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Fluoride Vol. 32 No. 2 84-90 1999 Research Report

SEPARATION OF INTERFERING ELEMENTS FROM COMMERCIAL PHOSPHORIC ACID FOR TITRIMETRIC DETERMINATION OF FLUORIDE
R Al-Merey and Z Hariri Damascus, Syria
a

SUMMARY: Ion exchange resin and precipitation techniques were used to separate aluminum, iron, vanadium, and phosphate ions from commercial phosphoric acid. These separations allowed titrimetric determination of fluoride, free from these interfering ions, in commercial phosphoric acid by thorium nitrate.
Keywords: Fluoride determination, Ion exchange resin, Phosphate precipitation, Phosphoric acid, Thorium nitrate titration.

INTRODUCTION

Commercial wet-process phosphoric acid is an important raw material used in the manufacture of fertilizers, animal feed, food, and pharmaceutical products. Unfortunately, it contains inseparable fluoride species, the presence of which severely limits its applications. Thus, in most industries, purification is necessary before wet-process phosphoric acid can be used. Moreover, the amount of fluoride that is present should be determined prior to purification. The most widely used method to determine fluoride in phosphoric acid is by ion selective electrode. 1-3 Ion chromatography4-7 and spectrophotometric techniques8 are two other methods that are recommended. In the present work, chemical separations were employed to eliminate the interfering ions of Al, Fe, V, and P from commercial phosphoric acid samples. The first three ions were separated by an ion exchange resin, while the phosphorus was precipitated as silver phosphate. Fluoride was then determined titrimetrically with thorium nitrate as the titration reagent. 9
MATERIALS AND METHODS

Analytical reagents: Analytical reagents used in this work were those of G.R. Merck.  Thorium nitrate pentahydrate [Th(NO 3)4·5H2O] solution: 11.40 g of this reagent was dissolved in a solution of 0.001 M nitric acid to give a volume of 1000 mL. This solution was standardized against the fluoride standard solution (see below).  Methylthymol blue indicator: 0.05 g of this indicator was dissolved in 100 mL of methanol, the solution was filtered, and 0.5 mL was taken for each titration. This solution was prepared daily.  Buffer solution: 6.7 g of glycine (H 2NCH2COOH), 11 g of sodium perchlorate monohydrate (NaClO4·H2O) and 10 mL of 1 M perchloric acid (HClO4) were dissolved in 90 mL of distilled water. The pH of the solu——————————————— a For Correspondence: Dr. R Al-Merey, Department of Chemistry, Atomic Energy Commission, P. O. Box 6091, Damascus, Syria. Fax: 0963-11-6112289.

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tion was adjusted to 3.35 ± 0.1 with 1 M perchloric acid or 1 M sodium hydroxide solution.  Fluoride standard solution: Standard fluoride solution (10 g L -1, NaF) for ion selective electrode was used. Preliminary investigation: Before investigating the titration method, the fluoride concentration in commercial phosphoric acid was found to be 12,000 µg g-1, as determined by ion selective electrode method. The thorium nitrate solution (2.06610-2 M) was standardized by titration against the above standard solution of fluoride and was used with methylthymol blue to titrate fluoride concentration in various fluoride standard solutions (Table 1). When the method was applied to commercial phosphoric acid samples, it was not suitable because the detection of the identifying color of the end point in the titration was not possible, evidently because of the presence of interfering ions.
Table 1. Titration of fluoride in fluoride standard solution
F taken -1 µg mL 15 20 25 30 35 40 45 50 100 150 200 250 300 350 400 500
-

Titration reagent consumed, mL 0.35 0.55 0.70 0.90 1.05 1.20 1.50 1.65 3.30 4.85 6.45 8.10 9.70 10.55 11.65 13.92

F titrated -1 µg mL 10.99 17.27 21.98 28.26 32.97 37.68 47.10 51.81 103.6 152.3 202.5 254.3 304.6 331.3 365.8 437.1

-

Error % -26.7 -13.7 -12.1 -5.80 -5.80 -5.80 +4.66 +3.62 +3.62 +1.53 +1.26 +1.73 +1.53 -5.35 -8.55 -12.6

APPLICATION OF PROCEDURE

Studying interference: A series of fluoride standard solutions was prepared, and to each standard solution a known concentration of each ion present in commercial phosphoric acid was added. The thorium nitrate titration was then performed. Table 2 shows that among added ionic forms of Al, Fe, V, Mg, Ca, Mn, Zn, U, B, Si, PO4, NO3, SO4, and Cl, only Al, Fe, V, and PO4 cause interference in the determination of fluoride.

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The interference caused by Al, Fe, and V was eliminated by their removal using ion exchange with Dowex 50X4 (H + form, 100-200 mesh). The resin was washed with double distilled water, transferred as a slurry in double distilled water into a polyethylene column (0.8 cm ID and 10 cm high) and left to settle. The column was washed successively with 50 mL of 4 N HNO 3, distilled water, and finally with 50 mL of 0.5 N HNO 3.
Table 2. Interfering ions added to fluoride standard solution
F taken -1 µg mL 50 50 50 50 50 50 50 50 50 50 50 50 50
-

ion added amount of ion F found after -1 -1 added µg g titration µg g PO4
3-

-

error %

Al

Fe

50

V

50 50 50 50 50 50 50 50 50 50

Mg Ca Mn Zn B Si U – NO3 2– SO4 Cl

4.0 7.5 10.0 25.0 50.0 100.0 1.0 2.0 5.0 10.0 20.0 25.0 5 10 15 20 25 35 40 1.5 3 6 18 30 48 up to 300 up to 300 up to 20 up to 300 up to 1800 up to 2000 up to 125 up to 2500 up to 300 up to 1500

50.16 + 0.32 54.70 +9.40 59.28 +18.56 65.94 +31.88 78.50 +57.00 100.50 +100.96 47.12 -5.76 45.60 -8.80 42.56 -14.88 35.00 -30.00 21.28 -57.44 13.68 -72.64 The solution becomes green before the addition of titration reagent. This color remains stable even after the addition of titration reagent.

The solution becomes blue before the addition of titration reagent, indicating the end point of titration has already been reached. No interference No interference No interference No interference No interference No interference No interference No interference No interference No interference

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A certain volume (Table 2) of fluoride standard solution in 0.5 N HNO 3 medium which contained the indicated amounts of Al, Fe, and V was passed through the column. The column was then washed with 50 mL of 0.5 M HNO3. The fluoride that passed through the column and was taken for determination, while Al, Fe and V were retained by the resin. The retained Al, Fe, and V were removed from the column by washing the resin with 50 mL of 4 N HNO3. The resulting solution was evaporated to a suitable volume (1-2 mL), and the concentrations of Al, Fe, and V were determined by atomic absorption spectrometry. Data in Tables 3 and 4 show the efficiency of ion exchange resin in the separation of Al, Fe, and V from fluoride standard solution. This step eliminates the interference of Al, Fe, and V with fluoride titration. The column was prepared for a second run of separation by regenerating the resin with 25 ml of 4 N HNO3, 50 mL of water, and 50 mL of 0.5 N HNO3.
Table 3. Separation of Al, Fe and V from fluoride standard solution by ion exchange resin
F taken -1 µg g 40.80 50.00 78.40
-

Al added -1 µg g 100 240 80

Fe added -1 µg g 120 120 60

V added -1 µg g 100 120 80

F recovered -1 µg g 40.20 49.38 78.10

-

*error % -1.47 -1.24 -0.40

*Each result is the mean of five replicates.

Table 4. AAS determination of Al, Fe, and V after ion exchange separation from fluoride standard solution
Al added Al found -1 -1 µg mL µg mL 100 100 240 238.8 80 80 *error % 0.0 -0.5 0.0 Fe added Fe found -1 -1 µg mL µg mL 100 100 120 119 60 60 *error % 0.0 -0.83 0.0 V added V found -1 -1 µg mL µg mL 100 100 120 119 80 80 *error % 0.0 -0.83 0.0

*Each result is the mean of five replicates.

Solvent extraction of phosphate as phosphomolybdate or phosphotungstate to eliminate the interference caused by phosphorus was unsuccessful. Therefore, phosphate was precipitated in the form of silver phosphate as follows: 2 mL of the phosphoric acid sample was adjusted to pH 13 with 30% aqueous sodium hydroxide. Silver nitrate solution (20%) was added until precipitation of brown silver phosphate was completed and the medium reached pH 7. The precipitation end point was determined by adding a few drops of 25% aqueous sodium chloride solution to the supernatant. The mixture was filtered the precipitate was washed several times with distilled water, and fluoride was de-

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termined in the filtrate. Results in Table 5 show quantitative elimination of the effect of phosphate on the titration of fluoride in samples of phosphoric acid.
Table 5. Precipitation of phosphorus as silver phosphate from fluoride standard solutions
F taken -1 µg g 1850 2400 5000 12000
-

PO4 added F recovered after PO4 -1 -1 mg g precipitation µg g 36.774 1816 61.290 2355 41.370 4910 45.967 11786

3-

-

3-

*error % -1.84 -1.87 -1.80 -1.78

*Each result is the mean of five replicates.

Determination of fluoride in commercial and pure phosphoric acid samples: A 1-mL sample of reagent grade phosphoric acid (G.R. Merck) was diluted to the same density as commercial phosphoric acid (1.27 g mL -1). To this diluted acid, 12000 µg g-1 fluoride, 800 µg g-1 aluminum, 125 µg g-1 iron, and 200 µg g-1 vanadium were added, and the above separation processes were applied, following which the concentration of fluoride was determined by titration. As seen in Table 6, the separation schemes (ion exchange and precipitation) were successfully applied to both reagent grade (Table 6) and commercial grade (Table 7) phosphoric acid for titrimetric determination of fluoride. t-Test evaluation: The t-test was carried out by comparing the experimental mean of six measurements of fluoride for the known value (5.263 mmol kg -1). The critical value of |t| is 2.57 at P = 0.05 for five degrees of freedom. Since the experimental value of |t| = 0.143 is less than the critical value, the null hypothesis is retained10 (Table 8).
Table 6. Determination of fluoride in reagent grade phosphoric acid after separation of the interfering ions (Al, Fe, V, and phosphate)
Phosphate F added Al added Fe added V added F recovered concentration -1 -1 -1 -1 -1 µg g µg g µg g µg g µg g -1 mg g 398.4 12000 800 125 200 11669 398.4 12000 800 125 200 11700 398.4 12000 800 125 200 11687 398.4 12000 800 125 200 11666 398.4 12000 800 125 200 11671 398.4 12000 800 125 200 11658 *Each result is the mean of five replicates. *error % -2.76 -2.50 -2.61 -2.78 -2.74 -2.85

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Table 7. Determination of fluoride in commercial phosphoric acid after chemical separation of the interfering ions (Al, Fe, V and phosphate).
F in phosphoric acid -1 µg g 12000 12000 12000 12000 12000 12000 12000 12000
-

F determined -1 µg g 11676 11664 11714 11727 11706 11688 11674 11659

-

*error % -2.70 -2.80 -2.38 -2.27 -2.45 -2.60 -2.72 -2.84

*Each result is the mean of five replicates.

Table 8. t-Test of the method
Fluoride found mmol kg 5.118 -1 Mean of measurements mmol kg -1 True value mmol kg Critical value of t t-Test value
-1

5.132

5.126 5.117 5.120 5.263 2.57 0.143

5.119

5.113

SUMMARY OF RESULTS

A resin ion exchange technique was used to achieve an efficient separation of the interfering ions of Al, Fe, and V from commercial phosphoric acid in the determination of fluoride by thorium nitrate titration. It was not possible, however, to eliminate the interference of phosphate in this method of fluoride determination since both ions behave similarly in the process of ion exchange separation. Therefore, phosphate was first precipitated as silver phosphate. The combined separation processes then allowed the determination of fluoride concentration in commercial phosphoric acid by thorium nitrate titration without interference.
ACKNOWLEDGEMENT

The authors would like to thank Prof. I. Othman, Director General of the Atomic Energy Commission of Syria, and Prof. Y. Koudsi, Head of the Chemistry Department, for their support and encouragement.
REFERENCES

1

Israel Y, Paschkes B. Determination of low levels of fluoride in concentrated phosphoric acid by use of fluoride selective electrode. Mikrochimica Acta 1981;II:69-81.

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3 4 5 6 7 8 9 10

Ciba L, Strzeszewska I, Matenko H. Purity of phosphoric acid salts with respect to fluorine and heavy-metal impurities. Prace Naukowe Instytutu Technologii Nieroganicznej i Nawozow Mineralnych Politechniki Wroclawskiej 1989;36:120-4 (in Polish) (Chem Abst 112:219821h 1990). Roldan R, De-Gyves J. Direct potentiometric determination of fluoride ions in wet-process phosphoric acid. Afinidad 1990;47:398-400. Sheridan RC. Simultaneous determination of fluoride, phosphate and sulphate in wet-process phosphoric acid by ion chromatography. Journal of Chromatography 1986;371:383-6. Luo Y, Cao D. Micro-determination of thiocyanate, fluoride, chloride, bromide phosphate, sulphate, and iodide anions by ion chromatography with conductivity detector. Fenxi Huaxue 1986;14:833-5. Pires MAF, Atalla LT, Abrao A. The use of ion chromatography for the determination of impurities in crude phosphoric acid. Sao Paulo; Instituto de Pesquisas Energeticase Nucleares, 1988. Sendra F, Canals A, Hernandis, V. Determination of fluoride by distillationion. Chromatography. Afinidad. 1990;47:127-30. Kostadinova LS, Elenkova NG, Nedelcheva TSK. Extraction - photometric determination of fluoride in phosphoric acid and forage phosphates. Zhurnal Analiticheskoi Khimii 1990;45:2235-8. Watson C. Official and Standardized Methods of Analysis. 3rd ed. Cambridge: The Royal Society of Chemistry, 1994:496-7. Miller JC, Miller JN. Statistics for Analytical Chemistry. 3rd ed. New York: Ellis Horwood PTR Prentice Hall, 1993.

—————————————————————— Published by the International Society for Fluoride Research Editorial Office: 17 Pioneer Crescent, Dunedin 9001, New Zealand

Fluoride 32 (2) 1999


				
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