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Acid decomposition method for visual colorimetric determination of molybdenum in geochemical exploration
Analytica Chimica Acta, 244 (1991) 245-246 Elsevier Science Publishers B.V.. Amsterdam
245
Acid decomposition method for visual calorimetric determination of molybdenum in geochemical exploration G.S. Reddi
* and C.R.M.
Rao
Geological Survey of India, Guindy, Madras-600 (Received
23rd October
032 (India)
1990)
Abstract
A mixture of nitric and sulphuric acids is used for the decomposition of geochemical samples. Determination of molybdenum at the ng g-’ level is effected by visually comparing the colour of the thiocyanate complex. The acid medium seems to be well suited for the calorimetric determination of molybdenum. The same solution can be used advantageously for atomic absorption measurements for larger amounts of molybdenum. The method is suitable for batch analysis and results in a high throughput. Keywords: Colorimetry;
Molybdenum;
Geological
samples
EXPERIMENTAL
The decomposition of silicate materials for the determination of molybdenum has been carried out by alkali mixture fusion [l], by potassium hydrogensulphate fusion [2,3] and by acid attack with nitric acid-perchloric acid [4,5]. The subsequent calorimetric determination of molybdenum in the resulting solution was based on the formation of the thiocyanate or dithiol complex. This paper describes a nitric acid-sulphuric acid decomposition procedure for the determination of molybdenum by the thiocyanate colorimetric method. A clear separation of the organic layer containing the molybdenum-thiocyanate complex is obtained in the acid medium used. This method results in a high throughput of up to 100 geochemical samples per man day. Organic matter, if present in the sample, is oxidized by the nitric acidsulphuric acid combination, thereby removing any interference in the final determination. 0003-2670/91/$03.50
0 1991 - Elsevier
Science Publishers
AND
RESULTS
A l-g amount of sample (smaller than 80 mesh) is weighed into a lOO-ml borosilicate glass beaker and 10 ml of nitric acid and 5 ml of sulphuric acid (1: 1) are added from an automatic dispenser. The beaker is heated on an electric hot-plate to obtain a syrupy mass. To the cooled contents, lo-20 ml of demineralized water are added and the mixture is digested for 30 min on low heat. The contents are then diluted to 40 ml with demineralized water. After allowing the insoluble matter to settle, a 2-ml aliquot of the clear solution is transferred into a borosilicate glass test-tube with a ground-in stopper. A l-ml volume of 10% potassium nitrate solution and 1 ml of 25% sodium tartrate solution are added, followed by 1 ml of 8% ammonium thiocyanate solution. The contents of the test-tube are mixed and 1 ml of tin(I1) chloride solution [lo B.V.
G.S. REDDI
246
of tin(I1) chloride dissolved in 25 ml of hydrochloric acid and made up to 100 ml with water] is added and shaken well. Then 0.3 ml of diisopropyl ether is delivered from a burette and the test-tube is shaken vigorously for 10 s. The layers are allowed to separate and the coloured organic layer is visually compared against standards (0.1, 0.2,. . . , 1.0 pg of MO). In some instances the aqueous layer may be yellow owing to unoxidized organic matter, which might affect the visual comparison of the coloured organic layer. In such an event the lower aqueous layer is blocked from view with a piece of paper before comparing the organic layers. For the amount of sample and aliquot taken in this method, a detection limit of 2 pg gg’ is obtained. The procedure can be applied to samples containing a maximum of 150 pg g- ’ of molybdenum, with dilution if necessary. For higher molybdenum contents the sample solution can be aspirated for atomic absorption spectrometric measurement using a nitrous oxide-acetylene flame. The molybdenum contents of several international standard samples were determined by the proposed method and the results agreed well with the certified values (Table 1). The decomposition with nitric acid-sulphuric acid mixture is more productive and cost effective than previous fusion or acid decomposition procedures. R
TABLE
AND
C.R.M.
RAO
1
Molybdenum contents of standard termined by the proposed method Sample
NBS 1474
samples
de-
Mo (pg gg’) Proposed
AMDEL-24610 CCRMP-3204 CCRMP-3207 AMDEL-1561 AMDEL-24424 BHl
reference
54 145 300 570 40 330 0.63%
a Each value is the average
method
a
Certified
value
50 140 290 580 38 330 0.64% of three determinations.
The authors thank Shri C.R. Narayanan, Geological Survey of India, for helpful suggestions and the Australian Mineral Development Laboratories (AMDEL), Adelaide, for providing the standards.
REFERENCES R.E. Stanton, Rapid Methods of Trace Analysis for Geochemical Application, Arnold, London, 1966, p. 71. R.E. Stanton and A.J. Hardwick, Analyst, 92 (1967) 387. R.E. Stanton, Analytical Methods for Use in Geochemical Exploration, Arnold, London, 1976, p. 9. R.E. Stanton, M. Mockler and S. Newton, J. Geochem. Explor., 2 (1973) 37. S.J. Hoffman and M.J. Waskett-Myers, J. Geochem. Explor., 3 (1973) 61.
245
Acid decomposition method for visual calorimetric determination of molybdenum in geochemical exploration G.S. Reddi
* and C.R.M.
Rao
Geological Survey of India, Guindy, Madras-600 (Received
23rd October
032 (India)
1990)
Abstract
A mixture of nitric and sulphuric acids is used for the decomposition of geochemical samples. Determination of molybdenum at the ng g-’ level is effected by visually comparing the colour of the thiocyanate complex. The acid medium seems to be well suited for the calorimetric determination of molybdenum. The same solution can be used advantageously for atomic absorption measurements for larger amounts of molybdenum. The method is suitable for batch analysis and results in a high throughput. Keywords: Colorimetry;
Molybdenum;
Geological
samples
EXPERIMENTAL
The decomposition of silicate materials for the determination of molybdenum has been carried out by alkali mixture fusion [l], by potassium hydrogensulphate fusion [2,3] and by acid attack with nitric acid-perchloric acid [4,5]. The subsequent calorimetric determination of molybdenum in the resulting solution was based on the formation of the thiocyanate or dithiol complex. This paper describes a nitric acid-sulphuric acid decomposition procedure for the determination of molybdenum by the thiocyanate colorimetric method. A clear separation of the organic layer containing the molybdenum-thiocyanate complex is obtained in the acid medium used. This method results in a high throughput of up to 100 geochemical samples per man day. Organic matter, if present in the sample, is oxidized by the nitric acidsulphuric acid combination, thereby removing any interference in the final determination. 0003-2670/91/$03.50
0 1991 - Elsevier
Science Publishers
AND
RESULTS
A l-g amount of sample (smaller than 80 mesh) is weighed into a lOO-ml borosilicate glass beaker and 10 ml of nitric acid and 5 ml of sulphuric acid (1: 1) are added from an automatic dispenser. The beaker is heated on an electric hot-plate to obtain a syrupy mass. To the cooled contents, lo-20 ml of demineralized water are added and the mixture is digested for 30 min on low heat. The contents are then diluted to 40 ml with demineralized water. After allowing the insoluble matter to settle, a 2-ml aliquot of the clear solution is transferred into a borosilicate glass test-tube with a ground-in stopper. A l-ml volume of 10% potassium nitrate solution and 1 ml of 25% sodium tartrate solution are added, followed by 1 ml of 8% ammonium thiocyanate solution. The contents of the test-tube are mixed and 1 ml of tin(I1) chloride solution [lo B.V.
G.S. REDDI
246
of tin(I1) chloride dissolved in 25 ml of hydrochloric acid and made up to 100 ml with water] is added and shaken well. Then 0.3 ml of diisopropyl ether is delivered from a burette and the test-tube is shaken vigorously for 10 s. The layers are allowed to separate and the coloured organic layer is visually compared against standards (0.1, 0.2,. . . , 1.0 pg of MO). In some instances the aqueous layer may be yellow owing to unoxidized organic matter, which might affect the visual comparison of the coloured organic layer. In such an event the lower aqueous layer is blocked from view with a piece of paper before comparing the organic layers. For the amount of sample and aliquot taken in this method, a detection limit of 2 pg gg’ is obtained. The procedure can be applied to samples containing a maximum of 150 pg g- ’ of molybdenum, with dilution if necessary. For higher molybdenum contents the sample solution can be aspirated for atomic absorption spectrometric measurement using a nitrous oxide-acetylene flame. The molybdenum contents of several international standard samples were determined by the proposed method and the results agreed well with the certified values (Table 1). The decomposition with nitric acid-sulphuric acid mixture is more productive and cost effective than previous fusion or acid decomposition procedures. R
TABLE
AND
C.R.M.
RAO
1
Molybdenum contents of standard termined by the proposed method Sample
NBS 1474
samples
de-
Mo (pg gg’) Proposed
AMDEL-24610 CCRMP-3204 CCRMP-3207 AMDEL-1561 AMDEL-24424 BHl
reference
54 145 300 570 40 330 0.63%
a Each value is the average
method
a
Certified
value
50 140 290 580 38 330 0.64% of three determinations.
The authors thank Shri C.R. Narayanan, Geological Survey of India, for helpful suggestions and the Australian Mineral Development Laboratories (AMDEL), Adelaide, for providing the standards.
REFERENCES R.E. Stanton, Rapid Methods of Trace Analysis for Geochemical Application, Arnold, London, 1966, p. 71. R.E. Stanton and A.J. Hardwick, Analyst, 92 (1967) 387. R.E. Stanton, Analytical Methods for Use in Geochemical Exploration, Arnold, London, 1976, p. 9. R.E. Stanton, M. Mockler and S. Newton, J. Geochem. Explor., 2 (1973) 37. S.J. Hoffman and M.J. Waskett-Myers, J. Geochem. Explor., 3 (1973) 61.