84
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-Mereya and Z Hariri
Damascus, Syria
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———————————————
aFor Correspondence: Dr. R Al-Merey, Department of Chemistry, Atomic Energy Commission, P. O. Box 6091, Damascus, Syria. Fax: 0963-11-6112289.
Fluoride determination in phosphoric acid
85
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
µg mL-1
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
µg mL-1
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.
Fluoride 32 (2) 1999
86
Al-Merey and Hariri
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
µg mL-1
ion added amount of ion F- found after
added µg g-1 titration µg g-1
50
50
50
50
50
50
50
50
50
50
50
50
50
PO43-
50
V
50
50
50
50
50
50
50
50
50
50
Mg
Ca
Mn
Zn
B
Si
U
NO3–
SO4 2–
Cl-
Fluoride 32 (2) 1999
Al
Fe
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
error
%
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
Fluoride determination in phosphoric acid
87
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
µg g-1
40.80
50.00
78.40
Al added
µg g-1
100
240
80
Fe added
µg g-1
120
120
60
V added
µg g-1
100
120
80
F- recovered
µg g-1
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
µg mL-1 µg mL-1
100
100
240
238.8
80
80
*error
%
0.0
-0.5
0.0
Fe added Fe found
µg mL-1 µg mL-1
100
100
120
119
60
60
*error
%
0.0
-0.83
0.0
V added V found
µg mL-1 µg mL-1
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-
Fluoride 32 (2) 1999
88
Al-Merey and Hariri
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
µg g-1
1850
2400
5000
12000
PO43- added F- recovered after PO43mg g-1
precipitation µg g-1
36.774
1816
61.290
2355
41.370
4910
45.967
11786
*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
µg g-1
µg g-1
µg g-1
µg g-1
µg g-1
mg g-1
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.
Fluoride 32 (2) 1999
*error
%
-2.76
-2.50
-2.61
-2.78
-2.74
-2.85
Fluoride determination in phosphoric acid
89
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
µg g-1
F- determined
µg g-1
*error
%
12000
12000
12000
12000
12000
12000
12000
12000
11676
11664
11714
11727
11706
11688
11674
11659
-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-1
5.118
Mean of measurements mmol kg-1
True value mmol kg-1
Critical value of t
t-Test value
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.
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——————————————————————
Published by the International Society for Fluoride Research
Editorial Office: 17 Pioneer Crescent, Dunedin 9001, New Zealand
Fluoride 32 (2) 1999