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Quantitative treatment of exchange equilibria involving complexans—III1: chelatometric determination of calcium in presence of magnesium with EGTA
325
Short communications
Quantitative treatment of exchange equilibria involving complexans-III* : Chelatometric determination of calcium in presence of magnesium with EGTA (Received 18 June 1962. Accepted 24 November 1962) THE great difference in stabilities of the EGTA [ethyleneglycol-bis-(~-aminoethylether)-N.W-tetraacetic acid] chelates of calcium and magnesium enables one to titrate calcium selectively in the presence of magnesium. Thus there have been published methods of visual titration of calcium in the presence of magnesium using either the Zn-EGTA-Zincon systemlrp or Calcon* as indicator. In this paper the possible use of the Zn-EGTA-PANT system as indicator in the selective chelatometry of calcium is reported. The method seems to be better than the earlier methods. RESULTS
AND
DISCUSSION
When the NY-HA system is used as indicator for the chelatometry of metal M, the pH and concentrations of NY and HAS are critical for the indicator transition. If the constants of the equilibria involved are all available, the end-point can be quantitatively accounted for by equation (27) of our previous paper,’ which is reproduced below :
WJ,“/[Ml, = 1 -
s
-j&j +$& *K+, 1
t
where K1 = Kr,rYKNb,/KNY#&) and K, = KN~,arhY~/KHY$, and [Ml, and [Ylt” denote the total concentration of M and of complexan used as titrant, respectively. Because Zn(PAN), participates in the titration, equation (1) can be rewritten as: w],“/[M],
PI
= 1 - [NA’l [Al ’ K 1&Yl[A] ‘- [NA]8 *
K,[NYl[Al [Ml, ’
(2)
in which the term lNA,]/[A] is a function of the indicator transition. Studying the effects of pH, concentrations of Zn-EGTA, PAN and magnesium using equation (2), the results are represented as photometric curves in Figs. 1-3. Taking into account these results and the fact that the indicator transition is somewhat irreversible because of the low solubility of Zn(PAN),, a procedure is established in which an excess of EGTA is titrated with calcium standard solution. As seen from Fig. 3, the present method allows the visual titration of calcium in a sample of which the ratio Mg:Ca does not exceed 10. For a higher proportion of magnesium, however, photometric titration is recommended. EXPERIMENTAL Solutions lo-=M EGTA standard solution: 3.8036 g of EGTA (Dojindo Co., Kumamoto, Japan) are dissolved in 20 ml of IM sodium hydroxide solution and the whole is diluted to 1 litre. The solution is standardised chelatometrically with 10AZM zinc standard solution using Erio T as indicator. lo-=M Calcium standard solution: 1WOO g of calcium carbonate is dissolved in a small amount of hydrochloric acid, of which the excess is expelled completely by evaporation to dryness on a steam bath. The residue is taken up in distilled water and diluted to 1 litre. The solution is standard&d chelatometrically with EDTA standard solution. 10-SM Zn-EGTA solution: An equivolume mixture of lo-=M EGTA solution and IO-=M zinc solution is diluted five fold. lo-‘M PAN solution: 25 mg of PAN (Dojindo Co., Kumamoto, Japan) are dissolved in 100 ml of methyl alcohol. Buffer solution (pH 8): 1M hydrochloric acid is added to a saturated solution of borax until the pH of the solution is brought to 8.0. Apparatus Filter photometer
and conventional
titration cell (Hirama Co., Tokyo, Japan).
* Part II: see reference 4. t PAN = l-(2-pyridylazo)-2-naphthol. $ If only the formation of NA has to be taken into consideration, [HA] does not influence the indicator transition as long as [HA] is sufficiently low. 8 Substituting aEfy) by an,nmy), the influence of a second metal (Mu) can be accounted for by means of the concept of “ligand buffer”.6
326
Short communications 0
0
FQG. l-Effect
([EGTA],“:
of pH on the titration of EGTA with calcium standard solution: 1. pH 7.0, 3. pH 9.0, 2. pH 8.0, 4. pH 10.0. lo-*M; FAN]: 2 x 10mSM; [Zn-EGTA]: 4 x lo-6M; 562 m,u)
-0.5 0.4 -
F .
E 3 b 0.3 0" a
0.2 -
0.1 -
1
0
05
I ka] ,;gmi,;
2-Effect of concentration of Zn-EGTA : 1. [Zn-EGTA] : 4 x lO+M, 2. [Zn-EGTA]: 8 x lO+M, 3. [Zn-EGTA]: 2.5-S x lO+M. ([EGTA],” : 10-aM; [PAN]: 2 x lO+M; pH: 8.0; 562 rnp) FIN.
327
Short communications
FIG. 3-Effect
([EGTA],“:
10-S&f; [PAN]:
of concentration of magnesium: 1. [Mg”+]: None, 2. [Mga+]: lo-*M, 3. [Mg2+]: 10-2M. 2 x lO+M; [Zn-EGTA] = 25 x lo-*M;
pH:
8.0; 562 w)
Recommended procedure 50 ml of buffer solution (pH 8) and 5-20 ml of lO-*M EGTA solution* are added to 25 ml of neutral sample solution containing l-4 mg of calcium and less than 10 mg of magnesimnt After the addition of 25-3 ml of lo-*It4 Zn-EGTA solution and 2 ml of 10-aM PAN solution, the excess EGTA is titrated with 10FM calchnn standard solution. Near the end-point, the solution is heated to about 50-60” and the titration is continued drop by drop under vigorous stirring. A colour change from pale yellow to pink is observed at the end-point. Titrate to a pink with no yellowish shade. Laboratory of Analytical Chemistry Nagoya Institute of Technology Showa-ku, Nagoya, Japan
GENKICHI NAKAGAWA HIROKO WADA
Laboratory of Ana!vtical Chemistry Faculty of Science Nagoya University Chikusa-ku, Nagoya, Japan
MOT~HARU TANAKA
Snnumuy-A method of selective chelatometry of calcium in the presence of magnesium is established using the Zn-EGTA-PAN system as indicator. Effects of uH and concentrations of Zn-EGTA. PAN and magnesium are studied by means of photometric titration curves. The method allows the visual titration of calcium in a sample of which the ratio Mg:Ca does not exceed 10. * Total concentration of EGTA to be added is preferably more than 1.5 times calcium or 0.5 magnesium in the sample solution. i By means of the photometric titration, the presence of about 100 mg of magnesium is tolerable.
328
Short communications Zusammenfassung-Eine Methode zur selektiven Bestimmung von Calcium in Gegenwart von Magnesium wurde ausgearbeitet. Das System Zink-AeGTE-PAN dient zur Anzeige des Endpunktes. Der Einfluss von pH, und der Konzentrationen des Indicatorsystems sowie des Magnesiums wurde mittels photometrischer Titrationskurven studiert. Die Methode gestattet die visuelle Bestimmung von Calcium in Proben, in denen das Verhlltnis Mg:Oa den Wert 10 nicht tiberschreitet. R&m&--Gn etablit une methode de dosage chelatometrique selective du calcium en presence du magnesium employant le syst6me ZnEGTA-PAN comme indicateur. On Ctudie les effets de pH et des concentrations de Zn-EGTA, PAN et de magnMum au moyen des courbes de titrage photometrique. Cette methode permet le titrage visuel du calcium contenu dam un echantillon dont Mg/Ca n’exc&de pas 10. REFERENCES
1 A. Ringbom, G. Pensar and E. Wanninen, Adyt. Chim. Acta, 1958,19,52X * F. S. Sadek, R. W. Schmid and C. N. Reilley, Tafanta, 1959, 2, 38. 8 R. A. Burg and H. F. Conaghan, Chemist-Analyst, 1959,48, 13. 4 G. Nakagawa and M. Tanaka, Talantq, !962,9,917. 6 M. Tanaka, Adyt. Chim. Acta, submitted for publication.
Short communications
Quantitative treatment of exchange equilibria involving complexans-III* : Chelatometric determination of calcium in presence of magnesium with EGTA (Received 18 June 1962. Accepted 24 November 1962) THE great difference in stabilities of the EGTA [ethyleneglycol-bis-(~-aminoethylether)-N.W-tetraacetic acid] chelates of calcium and magnesium enables one to titrate calcium selectively in the presence of magnesium. Thus there have been published methods of visual titration of calcium in the presence of magnesium using either the Zn-EGTA-Zincon systemlrp or Calcon* as indicator. In this paper the possible use of the Zn-EGTA-PANT system as indicator in the selective chelatometry of calcium is reported. The method seems to be better than the earlier methods. RESULTS
AND
DISCUSSION
When the NY-HA system is used as indicator for the chelatometry of metal M, the pH and concentrations of NY and HAS are critical for the indicator transition. If the constants of the equilibria involved are all available, the end-point can be quantitatively accounted for by equation (27) of our previous paper,’ which is reproduced below :
WJ,“/[Ml, = 1 -
s
-j&j +$& *K+, 1
t
where K1 = Kr,rYKNb,/KNY#&) and K, = KN~,arhY~/KHY$, and [Ml, and [Ylt” denote the total concentration of M and of complexan used as titrant, respectively. Because Zn(PAN), participates in the titration, equation (1) can be rewritten as: w],“/[M],
PI
= 1 - [NA’l [Al ’ K 1&Yl[A] ‘- [NA]8 *
K,[NYl[Al [Ml, ’
(2)
in which the term lNA,]/[A] is a function of the indicator transition. Studying the effects of pH, concentrations of Zn-EGTA, PAN and magnesium using equation (2), the results are represented as photometric curves in Figs. 1-3. Taking into account these results and the fact that the indicator transition is somewhat irreversible because of the low solubility of Zn(PAN),, a procedure is established in which an excess of EGTA is titrated with calcium standard solution. As seen from Fig. 3, the present method allows the visual titration of calcium in a sample of which the ratio Mg:Ca does not exceed 10. For a higher proportion of magnesium, however, photometric titration is recommended. EXPERIMENTAL Solutions lo-=M EGTA standard solution: 3.8036 g of EGTA (Dojindo Co., Kumamoto, Japan) are dissolved in 20 ml of IM sodium hydroxide solution and the whole is diluted to 1 litre. The solution is standardised chelatometrically with 10AZM zinc standard solution using Erio T as indicator. lo-=M Calcium standard solution: 1WOO g of calcium carbonate is dissolved in a small amount of hydrochloric acid, of which the excess is expelled completely by evaporation to dryness on a steam bath. The residue is taken up in distilled water and diluted to 1 litre. The solution is standard&d chelatometrically with EDTA standard solution. 10-SM Zn-EGTA solution: An equivolume mixture of lo-=M EGTA solution and IO-=M zinc solution is diluted five fold. lo-‘M PAN solution: 25 mg of PAN (Dojindo Co., Kumamoto, Japan) are dissolved in 100 ml of methyl alcohol. Buffer solution (pH 8): 1M hydrochloric acid is added to a saturated solution of borax until the pH of the solution is brought to 8.0. Apparatus Filter photometer
and conventional
titration cell (Hirama Co., Tokyo, Japan).
* Part II: see reference 4. t PAN = l-(2-pyridylazo)-2-naphthol. $ If only the formation of NA has to be taken into consideration, [HA] does not influence the indicator transition as long as [HA] is sufficiently low. 8 Substituting aEfy) by an,nmy), the influence of a second metal (Mu) can be accounted for by means of the concept of “ligand buffer”.6
326
Short communications 0
0
FQG. l-Effect
([EGTA],“:
of pH on the titration of EGTA with calcium standard solution: 1. pH 7.0, 3. pH 9.0, 2. pH 8.0, 4. pH 10.0. lo-*M; FAN]: 2 x 10mSM; [Zn-EGTA]: 4 x lo-6M; 562 m,u)
-0.5 0.4 -
F .
E 3 b 0.3 0" a
0.2 -
0.1 -
1
0
05
I ka] ,;gmi,;
2-Effect of concentration of Zn-EGTA : 1. [Zn-EGTA] : 4 x lO+M, 2. [Zn-EGTA]: 8 x lO+M, 3. [Zn-EGTA]: 2.5-S x lO+M. ([EGTA],” : 10-aM; [PAN]: 2 x lO+M; pH: 8.0; 562 rnp) FIN.
327
Short communications
FIG. 3-Effect
([EGTA],“:
10-S&f; [PAN]:
of concentration of magnesium: 1. [Mg”+]: None, 2. [Mga+]: lo-*M, 3. [Mg2+]: 10-2M. 2 x lO+M; [Zn-EGTA] = 25 x lo-*M;
pH:
8.0; 562 w)
Recommended procedure 50 ml of buffer solution (pH 8) and 5-20 ml of lO-*M EGTA solution* are added to 25 ml of neutral sample solution containing l-4 mg of calcium and less than 10 mg of magnesimnt After the addition of 25-3 ml of lo-*It4 Zn-EGTA solution and 2 ml of 10-aM PAN solution, the excess EGTA is titrated with 10FM calchnn standard solution. Near the end-point, the solution is heated to about 50-60” and the titration is continued drop by drop under vigorous stirring. A colour change from pale yellow to pink is observed at the end-point. Titrate to a pink with no yellowish shade. Laboratory of Analytical Chemistry Nagoya Institute of Technology Showa-ku, Nagoya, Japan
GENKICHI NAKAGAWA HIROKO WADA
Laboratory of Ana!vtical Chemistry Faculty of Science Nagoya University Chikusa-ku, Nagoya, Japan
MOT~HARU TANAKA
Snnumuy-A method of selective chelatometry of calcium in the presence of magnesium is established using the Zn-EGTA-PAN system as indicator. Effects of uH and concentrations of Zn-EGTA. PAN and magnesium are studied by means of photometric titration curves. The method allows the visual titration of calcium in a sample of which the ratio Mg:Ca does not exceed 10. * Total concentration of EGTA to be added is preferably more than 1.5 times calcium or 0.5 magnesium in the sample solution. i By means of the photometric titration, the presence of about 100 mg of magnesium is tolerable.
328
Short communications Zusammenfassung-Eine Methode zur selektiven Bestimmung von Calcium in Gegenwart von Magnesium wurde ausgearbeitet. Das System Zink-AeGTE-PAN dient zur Anzeige des Endpunktes. Der Einfluss von pH, und der Konzentrationen des Indicatorsystems sowie des Magnesiums wurde mittels photometrischer Titrationskurven studiert. Die Methode gestattet die visuelle Bestimmung von Calcium in Proben, in denen das Verhlltnis Mg:Oa den Wert 10 nicht tiberschreitet. R&m&--Gn etablit une methode de dosage chelatometrique selective du calcium en presence du magnesium employant le syst6me ZnEGTA-PAN comme indicateur. On Ctudie les effets de pH et des concentrations de Zn-EGTA, PAN et de magnMum au moyen des courbes de titrage photometrique. Cette methode permet le titrage visuel du calcium contenu dam un echantillon dont Mg/Ca n’exc&de pas 10. REFERENCES
1 A. Ringbom, G. Pensar and E. Wanninen, Adyt. Chim. Acta, 1958,19,52X * F. S. Sadek, R. W. Schmid and C. N. Reilley, Tafanta, 1959, 2, 38. 8 R. A. Burg and H. F. Conaghan, Chemist-Analyst, 1959,48, 13. 4 G. Nakagawa and M. Tanaka, Talantq, !962,9,917. 6 M. Tanaka, Adyt. Chim. Acta, submitted for publication.