Volumetric determination of calcium and phosphate ions in presence of iron, aluminum, fluoride, and sulfate

MICROCHEMICAL

JOURNAL

Volumetric

11, 1-12 (1966)

Determination

Ions in Presence

of Iron, and

L.

SZEKERES,E.

Chemical Institute

of Calcium Aluminum,

Phosphate Fluoride,

Sulfate

KARDOS,AND G.L.

of Veterinary

and

University,

SZEKERES

Budapest, Hungary

Received May 2.5, 1965

Hahn and Meyer (4) first described the precipitation titration of phosphate ions. The principle of this method is as follows: The phosphate ions are titrated with standard magnesium sulfate in presence of ammonium chloride-ammonia buffer . A precipitate of magnesium ammonium phosphate hexahydrate forms during the titration. The endpoint is indicated by quinalizarin. Subsequent precipitation titrations of phosphate utilizing lead nitrate (2, 6, 8,17, la), bismuth salt (3, 11,19, ZO), cerium (III) (6, 8-10) have been recommended. Magnesium sulfate or chloride has proved to be the best titrant among those listed since unlike the other titrants, it is insensitive to the presence of many anions except arsenate. During the past few years, eriochrome black-T (1,13), bromo pyrogallolred, methylthymolblue, pyrocatechol violet and thymolphthalexon (12, 14, 16), were suggested as alternate indicators for the phosphate determination. The method of Hahn and Meyer (4) can be applied directly only to alkali phosphates. Phosphates usually occur together with calcium and natural phosphates often contain iron, aluminum and fluoride impurities. Artificial phosphate fertilizers also contain sulfate. Accordingly, a method for phosphate and sulfate was worked out which gives good results in presence of calcium, iron, aluminum, and fluoride (13). A simultaneous determination of calcium is not possible with this method. A simple simultaneous determination of calcium and arsenate (1.5) utilizing precipitation titration was made possible by the discovery of

2

L.

SZEKERES,

E.

KARDOS,

AND

G. L.

SZEKERES

thymolphthalexone indicator (5, 7). The experiences gained while using this latter method led to the development of a simple simultaneous determination of calcium and phosphate, in presence of several foreign ions (12, 16). The principle of this method is as follows: EDTA, triethanolamine, and ammonia are added to the weakly acid solution containing calcium, phosphate, ferric, aluminum, sulfate, and fluoride ions. Ferric and aluminum ions are masked in this alkaline system, and calcium ions are masked by EDTA. Since thymolphthalexone indicator functions in presence of the triethanolamine-ferric complex, the excess EDTA may be titrated with standard magnesium chloride to the blue endpoint of this indicator. The calcium content may be calculated from this titration. The resultant blue solution is then decolorized with solid ammonium chloride and ethanol. Then one titrates with standard magnesium chloride to reappearance of the blue endpoint. Magnesium ammonium phosphate hexahydrate precipitates during this titration which measures the phosphate content. The procedure allows the determination of about 100 mg of phosphate (12, 16) in superphosphate fertilizers and the raw material from which they are manufactured. Later studies have shown that this method gives good accuracy on samples containing 2-8 mg of calcium and S-20 mg phosphate. As little as 2 mg of phosphate may be determined satisfactorily with a slightly modified procedure. The sample is dissolved in a few milliliters, exactly measured, of 0.1 M disodiumphosphate and a few drops of concentrated hydrochloric acid. The titration must be corrected for the phosphate added for sample dissolution. As seen in Tables 1-4, the quick method described yields satisfactory results. EXPERIMENTAL

Reagents Ammonia, concentrated Hydrochloric acid, concentrated Ammonium chloride, crystals Ethanol Triethanolamine, 20% EDTA, O.lM Magnesium chloride, 0.05 M Disodium phosphate, 0.1 M Thymolphthalexon indicator, 1: 100 in potassium nitrate

Al

-

-

-

PO,,

20.27

20.27

20.27

No.

1

2

3

DETERMINATION

-

-

-

Fe

Present

8.0

4.0

-

Ca

(mg)

OF CALCIUM

-

-

-

F

-

-

-

SO, -

3

2

(ml)

0.1 M EDTA

AND PHOSPHATE

2.020 2.008 1.996 2.000 2.004 1.992 1.982 2.033 1.980 2.006 2.033 1.970 1.980 2.040

-

2.006

2.000

-

Milliliters of 0.05 M MgCl, sol. used at the back titration of EDTA

4.316 4.329 4.329

4.343 4.356

20.62

7.99

+0.35

+0.41

-

-0.01

0.00

-

Ca

(mg)

s rn

5

5

8 0 r” 2 C g

E

z 5 2 5

i

4.343 4.369

4.341

4.0

-

PO,

Difference

IONS

4.369

20.68

20.27

Ca

(mg)

AND SULFATE

z F:

4.354

4.268

PO,

Found

FLUORIDE,

4.356 4.369 4.330 4.382

4.343 4.330

4.264 4.277

4.250 4.270

4.277 4.264 4.277

Milliliters of 0.05 M MgC12 sol. used at the titration of phosphate

TABLE 1 IN THE PRESENCE OF FERRIC, ALUMINUM,

Al

-

-

PO,

20.27

20.37

No.

4

5

-

1.2

Fe

-

8.0

Ca

Present (mg)

-

-

F

-

-

-

3

0.1 M EDTA SO, (ml)

1 (continued)

2.033 2.033 2.033 2.019 1.953 1.958 2.019 -

2.010

Milliliters of 0.05 M MgCl, sol. used at the back titration of EDTA

TABLE

4.316 4.329 4.329 4.355 4.368 4.342 4.316 4.290 4.290 4.290 4.290 4.290 4.300 4.300 4.292

4.340

Milliliters of MgCl, sol. used at the titration of phosphate 0.05 M

20.39

20.61

PO,

-

7.98

Ca

Found (mg)

+0.02

+0.34

PO,

Difference

-

-0.02

Ca

(mg)

.”

R E i-2 E

.F

z ”

E g 3

.M

J

R I? E

DETERMINATION

OF CALCIUM

AND

N

N. 0

h

PHOSPHATE

IONS

5

PO,

8.16

8.16

No.

9

10

0.5

0.5

Al

0.5

0.5

Fe

1.6

1.6

Ca

Present (mg)

9.6

-

0.1

SO,

F

0.1

2

2

0.1 M EDTA (ml)

2

3.168 3.181 3.102 3.115 3.141 3.155 3.155

3.168 3.181 3.141 3.168 3.115 3.194 3.168 3.145

3.161

1.716 1.755 1.716 1.769 1.755 1.755 1.755

1.746

1.749

Milliliters of 0.05 M MgCl, sol. used at the titration of phosphate 1.755 1.742 1.729 1.782 1.755 1.769 1.716

(continued)

Milliliters of 0.05 M MgCl, sol. used at the back titration of EDTA

TABLE

8.29

8.30

PO,

+0.14

+0.13

1.71

PO,

Difference

1.68

Ca

Found (mg)

+0.11

+O.OS

Ca

(mg)

.F

R z !i E

;

z

i w

?

,g

R z E

DETERMINATION

P

2 d

OF CALCIUM

AND

23 i

PHOSPHATE

IONS

r. N. d

1 0

v! 0

7

PO,

4.25

4.25

No.

14

15

0.5

0.5

Al

0.5

0.5

Fe

1.6

1.6

Ca

Present (mg)

1.1

0.1

F 9.6

2

2

SO,

-

EDTA (ml)

0.1 M

TABLE

3.169

3.168 3.181 3.154 3.181 3.168 3.181 3.154

0.937 0.924 0.910 0.937 0.910 0.924 0.897

0.910 0.897 0.924 0.924 0.910 0.897 0.910

3.164

3.128 3.181 3.168 3.181 3.168 3.181 3.141 0.917

0.901

Milliliters of 0.05 M MgCl, sol. used at the titration of phosphate

(continued)

Milliliters of 0.05 M MgCI, sol. used at the back titration of EDTA

3

4.35

1.68

1.67

+0.10

+0.03

PO,

4.28

Difference

PO,

Ca

Found (mg)

+0.08

$0.08

Ca

(mg)

co

2.12

2.12

16a

17a

0.5

0.5

0.5

Fe

0.5

Al

1.6

1.6

Ca

Present (mg)

OF CALCIUM

0.2

0.2

F

9.6

-

SO,

2

2

TABLE

4

3.207 3.207 3.181 3.194 3.168 3.181 3.194

3.155 3.168 3.155 3.168 3.194 3.207 3.181 3.190

3.175

4.765 4.765 4.791 4.765 4.765 4.778 4.791

4.791 4.765 4.754 4.752 4.791 4.778 4.752 4.780

4.770

Milliliters of Milliliters of 0.05 M MgCl, 0.05 M MgCl, sol. used at the sol. used at the back titration titration of of EDTA phosphate

IN THE PRESENCE OF FERRIC, ALUMINUM,

0.1 M EDTA (ml)

AND PHOSPHATE

a The sample was dissolved with the help of some drops of cc. HCI in 2 ml of Na,HPO, alent to 4.308 ml 0.05 M MgCI, sol.

PO,

No.

DETERMINATION

1.62

1.65

Ca

sol. Two ml Na,HPO,

2.24

2.19

PO,

IONS

+0.02

+o.os

Ca

(mg)

sol. is equiv-

+0.12

to.07

PO,

Difference

AND &JLFATE

Found (mg)

FLUORIDE,

EI 5

5

l is l

5

A 2 E

z $ 0

E !2 5 g

I2

iz

10

L. SZEKERES, E. KARDOS, AND G. L. SZEKERES

Procedure A DETERMINATION OF THE PHOSPHATE CONTENT OF ALKALI PHOSPHATES

Add 0.4 g of ammonium chloride, 4 ml of concentrated ammonia, 3 ml of ethanol, and thymolphthalexon indicator to 2 ml of sample containing 4 to 20 mg of phosphate. Titrate this colorless or light-grey solution with 0.05 M of magnesium chloride to a blue. Magnesium ammonium phosphate hexahydrate precipitate forms during the titration. It is very advantageous to add one-half of the calculated amount of magnesium chloride to the sample. This causes a strong blue color. After a minute of swirling, the blue fades and the precipitate forms in large crystals which settle easily. Then the titration is carried out as usual. The light blue endpoint is not obscured by the quickly settling crystals. Procedure B DETERMINATION

OF PHOSPHATE IN PRESENCE OF CALCIUM

Add 3 ml of 0.1 M EDTA and 5 ml of concentrated ammonia to 2 ml of a weakly acidic (hydrochloric acid) sample containing 4-20 mg of phosphate and 2-8 mg of calcium. Add thymolphthalexon and slowly titrate the colorless or pale-grey solution to a blue by using 0.05 M magnesium chloride. The excess EDTA is thus titrated. Occasionally the blue color fades after one to two minutes. Simply add more titrant to a stable endpoint. Calculate the calcium content from this titration. Add 0.4 g of ammonium chloride and 4-S ml of ethanol to the blue solution, which turns colorless. Titrate according to Procedure A to a blue endpoint. Procedure C DETERMINATION

OF PHOSPHATE IN PRESENCE OF CALCIUM, ALUMINUM, FERRIC, FLUORIDE, AND SULFATE IONS

Add 2 ml of thiethanolamine, 3 ml O.lM EDTA, and 5 ml of concentrated ammonia to 2-3 ml of a weakly acidic (HCl) sample containing 4-20 mg phosphate, 2-8 mg of calcium, 1 mg ferric, aluminum, and fluoride ions, and 10 mg sulfate. Titrate the colorless or pale-grey solution according to Procedure B. Then add 0.4 g of ammonium chloride and 5 ml of ethanol. Titrate according to Procedure A with O.OSM magnesium chloride.

DETERMINATION

OF CALCIUM

AND PHOSPHATE

IONS

I1

Procedure D DETERMINATION

OF LESS THAN

CALCIUM, ALUMINUM,

4 MG PHOSPHATE

FERRIC,

FLUORIDE,

IN PRESENCE OF

AND SULFATE IONS

Dissolve the sample, containing not more than 0.5-l mg of iron and aluminum, nor 1 mg of fluoride, by means of a few drops of concentrated HCl, 2-3 ml of water, and a measured amount of O.lM disodium phosphate. Titrate according to Procedure C. Correct the phosphate found for the amount added during dissolution of the sample. The determination takes only a few minutes. SUMMARY A procedure is described for the determination of calcium and phosphate in the presence of ferric, aluminum, fluoride, and sulfate ions. The ferric and aluminum ions in the weakly acid sample solution are masked with triethanolamine. If a precipitate forms it is redissolved with a few drops of concentrated HCI. A measured excess of EDTA is added to the clear solution and then the mixture is made alkaline with concentrated ammonia. The EDTA excess is titrated with standard magnesium chloride to a light-blue thymophthalexon endpoint. Then the solution is decolorized with ammonium chloride and ethanol. The titration is continued to a light-blue endpoint, which is not obscured by the precipitate of magnesium ammonium phosphate hexahydrate. The method is applicable to the determination of 4 mg of phosphate. REFERENCES 1.

2.

3.

4. 5. 6.

7.

E., AND SZEKERES, L., Schnelle Fallungsmethode zur Bestimmung der Phosphat- und Sulfationen nebeneinander. Z. Anal. Chem. 166, 406-410 (1959). EVANS, B. S., New color reagent for lead and its use as an indicator for the titration of various cations and anions (lead, zinc, tungstate, molybdate, vanadate, phosphate and arsenate). Analyst 64, 2-14 (1939). GENGE, J. A. R., AND SALMON, J. E., Volumetric determination of phosphates and of metals in the presence of phosphates. I. Volumetric determination of orthophosphates with bismuth oxyperchlorat solution. Lab. Pratt. 6, 325-326 (1957). HAHN, F. L., AND MEYER, H., Massanalytische Bestimmung von Phosphat und von Magnesium. Chem. Ber. 60, 975-977 (1927). K~~RBL, J., AND PRIBIL, R., Metallochromic indicators. VI. Analogues of o-cresolphtalein complexone. Coll. Czech. Chem. Commun. 23, 1213-1218 (1958). LASSNER, E., P~~SCHEL, R., AND SCHAW, R., Uber die Verwendung metallspezifische Indicatoren bei Fallungstitrationen. V. Exacte Schnellbestimmung geringer Mengen von Orthophosphat in Gegenwart von Sulfat. Z. Anal. Chew 170, 412-419 (1959). PRIBIL, R., KBRBL, J., KYSIL, B., AND VOBORA, J., Komplexometrische Titrationen (Chelatometrie). XXXVI. Beitrag zur Maskierung von Eisen mit Triathanola-

BAKACS-POLCAR,

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8.

9.

10.

11.

12.

13.

14. 15.

16.

17. 18.

19. 20.

L.

SZEKERES,

E.

KARDOS,

AND

G. L.

SZEKERES

min; Bestimmung vom Calcium mit Thymolphtaleinkomplexon. CoZZ. Czech. Chem. Commun. 24, 1799-1803 (1959). P~~SCHEL,R., tfher die Verwendung metallspezifischen Indikatoren bei Fallungstitrationen. VII. Eine neue direkte Schnellbestimmung von Mikrogrammengen Orthophosphat. Mikrochim. Acta 1960, 352-365. P%CHEL, R., AND LASSNER, E., uber die Verwendung von metallspezifischen Indicatoren bei Flllungstitrationen. VI. Direkte, volumetrische Schnellbestimmung von Phosphation mit Cer(III)-massliisung nach Abtrennung starender Metallionen mit Hilfe von Ionenaustauschern. 2. anal. Chem. 174, l-3 (1960). P~~SCHEL, R., AND WITTMAN, H., Massanalytische Schnellmikromethode zur Bestimmung von Phosphor in organischen Substanzen. Mikrochim. Acta 1960, 670674. SIJBBARAMAN, P. R.! A rapid volumetric method for the estimation of phosphate and its application to phosphate rocks, and artificial fertilizers. J. Sci. Znd. Res. (India) 12B, 300-302 (1953). SZEKFXES, L., Rapid volumetric determination of phosphate, sulphate and calcium amount in mineral raw phosphate and in superphosphate artificial-fertilizers. Magy. Ktm. Lapja 19, 321-324 (1964). SZEKERES, L., AND BAKACS-POLGAR, E., Simultaneous determination of phosphate and sulfate ions in the presence of metal contaminations. J. Agr. Food Chem. 8, 417-419 (1960). SZEKERES, L., AND KARDOS, E., Titrimetric

determination of phosphate, arsenate, and sulfate. Ann. Chim. (Rome) 52, 844-848 (1962). SZEKERES, L., KARDOS, E., AND SZEKER~, G. L., Titrimetric determination of calcium and arsenate in the presence of iron, aluminum, fluoride, and sulfate. Chemist-Analyst 53, 40-41 (1964). SZEKERES, L., KARDOS, E., AND SZEKERES, G. L., Volumetrische Bestimmung der Calcium- und Phosphationen neben Eisen(II1) -Aluminiumund anderen Ionen. J. Prakt. Chem. 28 (4R), 113-118 (1965). VANCEA, M., AND VOLUSNIUC, M., Direct volumetric determination of phosphates I. Acad. Rep. Populare Romine, Studii Cercetari Chim. 8, 85-88 (1957). VANCEA, M., AND VOLUSNIUC, M., Titration of assimilable phosphorus as lead phosphate dithizone indicator III. Acad Rep. Pop&are Romine, Studii Cercetari Chim. 8, 261-264 (19.57). VANCE, M., AND VOLUSNIUC, M., Direct volumetric determination of phosphates II. Acad. Rep. Populare Romine, Studii Cercetari Chim. 8, 89-92 (1957). VANCEA, M., AND VOLUSNIUC, M., Titration of assimilable phosphorus as lead phosphate dithizone indicator IV. Acad. Rep. Populare Romine, Studii Cerceturi Chim. 8, 265-268 (1957).