions The effect of diverse ions on the molybdenum determination was studied by adding 200 ppm of the ion in question to a solution containing 4 ppm of molybdenum and applying the recommended p&cedure. Ba, Ca, Mg, Sry -Al. TilIV). Sn(IV) and SWIII) do not interfere. Tolerance limits fo; ofher ioni are given’ in Table 1. The interference due to V(V) and Fe(II1) can be eliminated by addition of hydroxylamine hydrochloride and ascorbic acid respectively. Bi(III), Sn(II), and Re(V) obtained by reduction with stannous chloride, interfere seriqusly. Composition of the complex The composition of the complex was determined by Job’s method of continuous variations and was reproducible at different molybdenum concentrations. The Job plot (Fig. 1) showed some unusual features. At the mole ratio corresponding to the MR, species there
(a)
Table 1. Tolerance for foreign ions. (Molybdenum concentratlon: 1 ml of 1.042 x IO-‘M s 4ppm in final volume of 25 ml) Tolerance* limit, ppm
Ion added
loo@’ lOO@’ 100
Zn’+ CU2+ Cd’+ co2+ Ni2+ Mn2+ Fe3 + Cr3 + VS’ W6+
Tolerance* limit, ppm
Ion added
I$., :o,, $,c+J
loo 100 50 50 100 100 100 100 100
uo:+ Pd2+ pt*+ OS6+ Oxalate Tartrate Citrate EDTA Fluoride
100
* Amount causing < 2% error. (a) Masking with 3 ml of 0.2% EDTA. (b) Prior removal by extraction with ~m~by~iyoxime. (c) No interference in presence of ascorbic acid. (d) Using 2% NH,OH .HCI solution. Table
2. Determination of MO in (Mn-Ni-Mo-Cr-V)
high-speed
steel
Mote raTto of reawnt
Fig. 2. Mole-ratio plots: (a) [M] = [R] = 2.084 x 10m2M; (b) [M] = 1.402 x 10-‘&f, [R] = 7.01 x 10-‘&f.
MO. % Weight range studied mg
Certified
Found
Durgapur Steel (HSL, India) (0.49% Mn, 0.34% MO, 1.36% Cr, o.soo/, v BAS No. 60B
500-1000
0.34
0.338 0.337 0.337
5WlOOO
0.43
BAS No. 64B
100-200
4.95
0.427 0.427 0.429 0.429 4.94 4.94 4.94 4.94
Sample
0.6 8 i
was a ~imum in the absorbance and a sbarp ~nirn~ at tbe ratio corresponding to MR3. A mole-ratio plot (Fig. 2) gave a non-linear increase in absorbance up to an R:M ratio of about 3, then a steeper and linear increase followed by a maximum and constant absorbana when an R:M ratio of at least 500 was reached. This formation can be interpreted as indicating first an MR2 complex of low molar absorptivity (2.0 x IO* i.mole-‘.cm-‘) followed by formation of an MRs complex of low stability but high molar absorptivity (found by the Bag and Chakrabarti method5 to be 7.08 x lti I.mok-l.cm-l). The stability constant of the MR, complex was evaluated as 2.0 x 10’ at 25 k 1” by Yatsimirskii’s metbod.6 The reaction can be interpreted as MO O2 L2 + HL 2 MO O(OH) Ls where HL = 2-aminobenzenethiol. The MRs complex was isolated for verification of its composition. Elemental analysis supported the formula MoO(OH)L,. The complex gave negative tests for chloride. The infrared spectrum of the isolated complex showed a sharp absorption peak in the 3100 cm-’ region, suggesting the presence of an O-H stretching band The compound a&o showed an M& absorption band at 92Ocm-’ (s). This again supports the existence of the MoO(OH)3c species. The complexes Mo02L, in presence of a stoichiometric amount of l&and and MoO(OH)Ls in presence of a large excess of ligand are both extractable into chloroform.
0.6
*
REFERENCES
s 04
:: 3.
o-04 M
M+L * MO(PI) 10-Z M
Fig, 1. Job plot. [M] = [R] = 1.042 x lo-‘M.
4. 5. 6.
S. M. Kbopkar. J. Sci. Ind. Res. (India), 1972, 31, 233. R. G. Ch+rles and H. Freiser, J. Am. Chem. Sot., 1952, 74, 1885. A. I. Vogel, Quantitative Inorganic Analysis. 3rd Ed., Longmans, London. 1964. A. Ringbom, Z. Anal. Cttem., 1938, 115, 332. S. P. Bag and A. K. Cbakrabarti, J. Indian C&m. Sot., 1974, 51, 335. K. B. Yatsimirskii and V. P. Vasilev, Instability Constants of Complex Compounds, Consultants Bureau, New York, 1960.
738
SHORT
COMMUNlCATIONS
Summary-A new extractive photometric method IS described for estimation of molybdenum with 2-aminobenzenethiol. The green complex in chloroform has its absorbance maximum at 700nm and IS stable for 2 hr when extracted from a solution of optimum pH range 1.4-2.8. The extraction is quantitative. The sensitivity is 0.0075 &cmz. Beer’s law is obeyed over the range 0.25-10 ppm with optimum range 0.54.5 ppm. The molar absorptivity is 7.08 x 10’ 1.mole- I. cm- I. The overall stahlity constant is 2.0 x 10’ at 25 f 0.1”.
Tdanta.
Vol. 23. pp. 73X-740
Pergamon
Press. 1976 Rntcd
m Great Bntam.
ION-SELECTIVE ELECTRODES IN ORGANIC FUNCTIONAL GROUP ANALYSIS MICRODETERMINATION OF NITRATES AND NITRAMINES WITH USE OF THE IODIDE ELECTRODE Research Microanalytical
&AD s. hf. -AN Laboratory, Department of Chemistry, Faculty of Science Ain Shams University, Cairo, Egypt
(Received 20 February 1976. Accepted 12 April 1976) Organic nitrates and nitramines have been determined by titration with various reductants.’ However, these methods suffer from the defect that many nitrogenous and nonnitrogenous compounds interfere and the titrants need special precautions during preparation, storage and use.’ Methods based on spectrophotomet& and gravimetric* procedures are usually time-consuming and unreliable when used on a routine basis. Gasometric reactions using inorganic5-9 and organic’“,” reagents have also been suggested. Recently, the development of the nitrate-responsive electrodes’2-14 has made possible substantial improvement in the analysis of inorganic nitrates. Reduction of the nitrate ion followed by measurement of the liberated ammonia by means of the ammonium-responsive electrode has also been reported. I’ The use of both electrodes for the analysis of the nitrates by direct potential measurements requires careful adjustment of many variables and the resulting precision is not better than + 2%.“-” Potentiometric titration is more accurate provided that the titrant forms either a stable complex or a precipitate, but unfortunately not many such titrants are available for nitrate or ammonia. However, diphenylthallium(II1) sulphate has been applied for the titration of the nitrate ion, on the semi-micro scale only, with use of the nitrate electrode.16 On the other hand, the nitrate electrode is inapplicable to the determination of organic nitrates, and prior conversion of the organic nitrate or nitramine into inorganic nitrate by acid or alkaline hydrolysis is not quantitative.1’.‘8 The organic moiety of these compounds partially reduces the nitrate to various products such as ammonia and nitrogen oxides. The present work describes a new finish to the determination of organic nitrates and nitramines by reaction with mercury-sulphunc acid mixture, the mercurous ions released being titrated with iodide. and an iodide electrode used to detect the end-point. Several compounds used as high explosives, industrial intermediates and vasodilators have been analysed and the results obtained are accurate. EXPERIMENTAL
Reaqmts and materials
All reagents were analytical grade except where stated. Doublv-distilled water was used throughout. The nitrate and &amine samples used were of p&ty not less than 99”, as confirmed by the gasometric method.5
Apparatus
A Pye Unicam 292 MK2 pH-meter. an Orion 94-53 solid-state iodide-selective electrode and an Orion 90-02 double-junction reference electrode were used. Procedure
Weigh accurately 2-5mg of the ground dried nitrate, nitrite or nitramine sample and transfer it to a test-tube (10 x 21 cm). For smaller samples, transfer to the tube a portion of solution containing 0.1-1.0 mg of the sample and evaporate to complete dryness. Add 2-3 ml of 96% sulphuric acid and displace the air in the tube with pure nitrogen. Add 3 drops of mercury and shake the tube for 5-7min at room temperature, with a continuous flow of nitrogen. Transfer the contents of the tube to a 250-ml beaker, rinsing with co. SOml of doubly-distilled water, and stir. Insert the iodide and reference electrodes, titrate with 0.02M potassium iodide for sample sizes above 2 mg and with 0.002M solution for sample sizes below 2 mg and monitor the e.m.f. As the end-point is approached add the titrant in O.Ol-ml increments. For sample sizes above 5 mg, the titration has to be conducted slowly, with efficient stirnng from the beginning of the titration, since the equilibrium is reached slowly. Run a blank in the same manner. RESULTS
AND
DIBCUSSION
Nature of the reaction
Mercury in presence of concentrated sulphuric acid quantitatively reduces nitrates to nitric oxide,s-9 and is itself converted into mercurous and/or mercuric ions. It is found experimentally that three moles of potassium
Table 1. Effect of temperature on the reaction of mercury with 96% sulphuric acid (reaction time 15 min) Temperature, 20 30 40 60 80 100
‘C
Dissolved mercury, peel 0.4 0.6 1.6 3.6 6.4 90.0