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A rapid complexometric scheme for analysis of iron, aluminium, calcium and magnesium in slags
Talanro, Vol. 39, No. 6, pp. 6756679,1992
0039-9140/92$5.00+ 0.00 Copyright 0 1992Pergamon Press Ltd
Printed in Great Britain. All rights reserved
A RAPID COMPLEXOMETRIC SCHEME FOR ANALYSIS OF IRON, ALUMINIUM, CALCIUM AND MAGNESIUM IN SLAGS K. C. GHOSH, B. C. MUKHERJEE,N. N. GANGULY, M. YUSLJFand V. N. CHOUDHIJRY
National Metallurgical Laboratory, Jamshedpur 831007, India (Received 3 May 1991. Revised 17 October 1991. Accepted 17 October 1991) Summary-A
simple and rapid complexometric method has been developed for the simultaneous determination of iron, aluminium, calcium and magnesium in a single solution in slags. Phosphorous and small amounts of chromium (1.5 mg) and vanadium (1 mg) do not interfere in the titration. Titanium and manganese are suitably masked with lactic acid and tetra sodium pyrophosphate, respectively. In a suitable aliquot, iron is titrated at pH 2 with EDTA, using sulphosalicylic acid as indicator. To this solution, excess disodium 1,2-cyclohexane diamine tetra acetic acid (DCTA) is added and aluminium is titrated by titrating the excess DCTA with standard copper sulphate solution at pH 3.5, using I-(2-pyridylazo)-2-naphthol (PAN) as an indicator. A known excess of EDTA is added, the pH is raised to 10 and calcium and magnesium are jointly titrated by titrating the excess EDTA with copper sulphate solution, using PAN indicator. The Ca-EDTA complex is demasked with ammonium oxalate at pH 5 and the released EDTA equivalent to calcium is titrated with copper sulphate solution at pH 10 with PAN indicator. Results of analysis compare favourably with certified values and values obtained by standard methods for BCS and other slags. A set of five samples can be analysed for iron, aluminium, calcium and magnesium in four hours as compared to three days by the classical conventional method.
The chemical composition of slag depends on the raw materials used and the production conditions. Since the composition of the slag varies from lot to lot, a rapid and accurate evaluation of a slag composition by chemical analysis is essential for quality control of the raw materials and the finished products. The classical methods’ for determination of alumina, calcium and magnesium in slags are the well-known oxinate, oxalate and pyrophosphate methods, respectively, which are tedious and time consuming. In most of the complexometric methods of determination, calcium and magnesium are usually titrated together with EDTA at pH 10 in an ammoniacal buffer and then calcium alone in another aliquot at pH 12 when magnesium precipitates as hydroxide.2-6 Manganese, if present in slags, interferes seriously with the EDTA titration of calcium and magnesium by precipitating out as brown manganese hydroxide which absorbs metallochromic indicator to yield an unsatisfactory end point. Therefore, it is essential to separate manganese by the lengthy process of sulphide precipitation’ or mercury cathode electrolysis’ prior to the titration of calcium and magnesium with EDTA. Solvent extraction procedures for removal of manganese with diethyl dithiocarbamate* and
oxine’ have also been reported. Titanium,“*” chromium2 and vanadium’* seriously interfere with aluminium as they form stable complexes with EDTA at pH 3.5-5.5. The interference of phosphateI in the titration of calcium and magnesium at pH 10 has also been reported. However, no attempt has yet been made to develop a rapid complexometric method for the simultaneous determination of iron, aluminium, calcium and magnesium in a single solution in slags in the presence of titanium and manganese. As iron, aluminium, calcium and magnesium form stable complexes with EDTA at different pH values, a simple and rapid complexometric scheme has been developed based on this property for the simultaneous determination of these elements in slags in a single solution, using lactic acid” and tetra sodium pyro-phosphate’4 as masking agents for titanium and manganese respectively. EXPERIMENTAL
Reagents Unless analytical prepared redistilled 675
otherwise stated, all reagents were of reagent grade and all solutions were in a one-litre calibrated flask with water.
EDTA solution. Two solutions (0,025 and O.OlM) were prepared by dissolving 9.306 and 3.7225 g, respectively, of disodium ethylene diamine tetra acetate dihydrate in water and diluting to volume. DCTA solution. Two solutions (0.025 and O.OlM ) were prepared by dissolving 8.659 and 3.4636 g, respectively, of 1,2-diaminocyclohexane tetra acetic acid in 100 ml of 1M sodium hydroxide solution and 200 ml of water, and diluting to volume. Standard copper sutphate solution. Two solutions (0.025 and O.OlM) were prepared by dissolving 6.25 and 2.5 g, respectively, of CuSO,.SH,O in water and diluting to volume. A~umini~ solu?ian. A 0.025M solution was prepared by dissolving 0.675 g of aluminium foil (99.99%) in hydrochloric acid (1 + 1) and diluting to the mark. Calcium chloride solution. A 0.025M solution was prepared by dissolving 2.5 g of dried (150”) calcium carbonate in 5 ml of 12M hydrochloric acid and 25 ml of water. The solution was boiled, cooled and diluted up to the mark. Magnesium chloride solution. A O.OlM solution was prepared by dissolving 2.03 g of MgCl,. 6H,O in water and diluting to volume. Iron solution. A 0.0 1M solution was prepared by dissolving 0.56 g of pure iron wire in 10 ml of concentrated hydrochloric acid and diluting to volume. Ammonia-ammonium
chloride bufleer (pH IO).
Prepared by dissolving 67.5 g of ammonium chloride in 250 ml of water and 570 ml of liquid ammonia and diluting to volume. Ammonium acetate, 25%. Prepared by dissolving 25 g of ammonium acetate in 100 ml of water. Ammonium oxalate, 5%. Prepared by dissolving 5 g of ammonium oxalate in 100 ml of water. Lactic acid, 10%. Prepared by diluting 10 ml of lactic acid to 100 ml with water. Tetra sodium pyrophosphate, 10%. Prepared by dissolving 10 g of tetra sodium pyrophosphate in 100 ml of water. I-(2-Pyridylazo)-2-naphthol
(PAN),
0.05%.
Prepared by dissolving 50 mg of the indicator in 100 ml of methanol. Suiphosaiicylic acid (solid).
Foreign ion solution. All solutions were prepared in a 250-ml calibrated flask. Titanium (1 mg/ml). A 0.416-g sample of ignited titanium dioxide was fused with 4 g of potassium bisulphate. After cooling the fused
mass was dissolved in 50 ml of 10% sulphuric acid and diluted to volume with water. Chromium (1 mg/ml). A 0.707-g sample of dried (180”) potassium dichromate was dissolved in water and heated with 5 ml of concentrated hydrochloric acid and 15 ml of ethyl alcohol. The solution was cooled and diluted to volume with water. Phosphorous (1 mglml). A 1.258-g sample of sodium dihydrogen o~hophosphate was dissolved in water and diluted to volume. Manganese (1 mg/ml). A 0.687-g sample of anhydrous manganese sulphate was dissolved in water containing 1 ml of concentrated sulphuric acid and diluted to volume. Vanadium (1 mg/ml). A 0.574-g sample of ammonium metavanadate was dissolved in water and diluted to volume. Procedure Preliminary tests. A series of tests on synthetic solutions were carried out to ascertain the recovery of known amounts of iron, aluminium, calcium and magnesium in the presence of potential interferents like titanium, manganese, chromium, phosphorous and vanadium that would be encountered in the analyses of slags. Based on these experiments, a procedure was developed for the analyses of slags which is given below. Dissolution. The sample (0.5 g) was treated with 20 ml of 12M hydrochloric acid and 5 ml of 16M nitric acid in a beaker and the mixture was carefully evaporated to dryness. A few ml of hydrochloric acid and 100 ml of water were then added and the solution was boiled. The solution was cooled and filtered through Whatman No. 40 filter paper with pulps. The residue was washed with dilute hydrochloric acid (1 + 9) 5-6 times with hot water. After removing the silica with hydrofluoric acid, the residue was fused with 2-3 g of potassium bisulphate and the cooled melt was dissolved in 5% hydrochloric acid on heating, and added to the main filtrate. The solution was transferred to a 250-ml calibrated flask and diluted to volume. Titration of iron. A 50-ml portion of the sample solution was taken in a beaker and diluted with 50 ml of water. Ammonium hydroxide (1 + 1) was added until the solution became turbid. The turbidity cleared with a minimum quantity of dilute hydr~hlo~c acid (I + 3). A few drops were added in excess to adjust the pH to ca. 2. Then 50 mg of sulphosalicylic acid indicator were added and the solution
Analysis of Fe, Al, Ca and Mg in slags
was warmed. This solution was titrated with standard O.OlM EDTA solution with constant stirring to a sharp colour change from violet to a colourless or pale yellow solution. Titration of aluminium. After titration of iron, a known aliquot of 0.025 or O.OlM DCTA was added depending upon the alumina content of the sample. Then 10 ml of 10% lactic acid were added to mask titanium. The pH of the solution was adjusted to 3.5 with 25% ammonium acetate solution. The solution was just warmed and titrated the excess DCTA with 0.025 or O.OlM copper sulphate solution, using ten drops of PAN indicator to a sharp colour change from yellow to violet. Titration of calcium plus magnesium. A 50-ml portion of 0.025M excess EDTA was added to the titrated solution after aluminium determination and 5 ml of 10% tetra sodium pyrophosphate were added to mask manganese. The pH of the solution was adjusted to 10 with ammonia buffer. The solution was warmed and the excess of EDTA was titrated with 0.025M copper sulphate solution with ten drops of PAN indicator to a sharp colour change from yellow to violet. This titre value corresponds to the sum of calcium and magnesium. Titration of calcium. After the titration of calcium plus magnesium, the pH of the solution was reduced to 5 with dilute hydrochloric acid (1 + 3). Then 20 ml of 5% hot ammonium oxalate solution were added to precipitate calcium as calcium oxalate. The solution was digested for 20 min, cooled and transferred along with the precipitate to a 250-ml standard flask. The solution was diluted to the mark with the washings. A lOO-ml portion of the solution was filtered dry and then 10 ml of the ammonia buffer (pH 10) were added. The solution was warmed and the liberated EDTA equivalent to calcium was titrated with 0.025M copper sulphate solution
677
with ten drops of PAN indicator until the colour turned violet, which lasted for 15-30 sec. The volume of EDTA was subtracted from the sum of calcium and magnesium titre value. The difference is the volume of EDTA required to titrate the magnesium. A blank was also run but showed no significant amounts of iron, aluminium, calcium and magnesium. RESULTS AND DISCUSSION
Table 1 shows the results of determination of iron, aluminium, calcium and magnesium in the presence of potential interferents like phosphorus, chromium, vanadium, titanium and manganese in synthetic solutions. Chromium2 and vanadium’* seriously interfere with aluminium as they form stable complexes with EDTA at pH 3.5-5.5. In the present method DCTA was preferred to EDTA in the titration of aluminium at pH 3.5, for its better selectivity in the presence of chromium’5 and vanadium.16 It was found that chromium and vanadium up to 1.5 and 1 mg, respectively, did not interfere in the titration of aluminium at pH 3.5. More than 1.5 mg of chromium seriously interfered in the titration of aluminium due to the formation of a purple complex of Cr-DCTA. Vanadium more than 1 mg could not be tolerated as the transition of colour change of the indicator was not sharp. However, the chromium and vanadium content in slags is not usually more than 1% of each. Phosphate” interferes in the titration of calcium and magnesium at pH 10. In the present method the phosphate interference was obviated by back titration of excess EDTA with copper sulphate so that all calcium present combined with EDTA. There is no evidence of the complexation of copper with phosphate and as expected phosphorus up to 16 mg did not
Table 1. Titration of Fe, Al, Ca and Mg in the presence of various potential interferents Taken, mg Standard method
Element added, mg P Cr V Ti* Mnt
16.0 1.5 1.0 3.0 12.0
Found, mg Proposed method
Fe
Al
Ca
Mg
Fe
Al
Ca
Mg
2.8 2.8 2.8 2.8 2.8
13.5 13.5 13.5 13.5 13.5
20.0 20.0 20.0 20.0 20.0
3.6 3.6 3.6 3.6 3.6
2.78 2.78 2.82 2.82 2.82
13.52 13.53 13.56 15.60 13.48
20.08 20.10 20.05 20.10 20.08
3.58 3.57 3.58 3.57 9.53
*Interferes with Al. TInterferes with Mg.
K. C. GHOSH et al.
678 Table 2. Titration
of Fe, Al, Ca and Mg in the presence Ti: 10 ml of 10% lactic acid) Taken, mg Standard method
of Ti (masking
agent
for
Found, mg Proposed method
Ti added, mg
Fe
Al
Ca
Mg
Fe
Al
Ca
Mg
3.0 5.0 7.0
2.8 2.8 2.8
13.5 13.5 13.5
20.0 20.0 20.0
3.6 3.6 3.6
2.78 2.82 2.80
13.52 13.54 13.54
20.04 20.08 20.10
3.58 3.58 3.56
Table 3. Titration
of Fe, Al, Ca and Mg in the presence of Mn (masking Mn: 5 ml of 10% tetra sodium pyrophosphate) Taken, mg Standard method
agent for
Found, mg Proposed method
Mn added, w
Fe
Al
Ca
Mg
Fe
Al
Ca
Mg
8.0 10.0 12.0
2.8 2.8 2.8
13.5 13.5 13.5
20.0 20.0 20.0
3.6 3.6 3.6
2.82 2.80 2.80
13.50 13.54 13.56
19.97 20.04 19.97
3.62 3.58 3.62
interfere (Table 1). Actually in synthetic solution phosphorus was added in excess of that normally found in slags. Titanium and manganese seriously interfere with the determination of aluminium and magnesium, respectively, as they form stable complexes at pH 3.5 and 10 (Table 1). In the present method lactic acid” was used to mask titanium and addition of 10 ml of 10% lactic acid was found to mask up to 7 mg of titanium as shown in Table 2. However, for more than 7 mg of titanium, the end point was sluggish. Fog” used potassium cyanide for masking manganese in EDTA titration of magnesium. In the present method cyanide cannot be used as a masking agent because copper sulphate is used as back titrant. The interference due to manganese was Table 4. Determination
of Fe, Al,
Certified
value, %
Slags BCS No. 381 Basic slag
BCS No. 382/l Basic slag
BCS No. 367 Blast furnace slag
Submerged arc furnace slag
*Average iStandard
eliminated by complexing it with tetra sodium pyrophosphate. I4 The stability constant of the manganese-pyrophosphate complex is not known. However, it was observed that in the presence of tetra sodium pyrophosphate, manganese does not react with EDTA. This shows that the manganese-pyrophosphate complex must be more stable than the manganeseEDTA complex. In the present method it was observed that 5 ml of 10% tetrasodium pyrophosphate were able to mask up to 12 mg of manganese (Table 3). The concentration of masked manganese is well above the concentration of manganese found in slags. Table 4 shows the results of analysis of iron, aluminium, calcium and magnesium in some slags by the proposed method. The results of
Fe A&O, CaO MgO Fe A&O, CaO MgO Fe At,@ cao MgO Fe Al,O, CaO MnO
of three determinations. conventional method.
13.30 0.67 49.00 1.03 19.90 3.79 40.10 3.73 0.78 20.00 32.40 7.10 4.48t 14.33.t 38.30t 2.49t
0, , CaO and MgO in slags Found by proposed method,* %
Deviation
13.20 0.63 49.30 0.97 19.70 3.86 40.30 3.68 0.73 20.10 32.60 6.97 4.43 14.36 38.60 2.45
-0.1 -0.04 +0.3 -0.06 -0.2 +0.07 +0.2 -0.05 -0.05 +0.1 +0.2 -0.13 -0.05 f0.03 +0.3 -0.04
Analysis of Fe, Al, Ca and Mg in slags
analysis are in very good agreement with the certified values and the values obtained by the standard conventional method. Though the value of calcium oxide reported by the proposed method is higher than the certified value, the deviations reported in the Table are within the precision of the method. The proposed method is simple, rapid and accurate, and avoids multi-separation steps that are often involved in the standard conventional methods. A set of five samples can be analysed for iron, aluminium, calcium and magnesium in four hours as compared to three days by standard conventional methods. authors thank Prof. S. Banerjee, Director, National Metallurgical Laboratory, Jamshedpur for his kind permission to publish this paper.
Acknowledgement-The
REFERENCES 1. W. W. Scott, Standard Methods of Chemical Analysis, 5th Ed., Vol. I, p. 208. Technical Press, London, 1939.
679
2. G. Schwarzenbach and H. Flaschka, Complexometric Titrations, 2nd Ed., Trans. H.M.N.H. Irving, Methuen, London, 1969. 3. V. Williams, Iron Steel (London), 1955, Zs, 525. 4. I. N. Basilevskaya, Zavod. Lab., 1956, 22, 166. 5. P. Szarvas, J. Korondan and I. Raisz, Magy. Kern. Lab., 1967, 3, 149. 6. A. Hitchen and G. Z-echanowitsch, Talanra, 1980, 27, 269. 7. W. Westwood and R. Presser, Analyst, 1951, 76, 191. 8. V. Rett, Hum. Listy, 1965, Xl, 134. 9. N. N. Lapin, A. T. Slyusarev and N. S. Prilutskaya, Sb. Nauch, Trud. Zhdanovsk. Metallurgy, Inst., 1960, 393. 10. E. Vanninen and A. Ringbom, Anal. Chim. Acta, 1955, 12, 308.
11. Y. C. Chen and H. Y. Li, Acfa Chim. Sin., 1965,31,391. 12. D. Filipov and N. Kirtscheva, Compr. Rend. Acad. Bulg. Sri., 1964, 17, 467. 13. C. Cimerman, A. Alon and J. Mashall, Anal. Chim. Acta, 1958, 19, 461. 14. J. C. Bailar Jr., H. J. Emeleus, Sir R. Nyholm and A. F. Trotman Dickenson, Comprehensive Inorganic Chemisrry, 1st Ed., 1973, 830. 15. R. Pribil and V. Vesely, Talanta, 1962, 9, 23. 16. E. Lassner and R. Puschel, Mikrochim. Ichnoanal. Acta, 1963, 950. 17. H. M. Fog, Acla Chem. Stand., 1968, 22, 791.
0039-9140/92$5.00+ 0.00 Copyright 0 1992Pergamon Press Ltd
Printed in Great Britain. All rights reserved
A RAPID COMPLEXOMETRIC SCHEME FOR ANALYSIS OF IRON, ALUMINIUM, CALCIUM AND MAGNESIUM IN SLAGS K. C. GHOSH, B. C. MUKHERJEE,N. N. GANGULY, M. YUSLJFand V. N. CHOUDHIJRY
National Metallurgical Laboratory, Jamshedpur 831007, India (Received 3 May 1991. Revised 17 October 1991. Accepted 17 October 1991) Summary-A
simple and rapid complexometric method has been developed for the simultaneous determination of iron, aluminium, calcium and magnesium in a single solution in slags. Phosphorous and small amounts of chromium (1.5 mg) and vanadium (1 mg) do not interfere in the titration. Titanium and manganese are suitably masked with lactic acid and tetra sodium pyrophosphate, respectively. In a suitable aliquot, iron is titrated at pH 2 with EDTA, using sulphosalicylic acid as indicator. To this solution, excess disodium 1,2-cyclohexane diamine tetra acetic acid (DCTA) is added and aluminium is titrated by titrating the excess DCTA with standard copper sulphate solution at pH 3.5, using I-(2-pyridylazo)-2-naphthol (PAN) as an indicator. A known excess of EDTA is added, the pH is raised to 10 and calcium and magnesium are jointly titrated by titrating the excess EDTA with copper sulphate solution, using PAN indicator. The Ca-EDTA complex is demasked with ammonium oxalate at pH 5 and the released EDTA equivalent to calcium is titrated with copper sulphate solution at pH 10 with PAN indicator. Results of analysis compare favourably with certified values and values obtained by standard methods for BCS and other slags. A set of five samples can be analysed for iron, aluminium, calcium and magnesium in four hours as compared to three days by the classical conventional method.
The chemical composition of slag depends on the raw materials used and the production conditions. Since the composition of the slag varies from lot to lot, a rapid and accurate evaluation of a slag composition by chemical analysis is essential for quality control of the raw materials and the finished products. The classical methods’ for determination of alumina, calcium and magnesium in slags are the well-known oxinate, oxalate and pyrophosphate methods, respectively, which are tedious and time consuming. In most of the complexometric methods of determination, calcium and magnesium are usually titrated together with EDTA at pH 10 in an ammoniacal buffer and then calcium alone in another aliquot at pH 12 when magnesium precipitates as hydroxide.2-6 Manganese, if present in slags, interferes seriously with the EDTA titration of calcium and magnesium by precipitating out as brown manganese hydroxide which absorbs metallochromic indicator to yield an unsatisfactory end point. Therefore, it is essential to separate manganese by the lengthy process of sulphide precipitation’ or mercury cathode electrolysis’ prior to the titration of calcium and magnesium with EDTA. Solvent extraction procedures for removal of manganese with diethyl dithiocarbamate* and
oxine’ have also been reported. Titanium,“*” chromium2 and vanadium’* seriously interfere with aluminium as they form stable complexes with EDTA at pH 3.5-5.5. The interference of phosphateI in the titration of calcium and magnesium at pH 10 has also been reported. However, no attempt has yet been made to develop a rapid complexometric method for the simultaneous determination of iron, aluminium, calcium and magnesium in a single solution in slags in the presence of titanium and manganese. As iron, aluminium, calcium and magnesium form stable complexes with EDTA at different pH values, a simple and rapid complexometric scheme has been developed based on this property for the simultaneous determination of these elements in slags in a single solution, using lactic acid” and tetra sodium pyro-phosphate’4 as masking agents for titanium and manganese respectively. EXPERIMENTAL
Reagents Unless analytical prepared redistilled 675
otherwise stated, all reagents were of reagent grade and all solutions were in a one-litre calibrated flask with water.
EDTA solution. Two solutions (0,025 and O.OlM) were prepared by dissolving 9.306 and 3.7225 g, respectively, of disodium ethylene diamine tetra acetate dihydrate in water and diluting to volume. DCTA solution. Two solutions (0.025 and O.OlM ) were prepared by dissolving 8.659 and 3.4636 g, respectively, of 1,2-diaminocyclohexane tetra acetic acid in 100 ml of 1M sodium hydroxide solution and 200 ml of water, and diluting to volume. Standard copper sutphate solution. Two solutions (0.025 and O.OlM) were prepared by dissolving 6.25 and 2.5 g, respectively, of CuSO,.SH,O in water and diluting to volume. A~umini~ solu?ian. A 0.025M solution was prepared by dissolving 0.675 g of aluminium foil (99.99%) in hydrochloric acid (1 + 1) and diluting to the mark. Calcium chloride solution. A 0.025M solution was prepared by dissolving 2.5 g of dried (150”) calcium carbonate in 5 ml of 12M hydrochloric acid and 25 ml of water. The solution was boiled, cooled and diluted up to the mark. Magnesium chloride solution. A O.OlM solution was prepared by dissolving 2.03 g of MgCl,. 6H,O in water and diluting to volume. Iron solution. A 0.0 1M solution was prepared by dissolving 0.56 g of pure iron wire in 10 ml of concentrated hydrochloric acid and diluting to volume. Ammonia-ammonium
chloride bufleer (pH IO).
Prepared by dissolving 67.5 g of ammonium chloride in 250 ml of water and 570 ml of liquid ammonia and diluting to volume. Ammonium acetate, 25%. Prepared by dissolving 25 g of ammonium acetate in 100 ml of water. Ammonium oxalate, 5%. Prepared by dissolving 5 g of ammonium oxalate in 100 ml of water. Lactic acid, 10%. Prepared by diluting 10 ml of lactic acid to 100 ml with water. Tetra sodium pyrophosphate, 10%. Prepared by dissolving 10 g of tetra sodium pyrophosphate in 100 ml of water. I-(2-Pyridylazo)-2-naphthol
(PAN),
0.05%.
Prepared by dissolving 50 mg of the indicator in 100 ml of methanol. Suiphosaiicylic acid (solid).
Foreign ion solution. All solutions were prepared in a 250-ml calibrated flask. Titanium (1 mg/ml). A 0.416-g sample of ignited titanium dioxide was fused with 4 g of potassium bisulphate. After cooling the fused
mass was dissolved in 50 ml of 10% sulphuric acid and diluted to volume with water. Chromium (1 mg/ml). A 0.707-g sample of dried (180”) potassium dichromate was dissolved in water and heated with 5 ml of concentrated hydrochloric acid and 15 ml of ethyl alcohol. The solution was cooled and diluted to volume with water. Phosphorous (1 mglml). A 1.258-g sample of sodium dihydrogen o~hophosphate was dissolved in water and diluted to volume. Manganese (1 mg/ml). A 0.687-g sample of anhydrous manganese sulphate was dissolved in water containing 1 ml of concentrated sulphuric acid and diluted to volume. Vanadium (1 mg/ml). A 0.574-g sample of ammonium metavanadate was dissolved in water and diluted to volume. Procedure Preliminary tests. A series of tests on synthetic solutions were carried out to ascertain the recovery of known amounts of iron, aluminium, calcium and magnesium in the presence of potential interferents like titanium, manganese, chromium, phosphorous and vanadium that would be encountered in the analyses of slags. Based on these experiments, a procedure was developed for the analyses of slags which is given below. Dissolution. The sample (0.5 g) was treated with 20 ml of 12M hydrochloric acid and 5 ml of 16M nitric acid in a beaker and the mixture was carefully evaporated to dryness. A few ml of hydrochloric acid and 100 ml of water were then added and the solution was boiled. The solution was cooled and filtered through Whatman No. 40 filter paper with pulps. The residue was washed with dilute hydrochloric acid (1 + 9) 5-6 times with hot water. After removing the silica with hydrofluoric acid, the residue was fused with 2-3 g of potassium bisulphate and the cooled melt was dissolved in 5% hydrochloric acid on heating, and added to the main filtrate. The solution was transferred to a 250-ml calibrated flask and diluted to volume. Titration of iron. A 50-ml portion of the sample solution was taken in a beaker and diluted with 50 ml of water. Ammonium hydroxide (1 + 1) was added until the solution became turbid. The turbidity cleared with a minimum quantity of dilute hydr~hlo~c acid (I + 3). A few drops were added in excess to adjust the pH to ca. 2. Then 50 mg of sulphosalicylic acid indicator were added and the solution
Analysis of Fe, Al, Ca and Mg in slags
was warmed. This solution was titrated with standard O.OlM EDTA solution with constant stirring to a sharp colour change from violet to a colourless or pale yellow solution. Titration of aluminium. After titration of iron, a known aliquot of 0.025 or O.OlM DCTA was added depending upon the alumina content of the sample. Then 10 ml of 10% lactic acid were added to mask titanium. The pH of the solution was adjusted to 3.5 with 25% ammonium acetate solution. The solution was just warmed and titrated the excess DCTA with 0.025 or O.OlM copper sulphate solution, using ten drops of PAN indicator to a sharp colour change from yellow to violet. Titration of calcium plus magnesium. A 50-ml portion of 0.025M excess EDTA was added to the titrated solution after aluminium determination and 5 ml of 10% tetra sodium pyrophosphate were added to mask manganese. The pH of the solution was adjusted to 10 with ammonia buffer. The solution was warmed and the excess of EDTA was titrated with 0.025M copper sulphate solution with ten drops of PAN indicator to a sharp colour change from yellow to violet. This titre value corresponds to the sum of calcium and magnesium. Titration of calcium. After the titration of calcium plus magnesium, the pH of the solution was reduced to 5 with dilute hydrochloric acid (1 + 3). Then 20 ml of 5% hot ammonium oxalate solution were added to precipitate calcium as calcium oxalate. The solution was digested for 20 min, cooled and transferred along with the precipitate to a 250-ml standard flask. The solution was diluted to the mark with the washings. A lOO-ml portion of the solution was filtered dry and then 10 ml of the ammonia buffer (pH 10) were added. The solution was warmed and the liberated EDTA equivalent to calcium was titrated with 0.025M copper sulphate solution
677
with ten drops of PAN indicator until the colour turned violet, which lasted for 15-30 sec. The volume of EDTA was subtracted from the sum of calcium and magnesium titre value. The difference is the volume of EDTA required to titrate the magnesium. A blank was also run but showed no significant amounts of iron, aluminium, calcium and magnesium. RESULTS AND DISCUSSION
Table 1 shows the results of determination of iron, aluminium, calcium and magnesium in the presence of potential interferents like phosphorus, chromium, vanadium, titanium and manganese in synthetic solutions. Chromium2 and vanadium’* seriously interfere with aluminium as they form stable complexes with EDTA at pH 3.5-5.5. In the present method DCTA was preferred to EDTA in the titration of aluminium at pH 3.5, for its better selectivity in the presence of chromium’5 and vanadium.16 It was found that chromium and vanadium up to 1.5 and 1 mg, respectively, did not interfere in the titration of aluminium at pH 3.5. More than 1.5 mg of chromium seriously interfered in the titration of aluminium due to the formation of a purple complex of Cr-DCTA. Vanadium more than 1 mg could not be tolerated as the transition of colour change of the indicator was not sharp. However, the chromium and vanadium content in slags is not usually more than 1% of each. Phosphate” interferes in the titration of calcium and magnesium at pH 10. In the present method the phosphate interference was obviated by back titration of excess EDTA with copper sulphate so that all calcium present combined with EDTA. There is no evidence of the complexation of copper with phosphate and as expected phosphorus up to 16 mg did not
Table 1. Titration of Fe, Al, Ca and Mg in the presence of various potential interferents Taken, mg Standard method
Element added, mg P Cr V Ti* Mnt
16.0 1.5 1.0 3.0 12.0
Found, mg Proposed method
Fe
Al
Ca
Mg
Fe
Al
Ca
Mg
2.8 2.8 2.8 2.8 2.8
13.5 13.5 13.5 13.5 13.5
20.0 20.0 20.0 20.0 20.0
3.6 3.6 3.6 3.6 3.6
2.78 2.78 2.82 2.82 2.82
13.52 13.53 13.56 15.60 13.48
20.08 20.10 20.05 20.10 20.08
3.58 3.57 3.58 3.57 9.53
*Interferes with Al. TInterferes with Mg.
K. C. GHOSH et al.
678 Table 2. Titration
of Fe, Al, Ca and Mg in the presence Ti: 10 ml of 10% lactic acid) Taken, mg Standard method
of Ti (masking
agent
for
Found, mg Proposed method
Ti added, mg
Fe
Al
Ca
Mg
Fe
Al
Ca
Mg
3.0 5.0 7.0
2.8 2.8 2.8
13.5 13.5 13.5
20.0 20.0 20.0
3.6 3.6 3.6
2.78 2.82 2.80
13.52 13.54 13.54
20.04 20.08 20.10
3.58 3.58 3.56
Table 3. Titration
of Fe, Al, Ca and Mg in the presence of Mn (masking Mn: 5 ml of 10% tetra sodium pyrophosphate) Taken, mg Standard method
agent for
Found, mg Proposed method
Mn added, w
Fe
Al
Ca
Mg
Fe
Al
Ca
Mg
8.0 10.0 12.0
2.8 2.8 2.8
13.5 13.5 13.5
20.0 20.0 20.0
3.6 3.6 3.6
2.82 2.80 2.80
13.50 13.54 13.56
19.97 20.04 19.97
3.62 3.58 3.62
interfere (Table 1). Actually in synthetic solution phosphorus was added in excess of that normally found in slags. Titanium and manganese seriously interfere with the determination of aluminium and magnesium, respectively, as they form stable complexes at pH 3.5 and 10 (Table 1). In the present method lactic acid” was used to mask titanium and addition of 10 ml of 10% lactic acid was found to mask up to 7 mg of titanium as shown in Table 2. However, for more than 7 mg of titanium, the end point was sluggish. Fog” used potassium cyanide for masking manganese in EDTA titration of magnesium. In the present method cyanide cannot be used as a masking agent because copper sulphate is used as back titrant. The interference due to manganese was Table 4. Determination
of Fe, Al,
Certified
value, %
Slags BCS No. 381 Basic slag
BCS No. 382/l Basic slag
BCS No. 367 Blast furnace slag
Submerged arc furnace slag
*Average iStandard
eliminated by complexing it with tetra sodium pyrophosphate. I4 The stability constant of the manganese-pyrophosphate complex is not known. However, it was observed that in the presence of tetra sodium pyrophosphate, manganese does not react with EDTA. This shows that the manganese-pyrophosphate complex must be more stable than the manganeseEDTA complex. In the present method it was observed that 5 ml of 10% tetrasodium pyrophosphate were able to mask up to 12 mg of manganese (Table 3). The concentration of masked manganese is well above the concentration of manganese found in slags. Table 4 shows the results of analysis of iron, aluminium, calcium and magnesium in some slags by the proposed method. The results of
Fe A&O, CaO MgO Fe A&O, CaO MgO Fe At,@ cao MgO Fe Al,O, CaO MnO
of three determinations. conventional method.
13.30 0.67 49.00 1.03 19.90 3.79 40.10 3.73 0.78 20.00 32.40 7.10 4.48t 14.33.t 38.30t 2.49t
0, , CaO and MgO in slags Found by proposed method,* %
Deviation
13.20 0.63 49.30 0.97 19.70 3.86 40.30 3.68 0.73 20.10 32.60 6.97 4.43 14.36 38.60 2.45
-0.1 -0.04 +0.3 -0.06 -0.2 +0.07 +0.2 -0.05 -0.05 +0.1 +0.2 -0.13 -0.05 f0.03 +0.3 -0.04
Analysis of Fe, Al, Ca and Mg in slags
analysis are in very good agreement with the certified values and the values obtained by the standard conventional method. Though the value of calcium oxide reported by the proposed method is higher than the certified value, the deviations reported in the Table are within the precision of the method. The proposed method is simple, rapid and accurate, and avoids multi-separation steps that are often involved in the standard conventional methods. A set of five samples can be analysed for iron, aluminium, calcium and magnesium in four hours as compared to three days by standard conventional methods. authors thank Prof. S. Banerjee, Director, National Metallurgical Laboratory, Jamshedpur for his kind permission to publish this paper.
Acknowledgement-The
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