- Email: [email protected]
Determinations of traces of calcium in phosphoric acid and its salts by atomic-absorption spectrometry
Talanta. 1967. Vol. 14. pp. 933 to 936. Pergamon Press Ltd. Prhtcd in Northern Ireland
DETERMINATIONS OF TRACES OF CALCIUM IN PHOSPHORIC ACID AND ITS SALTS BY ATOMICABSORPTION SPECTROMETRY MASAAKI YANAGISAWA, MASAMI SUZUKI and TSUGIO TAKEUCHI Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University’, Chikusa-ku, Nagoya, Japan (Received 13 December 1966. Accepted 29 March 1967) Summary-A method for the determination of traces of calcium by atomic-absorption spectrometry after extraction into an organic solvent, has been investigated for the determhration of traces of calcium in phosphoric acid and its salts. Calcium is extracted as the hydroxyquinolate complex, the extract is introduced into the flame, and the calcium atomic-absorption is measured. The effects of PI-I, concentration of reagent, and diverse ions on the calcium extraction were investigated.
spectrometry has been applied to the determination of calcium in various kinds of sample. However, difhculty may be encountered in the determination of traces of calcium in phosphoric acid and its salts because of the effect of phosphate on the calcium absorption. For this kind of sample, it was considered preferable to combine the techniques of solvent extraction and atomic absorption, as had been done previously for magnesiuml, and this paper describes the results. ATOMIC-ABSORPTION
EXPERIMENTAL Apparatus A Hitachi photoelectric spectrophotometer EPUGA was used with the atomic-absorption attachment RA-1. An air-acetylene flame was used with a 9-cm fishtail burner. Hitachi and Westinghouse hollow cathode lamps were used as light sources. The operating parameters were: 1.5 kg/cm* Air pressure Acetylene pressure 0.04 kg/cm* Wavelength 4227 A 0.03 mm Slit-width 40 mA (Hitachi), 10 mA (Westinghouse). Lamp current Reagents Calcium solution. Dissolve 0.250 g of calcium carbonate in a minimum excess of hydrochloric acid, and dilute to 100 ml with water. S-Hy&oxyqz&ofine solution. Dissolve 5 g of I-hydroxyquinoline in 100 ml of ethanol. Sodium tartrate solution, 10 % w/v. Saturated sodium chloride s&t&. Bu$ersolution. Mix 1Mpotassium chloride solution and 1Msodium hydroxide solution to give a buffer of pH 13. Reagents were of analytical grade unless otherwise stated.
To 9.80 g of phosphoric acid in a polyethylene beaker, add 1 ml of 6M hydrochloric acid and 20 ml of sodium tartrate solution. Neutralize the solution by slowly adding 6M sodium hydroxide and dilute to 100 ml with water in a standard flask. Place 10 ml of this samule solution in a SO-ml separatory funnel, and add 10 ml of sodium chloride solution. Mix well, and’then add 2 ml of butyl cellosolve, 3 ml of I-hydroxyquinolme solution, and 5 ml of buffer solution. Adjust the volume to 40 ml, add exactly 5 ml of 3-methyl-l-butanol and shake vigorously for 1 min. Drain off and discard the aqueous phase. Spray the organic phase into the flame and measure the calcium absorption. 933
934
SUZUKI and TSUGIO TAKEUCHI
MASAAKI YANAGISAWA, MB~I
For phosphate samples, dissolve a suitable weight (3 g for diammonium hydrogen phosphate and 8.9 g for sodium pyrophosphate) in 2 ml of 1M hydrochloric acid and 20 ml of sodium tartrate solution, and dilute to 100 ml with water. Place 10 ml of the sample solution in a separato funnel and proceed as above. If the pH of the solution is lower than 13, adlust it with sodium hydroxr 7 ebefore extraction. Prepare a calibration curve by taking appropriate volumes of standard calcium solution in 50-ml separatory funnels, adding 10 ml of sodium chloride solution and 2 ml of sodium tartrate solution to each, mix, and apply the procedure. RESULTS
AND
DISCUSSION
Atomic-absorption of calcium The air-fuel ratio had a significant effect on the calcium absorption. The air pressure was set at a constant value (1.5 kg/cm2), and the acetylene pressure was varied. The results are shown in Fig. 1. Although the acetylene pressure which gives maximum
I
LiObl
I
I
003 Amtylcne presurq 002
Air pressure 1.5,
I
,
004 005 kg/cm2
I
kg/cm2
FIG. 1.-Effect of acetylene pressure on calcium absorption Beam height above burner top: x-2.0 mm; 0-3~0 mm; 0-45
mm.
absorption depends on the beam height above the burner-top, the maximum absorption was constant irrespective of the beam height. Extraction of calcium Extraction of calcium hydroxyquinolate has been used for spectrophotometric determinations,2~ and we investigated the use of various solvents such as 3-methyl-lbutanol, 4-methyl-2-pentanone, butyl acetate, I-butanol and chloroform. Of all the solvents tested 3-methyl-1-butanol was found to be best for extraction because it gave a higher sensitivity of measurement. The extraction efficiency was estimated by extracting with successive 5-ml portions of 3-methyl-1-butanol. It was found that 89 % of the calcium was extracted in a single pass, over the range of calcium concentration studied. In the present work, a single extraction was used because it was highly reproducible. Shaking vigorously for longer than a minute did not improve the extraction. Although calcium hydroxyquinolate could be extracted into 3-methyl-lbutanol, the addition of butyl cellosolve resulted in better extraction and repeatability for increasing amounts of calcium. Some workers2n4 have successfully extracted calcium hydroxyquinolate complex with chloroform by using butyl cellosolve as a supplementary solvent. However, chloroform gives an unstable flame and noxious gases during the combustion. Solvents other than esters, ketones and alcohols were not suitable for atomic-absorption measurements because of incomplete combustion and self-absorption of radiation.
Traces of calcium in phosphoric acid
935
The extraction of calcium hydroxyquinolate was examined over a pH range of lOG13.6. The degree of extraction was constant when the pH was greater than 12.5. The effect of reagent concentration on the degree of extraction was also investigated. The degree of extraction was constant if more than 100 mg of 8-hydroxyquinoline were added. Sodium chloride was useful for salting-out and preventing emulsification, the minimum necessary for reproducible results being 5 ml of saturated sodium chloride solution in 40 ml of aqueous phase. Tartrate, which served to prevent the formation of calcium phosphate at higher pH, had no effect on the extraction of calcium when present in concentrations of O*Ol-O.O5M. The calibration curves are almost linear and were corrected for a very smaIl blank value which we attributed to absorption caused by 8-hydroxyquinoline. Various metals form hydroxyquinolate complexes at pH 13, some of which are extractable into 3-methyl-1-butanol. To ascertain the influence of diverse ions on extraction and on the calcium atomioabsorption in the flame, calcium was extracted with 8-hydroxyquinoline from solutions containing diverse ions; Al (489 pug), Ba (1063 rug), Cr (456 pg), Cu (532 Icg), Fe (989 ,&, Mg (1004 cl&, Mn (511 pg), MO (320 ,ug), Ni (964 pg), Pb (801 pg), Si (1554 ,L.&,Sn (654 pug),Sr (1440 rug), Ti (497 pg), W (1000 ,ug) and Zn (1266 ,ug) did not interfere in the determination of 40 pug of calcium. Aluminium and titanium depressed the calcium absorption when an aqueous solution was sprayed into the flame, but no interference was observed in the present method, even though both metals were extracted into the organic phase as hydroxyquinolates. It may be considered that 8-hydroxyquinoline in the extracts counteracts the depressive effect of ahuninium and titanium on the calcium absorption. Larger amount of these metals did cause depression of the calcium absorption. Large amounts of chromium and molybdenum also affected the calcium absorption, but this effect could be controlled by changing the air-fuel ratio, a fuel-rich flame being recommended. The copper complex remained in the aqueous phase as a precipitate and did not dissolve in 3-methyl-1-butanol, but resulted in a lower result for the calcium. A complexing agent such as cyanide was necessary to keep copper in solution. Traces of calcium could be extracted from solutions containing large amounts of phosphate (100 mg of P) without loss. Attempts to use 8-hydroxyquinaldine as complexing agent were unsuccessful. The atomic-absorption of the calcium in the flame was very low, and it was concluded that the calcium was very incompletely extracted into the solvents tried (3-methyl-lbutanol, 4-methyl-2-pentanone, butyl acetate, amyl acetate and I-butanol). Determination of traces of calcium in phosphoric acid and its salts Table I shows the results for some samples of phosphoric acid and phosphates. One sample was analysed five times to evaluate the reproducibility of the method. The mean value was found to be 13.5 ppm and the coefficient of variation was 3.7 %. Recovery of added calcium was excellent. Earlier workersb*6 have described the determination of calcium in phosphoric acid, but the present method appeared to be very convenient for traces of calcium in phosphoric acid and its salts. Application of the method to polypropylene polymers Polypropylene polymers contain titanium, aluminium and phosphorus together with calcium derived from additives or the polymerization catalyst. The present
MAWAKI YANAGISAWA,MA~AMI SUSUKI and TSUGIO TAKEUCHI
936
TABLEI.-DETERMINATION OF TRACESOF CALCIUMIN PHOSPHOIUCACID AND
ITS SALTS
Calcium, ppm Sample Phosphoric
acid A
Phosphoric
acid B
Diammonium hydrogen phosphate
Added
Found
20.0 20.0
13.2, 13.3 32.9 1.6, 1.6 21.5
133.5
15.4, 15.4 149 3.2,3.2 25.6
Sodium pyrophosphate <4
extraction method was applied to the determination of calcium in such samples. For estimation 1 g of sample was ashed in a platinum crucible with O-3 g of sodium carbonate. After addition of 6M hydrochloric acid to acidify the residue, the contents of the crucible were heated with 20 ml of sodium tartrate solution to give a clear solution which was then diluted to 100 ml, and 10 ml of this solution were used for analysis. TABLE II.-DETERMINATION OF CALCIUMIN POLYPROPYLENE WLYMERS Calcium, ppm Polypropylene A B C
The ers can content present
Added 100-O 200.0 200.0
Found 45,45 146 120,120 324 267,267 460
results obtained for some polymers are shown in Table II. Calcium in polymbe determined only with diflkulty by the usual method because of the low of calcium and the presence of titanium, aluminium and phosphorus. The method is convenient for measuring the calcium. R&un&-On a &udiQ une methode de dosage de traces de calcium par spectrometrie d’absorption atomique aprb extraction en solvant organique pour doser des traces de calcium dans l’acide phosphorique et ses sels. On extrait le calcium a l’etat de complexe hydroxyquinoMique, introduit l’extrait darts la flamme et mesure l’absorption atomique du calcium. On a etudie les influences du pH, de la concentration du reactif et de divers ions sur l’extraction du calcium. Zusammenfassung-Ein Verfahren zur Bestimmung von Calciumspuren durch Atomabsorptionsspektrometrie nach Extraktion in ein organisches Losungsmittel wurde zur Bestimmung von Calciumspuren in Phosphorsaure und ihren Salzen geprtift. Calcium wird als Hydroxychinolatkomplex extrahiert, der Extrakt in die Flamme gebracht und die Atomabsorption von Calcium gemessen. Der EinfIuh von pH, Reagenskonzentration und verschiedenen Ionen auf die Extraktion von Calcium wurden untersucht.
1. 2. 3. 4. 5. 6.
REFERENCES M. Suzuki, M. Yanagisawa and T. Takeuchi, Tu[unra, 1965,12,989. F. Urnland and K. U. Meckenstock, 2. Anal. Chem., 1959,X5,161. H. Goto and E. Sudo, Japan Analyst, 1961,10, 171. C. L. Luke, Anal. Chim. Actu, 1965,32,221. D. K. Banerjee, C. C. Budke and F. D. Miller, Anal. Chem., 1962,34,440. I. Johnston and M. Stow, Analyst, 1964, 89,290.
DETERMINATIONS OF TRACES OF CALCIUM IN PHOSPHORIC ACID AND ITS SALTS BY ATOMICABSORPTION SPECTROMETRY MASAAKI YANAGISAWA, MASAMI SUZUKI and TSUGIO TAKEUCHI Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University’, Chikusa-ku, Nagoya, Japan (Received 13 December 1966. Accepted 29 March 1967) Summary-A method for the determination of traces of calcium by atomic-absorption spectrometry after extraction into an organic solvent, has been investigated for the determhration of traces of calcium in phosphoric acid and its salts. Calcium is extracted as the hydroxyquinolate complex, the extract is introduced into the flame, and the calcium atomic-absorption is measured. The effects of PI-I, concentration of reagent, and diverse ions on the calcium extraction were investigated.
spectrometry has been applied to the determination of calcium in various kinds of sample. However, difhculty may be encountered in the determination of traces of calcium in phosphoric acid and its salts because of the effect of phosphate on the calcium absorption. For this kind of sample, it was considered preferable to combine the techniques of solvent extraction and atomic absorption, as had been done previously for magnesiuml, and this paper describes the results. ATOMIC-ABSORPTION
EXPERIMENTAL Apparatus A Hitachi photoelectric spectrophotometer EPUGA was used with the atomic-absorption attachment RA-1. An air-acetylene flame was used with a 9-cm fishtail burner. Hitachi and Westinghouse hollow cathode lamps were used as light sources. The operating parameters were: 1.5 kg/cm* Air pressure Acetylene pressure 0.04 kg/cm* Wavelength 4227 A 0.03 mm Slit-width 40 mA (Hitachi), 10 mA (Westinghouse). Lamp current Reagents Calcium solution. Dissolve 0.250 g of calcium carbonate in a minimum excess of hydrochloric acid, and dilute to 100 ml with water. S-Hy&oxyqz&ofine solution. Dissolve 5 g of I-hydroxyquinoline in 100 ml of ethanol. Sodium tartrate solution, 10 % w/v. Saturated sodium chloride s&t&. Bu$ersolution. Mix 1Mpotassium chloride solution and 1Msodium hydroxide solution to give a buffer of pH 13. Reagents were of analytical grade unless otherwise stated.
To 9.80 g of phosphoric acid in a polyethylene beaker, add 1 ml of 6M hydrochloric acid and 20 ml of sodium tartrate solution. Neutralize the solution by slowly adding 6M sodium hydroxide and dilute to 100 ml with water in a standard flask. Place 10 ml of this samule solution in a SO-ml separatory funnel, and add 10 ml of sodium chloride solution. Mix well, and’then add 2 ml of butyl cellosolve, 3 ml of I-hydroxyquinolme solution, and 5 ml of buffer solution. Adjust the volume to 40 ml, add exactly 5 ml of 3-methyl-l-butanol and shake vigorously for 1 min. Drain off and discard the aqueous phase. Spray the organic phase into the flame and measure the calcium absorption. 933
934
SUZUKI and TSUGIO TAKEUCHI
MASAAKI YANAGISAWA, MB~I
For phosphate samples, dissolve a suitable weight (3 g for diammonium hydrogen phosphate and 8.9 g for sodium pyrophosphate) in 2 ml of 1M hydrochloric acid and 20 ml of sodium tartrate solution, and dilute to 100 ml with water. Place 10 ml of the sample solution in a separato funnel and proceed as above. If the pH of the solution is lower than 13, adlust it with sodium hydroxr 7 ebefore extraction. Prepare a calibration curve by taking appropriate volumes of standard calcium solution in 50-ml separatory funnels, adding 10 ml of sodium chloride solution and 2 ml of sodium tartrate solution to each, mix, and apply the procedure. RESULTS
AND
DISCUSSION
Atomic-absorption of calcium The air-fuel ratio had a significant effect on the calcium absorption. The air pressure was set at a constant value (1.5 kg/cm2), and the acetylene pressure was varied. The results are shown in Fig. 1. Although the acetylene pressure which gives maximum
I
LiObl
I
I
003 Amtylcne presurq 002
Air pressure 1.5,
I
,
004 005 kg/cm2
I
kg/cm2
FIG. 1.-Effect of acetylene pressure on calcium absorption Beam height above burner top: x-2.0 mm; 0-3~0 mm; 0-45
mm.
absorption depends on the beam height above the burner-top, the maximum absorption was constant irrespective of the beam height. Extraction of calcium Extraction of calcium hydroxyquinolate has been used for spectrophotometric determinations,2~ and we investigated the use of various solvents such as 3-methyl-lbutanol, 4-methyl-2-pentanone, butyl acetate, I-butanol and chloroform. Of all the solvents tested 3-methyl-1-butanol was found to be best for extraction because it gave a higher sensitivity of measurement. The extraction efficiency was estimated by extracting with successive 5-ml portions of 3-methyl-1-butanol. It was found that 89 % of the calcium was extracted in a single pass, over the range of calcium concentration studied. In the present work, a single extraction was used because it was highly reproducible. Shaking vigorously for longer than a minute did not improve the extraction. Although calcium hydroxyquinolate could be extracted into 3-methyl-lbutanol, the addition of butyl cellosolve resulted in better extraction and repeatability for increasing amounts of calcium. Some workers2n4 have successfully extracted calcium hydroxyquinolate complex with chloroform by using butyl cellosolve as a supplementary solvent. However, chloroform gives an unstable flame and noxious gases during the combustion. Solvents other than esters, ketones and alcohols were not suitable for atomic-absorption measurements because of incomplete combustion and self-absorption of radiation.
Traces of calcium in phosphoric acid
935
The extraction of calcium hydroxyquinolate was examined over a pH range of lOG13.6. The degree of extraction was constant when the pH was greater than 12.5. The effect of reagent concentration on the degree of extraction was also investigated. The degree of extraction was constant if more than 100 mg of 8-hydroxyquinoline were added. Sodium chloride was useful for salting-out and preventing emulsification, the minimum necessary for reproducible results being 5 ml of saturated sodium chloride solution in 40 ml of aqueous phase. Tartrate, which served to prevent the formation of calcium phosphate at higher pH, had no effect on the extraction of calcium when present in concentrations of O*Ol-O.O5M. The calibration curves are almost linear and were corrected for a very smaIl blank value which we attributed to absorption caused by 8-hydroxyquinoline. Various metals form hydroxyquinolate complexes at pH 13, some of which are extractable into 3-methyl-1-butanol. To ascertain the influence of diverse ions on extraction and on the calcium atomioabsorption in the flame, calcium was extracted with 8-hydroxyquinoline from solutions containing diverse ions; Al (489 pug), Ba (1063 rug), Cr (456 pg), Cu (532 Icg), Fe (989 ,&, Mg (1004 cl&, Mn (511 pg), MO (320 ,ug), Ni (964 pg), Pb (801 pg), Si (1554 ,L.&,Sn (654 pug),Sr (1440 rug), Ti (497 pg), W (1000 ,ug) and Zn (1266 ,ug) did not interfere in the determination of 40 pug of calcium. Aluminium and titanium depressed the calcium absorption when an aqueous solution was sprayed into the flame, but no interference was observed in the present method, even though both metals were extracted into the organic phase as hydroxyquinolates. It may be considered that 8-hydroxyquinoline in the extracts counteracts the depressive effect of ahuninium and titanium on the calcium absorption. Larger amount of these metals did cause depression of the calcium absorption. Large amounts of chromium and molybdenum also affected the calcium absorption, but this effect could be controlled by changing the air-fuel ratio, a fuel-rich flame being recommended. The copper complex remained in the aqueous phase as a precipitate and did not dissolve in 3-methyl-1-butanol, but resulted in a lower result for the calcium. A complexing agent such as cyanide was necessary to keep copper in solution. Traces of calcium could be extracted from solutions containing large amounts of phosphate (100 mg of P) without loss. Attempts to use 8-hydroxyquinaldine as complexing agent were unsuccessful. The atomic-absorption of the calcium in the flame was very low, and it was concluded that the calcium was very incompletely extracted into the solvents tried (3-methyl-lbutanol, 4-methyl-2-pentanone, butyl acetate, amyl acetate and I-butanol). Determination of traces of calcium in phosphoric acid and its salts Table I shows the results for some samples of phosphoric acid and phosphates. One sample was analysed five times to evaluate the reproducibility of the method. The mean value was found to be 13.5 ppm and the coefficient of variation was 3.7 %. Recovery of added calcium was excellent. Earlier workersb*6 have described the determination of calcium in phosphoric acid, but the present method appeared to be very convenient for traces of calcium in phosphoric acid and its salts. Application of the method to polypropylene polymers Polypropylene polymers contain titanium, aluminium and phosphorus together with calcium derived from additives or the polymerization catalyst. The present
MAWAKI YANAGISAWA,MA~AMI SUSUKI and TSUGIO TAKEUCHI
936
TABLEI.-DETERMINATION OF TRACESOF CALCIUMIN PHOSPHOIUCACID AND
ITS SALTS
Calcium, ppm Sample Phosphoric
acid A
Phosphoric
acid B
Diammonium hydrogen phosphate
Added
Found
20.0 20.0
13.2, 13.3 32.9 1.6, 1.6 21.5
133.5
15.4, 15.4 149 3.2,3.2 25.6
Sodium pyrophosphate <4
extraction method was applied to the determination of calcium in such samples. For estimation 1 g of sample was ashed in a platinum crucible with O-3 g of sodium carbonate. After addition of 6M hydrochloric acid to acidify the residue, the contents of the crucible were heated with 20 ml of sodium tartrate solution to give a clear solution which was then diluted to 100 ml, and 10 ml of this solution were used for analysis. TABLE II.-DETERMINATION OF CALCIUMIN POLYPROPYLENE WLYMERS Calcium, ppm Polypropylene A B C
The ers can content present
Added 100-O 200.0 200.0
Found 45,45 146 120,120 324 267,267 460
results obtained for some polymers are shown in Table II. Calcium in polymbe determined only with diflkulty by the usual method because of the low of calcium and the presence of titanium, aluminium and phosphorus. The method is convenient for measuring the calcium. R&un&-On a &udiQ une methode de dosage de traces de calcium par spectrometrie d’absorption atomique aprb extraction en solvant organique pour doser des traces de calcium dans l’acide phosphorique et ses sels. On extrait le calcium a l’etat de complexe hydroxyquinoMique, introduit l’extrait darts la flamme et mesure l’absorption atomique du calcium. On a etudie les influences du pH, de la concentration du reactif et de divers ions sur l’extraction du calcium. Zusammenfassung-Ein Verfahren zur Bestimmung von Calciumspuren durch Atomabsorptionsspektrometrie nach Extraktion in ein organisches Losungsmittel wurde zur Bestimmung von Calciumspuren in Phosphorsaure und ihren Salzen geprtift. Calcium wird als Hydroxychinolatkomplex extrahiert, der Extrakt in die Flamme gebracht und die Atomabsorption von Calcium gemessen. Der EinfIuh von pH, Reagenskonzentration und verschiedenen Ionen auf die Extraktion von Calcium wurden untersucht.
1. 2. 3. 4. 5. 6.
REFERENCES M. Suzuki, M. Yanagisawa and T. Takeuchi, Tu[unra, 1965,12,989. F. Urnland and K. U. Meckenstock, 2. Anal. Chem., 1959,X5,161. H. Goto and E. Sudo, Japan Analyst, 1961,10, 171. C. L. Luke, Anal. Chim. Actu, 1965,32,221. D. K. Banerjee, C. C. Budke and F. D. Miller, Anal. Chem., 1962,34,440. I. Johnston and M. Stow, Analyst, 1964, 89,290.