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Studies on determination of fluoride in zinc and lead concentrates by using a fluoride ion-selective electrode
0039-91401x5 53.00+ooo Pergamon Press Ltd
STUDIES ON DETERMINATION OF FLUORIDE IN ZINC AND LEAD CONCENTRATES BY USING A FLUORIDE ION-SELECTIVE ELECTRODE S. C.
S. RAJAN*, L. M. BHANDARI~ and B. R. L. Row$ Hindustan Zinc Limited, Visakhapatuam-530015, India
(Received 8 June 1984. Revised 18 September
1984. Accepted 14 June 1985)
Summary-A modification of the potentiometric determination of fluoride has been developed, which allows use of aqueous fluoride standards in analysis of lead or zinc concentrates. instead of the need to use matrix-matching or standard additions.
Determination of fluoride in zinc concentrates is important since fluoride can cause problems in the electrolytic production of high-purity zinc with aluminium cathodes.’ Work was taken up in our laboratories some years ago to develop a procedure for the determination of fluoride in zinc concentrates by use of a fluoride ion-selective electrode, and preliminary results were presented.2 The work also included determination of fluoride in lead concentrates, since Hindustan Zinc Limited treats these for recovery of lead. The procedure consisted of fusion of the concentrates with sodium hydroxide, extraction of the melt with water, adjustment of the extract to pH 8-9, filtration and measurement of fluoride in the filtrate. The working standards contained similar quantities of sodium hydroxide and acid. Almost immediately afterwards, a paper appeared on the determination of fluoride in zinc concentrate, by use of an ion-selective electrode.3 The authors, after extensive study of various procedures for decomposition of the sample, also finally recommended fusion with alkali but preferred a single-point standard-addition technique to deal with matrix interference. However, it had earlier been found that the matrix interference could be avoided by neutralizing the alkaline extract to pH 8-9 and then filtering.4 so it seemed to us possible to use aqueous standards for measurement of fluoride in the sample solution and not to need the single-point standard-addition calibration. EXPERIMENTAL
diluting to volume in a 1-litre standard flask. Lower concentrations were prepared by serial dilution. Buffers.’ A IM citrate buffer was made by dissolving 294 g of sodium citrate and 20.2 g of potassium nitrate in 800 ml of water, adjusting the pH_to 51tX5.5 with hydrochloric acid (1 + 1) and diluting to 1 litre. A 0.2M citrate buffer’ was similarly made with 58.8 g of sodium citrate and 20.2 g of potassium nitrate. TEAK4 A total ionic strength adjustment buffer was made by adding 58 ml of glacial acetic acid to 12 g of sodium citrate dissolved in 300 ml of water, adjusting the pH to 5S5.5 with 25% sodium hydroxide solutton and diluting to I litre with distilled water. Procedure To a l-g sample in a lOO-ml nickel crucible add as nearly as possible (i.e., within I pellet) 5 g of sodium hydroxide pellets, put the crucible in a cold electric furnace, and switch on the power. When the temperature has reached 500”, maintain that temperature for 30 min. Occasionally remove the crucible and swirl its contents to ensure uniform dispersion. When the fusion is completed, cool the crucible, add about 30 ml of distilled water, and warm the crucible gently to facilitate dissolution of the cake. Adjust the pH to 8-9 with glacial acetic acid, taking care to avoid development of neutral or acidic condittons during the process. Cool to room temperature, filter through a Whatman No. 41 paper into a lOO-ml standard flask, washmg the residue and paper with distilled water, and finally dilute to volume. Prepare a blank solutton in the same way. Mix a suitable aliquot (e.g., 25 ml) of this sample solution with an exactly equal volume of buffer. The pH of the buffered sample should be 5.45.6. Measure the fluoride content potentiometrically with a fluoride-sensitive electrode, calibrated with standard fluoride solutions (I-IO ppm) mixed m exactly I : 1 ratio with the buffer used. Apply a correction for the fluonde content of the blank, measured in the same way.
Apparatus An Orion model 407 A ion-meter, 901 ion-analyser, 9409 fluoride electrode and 9001 reference electrode were used. Reagents Analytical grade materials were used whenever possible. Stock fluorrde solution. 1000 ppm. Prepared by dissolving 2.210 g of dried sodium fluoride in distrlled water and
*Author for correspondence. tProcess Laboratory, Zinc Smelter. @Regional Office, Hyderabad.
Udaipur.
RESULTS
AND
DISCUSSION
The results obtained for three zinc-concentrate and two lead-concentrate samples by the procedure above are presented in Table 1. As no standard samples were available, the accuracy of the method was checked by determining the recovery of standard additions of fluoride. One of our main concerns was to see whether calibration with pure aqueous standards would give 1064
SHORT
Table
I. Analysis
Sample
Added I
Lead Concentrate
2
Zinc Concentrate
I
Zinc Concentrate
*Average
2
/wig Found
Relative standard deviation, % 3.5
100
I so* 240
6.0
150+
2.0
275 i 6
200 300
27S 460 580
200 200
l78* 370 360
2.9
of six replicate
293+
for the
Recovery,
y0
II 93 102
IO
90 93 IO1 l78k6 96 90
analyses.
Table 2. Fluoride standards prepared wth sodium hydroxrde and acetrc acid*
F ~, /.~e/nrl Found?
respectively, indicating that more than 98% of the zinc and 99% of the lead remained in the residue. Three buffers (IM citrat&.2M nitrate, 0.2M citrate-0.2M nitrate, and TISAB) were tried, and all gave similar results. The 0.2M citrate buffer will obviously contain less fluoride than the IM citrate buffer and hence be more suitable for low-level fluoride measurements. The fluoride content of TISAB will depend on the relative fluoride content of the individual buffer components. For those used in the work described, we found 0.5 pg/g in the sodium citrate and 4 pg/g in the sodium hydroxide, and ~0.02 pg/ml in the 0.2M citrate buffer, 0.08 pg/ml in the IM citrate buffer and 0.05 pg/ml in the TISAB. We modified the procedure of McQuaker and Gurney4 by using solid sodium hydroxide for the sample decomposition, instead of adding sodium hydroxide solution and evaporating the water (which takes rather a long time). We found that the pellets weighed about 0.2 g each, and the maximum error in weighing the 5 g was about kO.2 g. If the sodium hydroxide used contains say 5 pg of fluoride per g, the error introduced by weighing the pellets should not exceed 0.01 ppm fluoride in the value obtained. We also preferred acetic acid to hydrochloric for the pH adjustment, because the buffering action of the acetate system gave better control in preventing formation of acidic or neutral conditions during the adjustment. The Willard and Winter distillation technique with perchloric acid was also tested and good recoveries of fluoride were obtained. About 400 ml of distillate was found to be sufficient for quantitative recovery from a l-g sample. Acknowledgemen/s-The authors assistance in the investigations.
I I8 f 0.03 2.18 * 0.03 4. I7 + 0.03 5.13+0.03
*I A4crtric XXI buffer used tMcan &standard dcwatwn rcphcates)
and results
Value for 95% confidence, /wcg/g
298* 480 600
measurement of fluoride in sample solutions prepared by the procedure described. To do this, fluoride standards were prepared, each by adding 5 g of sodium hydroxide to a suitable volume of a standard solution, diluting to about 30 ml, adjusting to pH 8-9 with glacial acetic acid, and diluting to volume in a IOO-ml standard flask. These standards were then mixed in I : I ratio with I it4 citrate buffer and measured potentiometrically for fluoride content. A similar range of purely aqueous standards was prepared, mixed in 1:l ratio with the buffer and measured. Comparrson of the two sets of results showed that the sodium hydroxide and glacial acetic acid used produced between them an apparent increase of 0 1550.20 pg/ml in the fluoride concentration. Hence purely aqueous standards can be used for calibration, provided a correction is applied for the reagent blank. As reported by McQuaker and Gurney,4 adjustment of the test solution to pH 889 and filtration removes most of the interference from matrix element\ such as aluminium, iron, silicon. calcium and mapnesrum. Except for iron, which is usually present :II around 5-Y’,, level, these elements occur only at low levels m the concentrates of interest. Since these concentrates contain zinc (cu. SO”/,) or lead (cu. 60%) as the major elements. the filtrates were analysed for these two elements by AAS. The zinc and lead values found were in the ranges 4&80 and 5-7 pg/ml
I.0 2.0 4.0 5.0
method
200 300
accurate
Taken
1065
of lead and zinc concentrates by the proposed recovery of added fluoride
F-.
Lead Concentrate
COMMUNICATIONS
thank
Shri K S. Bolia for
REFERENCES
(3
R. M. Farmer, Anode pre-conditroning and other changes in Cominco’s electrolytrc zinc operatrons, AIME Symposium on Electrometallurgy, 2 December 1968.
1066
SHORT
COMMUNICATIONS
2. S. C. S. Rajan, L. M. Bhandari and B. R. L. Row, paper presented at Indian Science Congress, Calcutta, 1980. 3. D. S. Russell, H. B. Macpherson and V. P. Clancy, Talanra, 1980, 27, 403.
4. N. R. McQuaker and H. Gurney, And. 49, 53 5. B. L Ingram, i/w/.. 1970. 42, 1825.
Chem..
1977,
STUDIES ON DETERMINATION OF FLUORIDE IN ZINC AND LEAD CONCENTRATES BY USING A FLUORIDE ION-SELECTIVE ELECTRODE S. C.
S. RAJAN*, L. M. BHANDARI~ and B. R. L. Row$ Hindustan Zinc Limited, Visakhapatuam-530015, India
(Received 8 June 1984. Revised 18 September
1984. Accepted 14 June 1985)
Summary-A modification of the potentiometric determination of fluoride has been developed, which allows use of aqueous fluoride standards in analysis of lead or zinc concentrates. instead of the need to use matrix-matching or standard additions.
Determination of fluoride in zinc concentrates is important since fluoride can cause problems in the electrolytic production of high-purity zinc with aluminium cathodes.’ Work was taken up in our laboratories some years ago to develop a procedure for the determination of fluoride in zinc concentrates by use of a fluoride ion-selective electrode, and preliminary results were presented.2 The work also included determination of fluoride in lead concentrates, since Hindustan Zinc Limited treats these for recovery of lead. The procedure consisted of fusion of the concentrates with sodium hydroxide, extraction of the melt with water, adjustment of the extract to pH 8-9, filtration and measurement of fluoride in the filtrate. The working standards contained similar quantities of sodium hydroxide and acid. Almost immediately afterwards, a paper appeared on the determination of fluoride in zinc concentrate, by use of an ion-selective electrode.3 The authors, after extensive study of various procedures for decomposition of the sample, also finally recommended fusion with alkali but preferred a single-point standard-addition technique to deal with matrix interference. However, it had earlier been found that the matrix interference could be avoided by neutralizing the alkaline extract to pH 8-9 and then filtering.4 so it seemed to us possible to use aqueous standards for measurement of fluoride in the sample solution and not to need the single-point standard-addition calibration. EXPERIMENTAL
diluting to volume in a 1-litre standard flask. Lower concentrations were prepared by serial dilution. Buffers.’ A IM citrate buffer was made by dissolving 294 g of sodium citrate and 20.2 g of potassium nitrate in 800 ml of water, adjusting the pH_to 51tX5.5 with hydrochloric acid (1 + 1) and diluting to 1 litre. A 0.2M citrate buffer’ was similarly made with 58.8 g of sodium citrate and 20.2 g of potassium nitrate. TEAK4 A total ionic strength adjustment buffer was made by adding 58 ml of glacial acetic acid to 12 g of sodium citrate dissolved in 300 ml of water, adjusting the pH to 5S5.5 with 25% sodium hydroxide solutton and diluting to I litre with distilled water. Procedure To a l-g sample in a lOO-ml nickel crucible add as nearly as possible (i.e., within I pellet) 5 g of sodium hydroxide pellets, put the crucible in a cold electric furnace, and switch on the power. When the temperature has reached 500”, maintain that temperature for 30 min. Occasionally remove the crucible and swirl its contents to ensure uniform dispersion. When the fusion is completed, cool the crucible, add about 30 ml of distilled water, and warm the crucible gently to facilitate dissolution of the cake. Adjust the pH to 8-9 with glacial acetic acid, taking care to avoid development of neutral or acidic condittons during the process. Cool to room temperature, filter through a Whatman No. 41 paper into a lOO-ml standard flask, washmg the residue and paper with distilled water, and finally dilute to volume. Prepare a blank solutton in the same way. Mix a suitable aliquot (e.g., 25 ml) of this sample solution with an exactly equal volume of buffer. The pH of the buffered sample should be 5.45.6. Measure the fluoride content potentiometrically with a fluoride-sensitive electrode, calibrated with standard fluoride solutions (I-IO ppm) mixed m exactly I : 1 ratio with the buffer used. Apply a correction for the fluonde content of the blank, measured in the same way.
Apparatus An Orion model 407 A ion-meter, 901 ion-analyser, 9409 fluoride electrode and 9001 reference electrode were used. Reagents Analytical grade materials were used whenever possible. Stock fluorrde solution. 1000 ppm. Prepared by dissolving 2.210 g of dried sodium fluoride in distrlled water and
*Author for correspondence. tProcess Laboratory, Zinc Smelter. @Regional Office, Hyderabad.
Udaipur.
RESULTS
AND
DISCUSSION
The results obtained for three zinc-concentrate and two lead-concentrate samples by the procedure above are presented in Table 1. As no standard samples were available, the accuracy of the method was checked by determining the recovery of standard additions of fluoride. One of our main concerns was to see whether calibration with pure aqueous standards would give 1064
SHORT
Table
I. Analysis
Sample
Added I
Lead Concentrate
2
Zinc Concentrate
I
Zinc Concentrate
*Average
2
/wig Found
Relative standard deviation, % 3.5
100
I so* 240
6.0
150+
2.0
275 i 6
200 300
27S 460 580
200 200
l78* 370 360
2.9
of six replicate
293+
for the
Recovery,
y0
II 93 102
IO
90 93 IO1 l78k6 96 90
analyses.
Table 2. Fluoride standards prepared wth sodium hydroxrde and acetrc acid*
F ~, /.~e/nrl Found?
respectively, indicating that more than 98% of the zinc and 99% of the lead remained in the residue. Three buffers (IM citrat&.2M nitrate, 0.2M citrate-0.2M nitrate, and TISAB) were tried, and all gave similar results. The 0.2M citrate buffer will obviously contain less fluoride than the IM citrate buffer and hence be more suitable for low-level fluoride measurements. The fluoride content of TISAB will depend on the relative fluoride content of the individual buffer components. For those used in the work described, we found 0.5 pg/g in the sodium citrate and 4 pg/g in the sodium hydroxide, and ~0.02 pg/ml in the 0.2M citrate buffer, 0.08 pg/ml in the IM citrate buffer and 0.05 pg/ml in the TISAB. We modified the procedure of McQuaker and Gurney4 by using solid sodium hydroxide for the sample decomposition, instead of adding sodium hydroxide solution and evaporating the water (which takes rather a long time). We found that the pellets weighed about 0.2 g each, and the maximum error in weighing the 5 g was about kO.2 g. If the sodium hydroxide used contains say 5 pg of fluoride per g, the error introduced by weighing the pellets should not exceed 0.01 ppm fluoride in the value obtained. We also preferred acetic acid to hydrochloric for the pH adjustment, because the buffering action of the acetate system gave better control in preventing formation of acidic or neutral conditions during the adjustment. The Willard and Winter distillation technique with perchloric acid was also tested and good recoveries of fluoride were obtained. About 400 ml of distillate was found to be sufficient for quantitative recovery from a l-g sample. Acknowledgemen/s-The authors assistance in the investigations.
I I8 f 0.03 2.18 * 0.03 4. I7 + 0.03 5.13+0.03
*I A4crtric XXI buffer used tMcan &standard dcwatwn rcphcates)
and results
Value for 95% confidence, /wcg/g
298* 480 600
measurement of fluoride in sample solutions prepared by the procedure described. To do this, fluoride standards were prepared, each by adding 5 g of sodium hydroxide to a suitable volume of a standard solution, diluting to about 30 ml, adjusting to pH 8-9 with glacial acetic acid, and diluting to volume in a IOO-ml standard flask. These standards were then mixed in I : I ratio with I it4 citrate buffer and measured potentiometrically for fluoride content. A similar range of purely aqueous standards was prepared, mixed in 1:l ratio with the buffer and measured. Comparrson of the two sets of results showed that the sodium hydroxide and glacial acetic acid used produced between them an apparent increase of 0 1550.20 pg/ml in the fluoride concentration. Hence purely aqueous standards can be used for calibration, provided a correction is applied for the reagent blank. As reported by McQuaker and Gurney,4 adjustment of the test solution to pH 889 and filtration removes most of the interference from matrix element\ such as aluminium, iron, silicon. calcium and mapnesrum. Except for iron, which is usually present :II around 5-Y’,, level, these elements occur only at low levels m the concentrates of interest. Since these concentrates contain zinc (cu. SO”/,) or lead (cu. 60%) as the major elements. the filtrates were analysed for these two elements by AAS. The zinc and lead values found were in the ranges 4&80 and 5-7 pg/ml
I.0 2.0 4.0 5.0
method
200 300
accurate
Taken
1065
of lead and zinc concentrates by the proposed recovery of added fluoride
F-.
Lead Concentrate
COMMUNICATIONS
thank
Shri K S. Bolia for
REFERENCES
(3
R. M. Farmer, Anode pre-conditroning and other changes in Cominco’s electrolytrc zinc operatrons, AIME Symposium on Electrometallurgy, 2 December 1968.
1066
SHORT
COMMUNICATIONS
2. S. C. S. Rajan, L. M. Bhandari and B. R. L. Row, paper presented at Indian Science Congress, Calcutta, 1980. 3. D. S. Russell, H. B. Macpherson and V. P. Clancy, Talanra, 1980, 27, 403.
4. N. R. McQuaker and H. Gurney, And. 49, 53 5. B. L Ingram, i/w/.. 1970. 42, 1825.
Chem..
1977,