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Amperometric complex-formation titration of traces of alkaline earths
Short communications
1006
titration with silver in the amperometric methods:*’ in which positive errors were attributed to reactions with groups other than tbiol. Since the electrode used in the present study responds directly to the thiol level, it is apparent from the results in Table II that silver gives correct results. Department of Radiation BioIogy and Biophysics University of Rochester School of Medicine and Dentistry Rochester, New York 14620, U.S.A.
T~pr Y. TOIUBARA hRYSA KOVAL
Summary-A silver wire immersed in a thiol solution gives a potential responsive to the thiol concentration, and is a sensitive indicator electrode in the potentiometric titration of thiols with mercury(H) chloride, p-chloromercuryphenyl sulphonate, and silver nitrate at pH 4%95,7-9.5 and 95 respectively. Titrations of simple thiols such as cysteine or a protein such as albumin are equally successful, but the potential break was smaller for the protein. The end-point could be determined within an increment of titrant equal to 5 nmole of thiol. An inert atmosphere is needed for titration at pH > 7. Zusammenfassuug-Ein in eine Thiolliisung tauchender Silberdraht aibt em Potential. das ein Ma8 filr die Thiolkonzentration darstellt. ber Draht ist eine empfindliche Indikatorelektrode bei der potentiometrischen Titration von Thiolen mit Quecksilber(H)chlorid, pChlormercuriphenylsulfonat und Silbernitrat bei pH 4,5-9,5: 7-9,5 bnv. 9.5. Titrationen einfacher Thiole wie Cvstein oder elnes Proteins wie Albumin sind gleichermahen moglich; her Potentialsprung war iedoch beim Protein klemer. Der Endnunkt lien sich hmerhalb einer iugabe von Titrant feststellen, die $ nMol Thiol entspricht. Bei Titrationen bei pH > 7 benatigt man eine inerte Atmosphere. R&mm&Un fll d’argent immergd dans une solution de thiol donne un potentiel sensible B a concentration en thiol, et est une electrode indicatrice sensible dans le titrage potentiometrique des thiols par le chlorure de mercure(II), le p-chloromercuriphenyl sulfonate et le nitrate d’argent il pH 4.5-9.5, 7-9,5 et 9,5 respectivement. Les titrages de thiols simples tels que la cyst&me ou d’une proteine telle que l’albumine sont egalement couronnes de suc&, mais la cassure de potentiel est plus petite pour la proteine. Le point de tin de dosage peut &tre d&ermine avec un increment d’agent de titrage &gal ii 5 nmole de thiol. Une atmosphere merte est necessaire pour le titrage a pH > 7. REFERENCES 1. 2. 3. 4.
R. Benesch and R. E. Benesch, Arch. Biochem., 1948,19,35. I. M. Kolthoff, W. S. Shore, B. H. Tan and M. Matsuoka, Anal. Biochem., 1965,12,497. R. Cecil and J. R. McPhee, Biochem. J., 1955,59,234. A. C. Allison and R. Cecil, ibid., 1958,69,27.
Talmta, 1970, Vol. 17, pp. 1006 to 1009. Perpamon Press. Printed in Northern Ireland
Amperometric
complex-formation
titration
of traces of alkaline earths
(Received 17 February 1970. Accepted 17 April 1970) IN A PREVIOUS communication1 the principles of the amperometric complex-formation titration of metal ions with indication by following the anodic wave of the excess of ligand, have been presented. The work of Campbell and Reilley,* who used a dropping mercury electrode, could be extended to a higher sensitivity by using a rotating mercury electrode as the indicator electrode. The criterion for a sharp end-point of a complexometric titration curve appeared to be
Short
1007
communications
where K’ is the conditional stability constant of the metal complex involved and c is the original concemration of the metal ion. When errors of 1% or slightly more are tolerable values of K’c of l(r or even 10” can be used. In the present paper the determination of the alkaline earth metals with EDTA, EGTA and DTPA as chelating agents* is described. A prediction of the limit of determination can easily be made by means of the sharpness criterion mentioned above. In Table I the conditional stability constants TABM I.--vALuBs Metal
EDTA
OF log
K&=
EGTA
AT
pH 10 DTPA
ca Mg Sr Ra of the various complexes are given for a pH-value of 19. The Table shows that the determination of the alkaline earths should be possible at the 10-5M-concentration level, when the appropriate chelatin agent is used, Le., any of the three for calcium, EDTA or DTPA for magnesium, DTPA or EGTA Bor strontium or barium. When errors of about 1% are admissible determinations at the lO-a&f-level should be possible. As the titration cell is constructed for determinations in a volume of 10 ml the minimum amount of these metals that can be determined should be about 10 nmole, which corresponds to about @2-14 pg, depending on the metal. EXPERIMENTAL Apparatus The titration cell has a volume of 10 ml. It is connected by means of fritted glass discs on the one
side with a saturated calomel electrode and on the other side with a compartment containing an auxiliary tungsten electrode. All three compartments of the titration cell are fIlled with the same supporting electrolyte solution. The rotating mercury electrode (600 rpm) used as the indicator electrode was described in the previous paper. The Reckmann Electroscan 30 supplied the voltage for the cell and also served for the measurement of the current. Titrations were carried out by adding the titrant solution from a l-ml syringe microburette. Selection of the potential
The selection of the potential to be applied to the indicator electrode is very important. The most favourable potential depends on the concentration of the metal ion to be titrated and on the condition of the electrode. The correct selection of the potential can be made by recording polarograms with the indicator electrode of solutions containing the supporting electrolyte solution and small but different amounts of the titrant. Reagents
Potassium nitrate (10 g) and ammonium nitrate (8 g) were dissolved in 1 litre of distilled water. The pH was adjusted to 10 by addition of potassium hydroxide to give a O*lM buffer solution. A O+OlM buffer solution was prepared in a similar way. Stock solutions of the alkaline-earth metals were prepared from the chemically pure nitrates or sulphates and were standardized complexometrically. Solutions of EDTA, DTPA, and EGTA were standardized against calcium carbonate. Procedure The sample, containing l-100 pg of the alkaline earth metal, is dissolved in 10 ml of 04OlM0.1 M buffer solution. The solution is put in the titration cell and titrated with a lo-“-l O-aM solution
of the appropriate complexing agent; the titration graph is recorded on the Electroscan. In order to avoid too large an i&drop across the cell, the titration is stopped when a 20% excess of the titrant has been added. The end-point is obtained as the point of intersection of the two straight parts of the curve. l
EDTA = ethylenediaminetetra-acetic acid. EGTA = ethyleneglycol bis(&rnhroethyl ether)-iV,N,N’,N’-tetra-acetic acid. DTPA = diethylenetriaminepenta-acetic acid.
Short communications
1008
RESULTS In Table II some typical results are given for determinations in the NO-,q range, and in Table III for the l+g range. It can be seen from Table III that 1O-o&fsolutionsof the alkaline earth metals can be titrated with fair accuracy. The time required for a titration is between 3 and 5 min. TABLEIL-D
I~TBWINA~ONS OF ALKALWS EARTH MSTALS IN THE lOI%@ RANGE AT pH 10
taken
found
487 Mg 48-7 Mg 59.3 Ca 79.0 Ca 128.8 Sr 128.8 Sr 128.8 Sr 2697 Ba 202.4 Ba
47.7 47.8 58-8 77.1 128.4 1270 128.8 261.3 201.3
TN=
Error, %
Std. devn., % (no. of detns.)
1;
0.6 (4) 0.5
-O+I -2.2 -0.3 -1.4 -3
:;. g; 200 2.2 (5) 5.2 1.6 (4) (5)
-05
37 (S)
Titrant EDTA DTPA EGTA DTPA EDTA EGTA DTPA EGTA DTPA
TION OF THE ALKALINE EARTH METALS IN THE 1-,Ug RANGE AT PH 10
~.-~
Met& iug
Errof,
Std. de-m., % (5 variates)
taken
found
%
@365 Mg t-J=; ES
&371 0.374 0.416 0.830 160 1.29 l-37 2.04 2.05
+1.6 +24 $5 -t-7 -!-l-2 +6
;:; 1.3 3.5 20
::.5
;:;
0.790 ca 1.58 ca 1.29 Sr 1.29 Sr 2.02 Ba 202 Ba
4.1 2.3
T&rant DTPA EDTA EDTA EDTA EDTA EGTA DTPA EGTA DTPA
Determittath of calcium in the presence of magnesium The investigations have been extended to incIude the determination of calcium with EGTA in the
presence of magnesinm. The maximum admissible concentration of magnesium can be found from the sensitivity condition mentioned in the previous paper [equation (W)]:’ &Qf,
< (x - 1)
where n is the factor by which the sensitivity may be reduced. The determination of 1.5 x lo-@M calcium has been carried out in the presence of a lOO-foldand a lOOO-foldamount of magneshrm. A considerable decrease in the slope of the titration curve after the equivalence point and thus a considerable decrease in sensitivity was to be expected in these cases. The experimental results agree reasonably with the predictions, but as the decrease of linearity observed was smaher than predicted, TABLBN.-DETERMINATIONS
OF CAL~M wrm PRESENCE OF MACmESIUM
Calcium, fig present
found
Magnesium W
0.596 0,596
0.581 0.589
37.6 364
EGTA
IN
THE
Error, %
Std. devn., % (no. of detns.)
-2.5 -1-2
58 (6) 49 (9)
1009
Short communications
the experiments turned out to be better than expected. An exact agreement could not be expected as the stability constants used in the calculations were determined under conditions other than the actual experimental conditions. The analytical results are given in Table IV. Although a great number of complexometric titrations of the alkaline earth metals have been described, the amperometric indication of the excess of ligand with a rotating mercury electrode would seem to offer improvements in sensitivity and speed. Only a few minutes are required for each titration. L&oratory for Analytical Chemistry University of Amsterdam Nieuwe Achtergracht 125 Amsterdam-C
G.
DEN Bow
F. Fna~sa M. M. P. F. KRAMBR H. POPPE
Summary-Alkaline earth metals were determined in microgram ouantities bv comulexometric titration with EDTA. EGTA and DTPA. ?he~end@int w’s_ detected by following the anodic wave of the chelating agent at the rotating mercury electrode. All the alkaline earths can be titrated at the microgram level with reasonable accuracy, and calcium may be titrated with EGTA in the presence of a lOOO-fold excess of magnesium. R&m&-Gn a dose les m&aux alcalino-terreux en quantites de l’ordre du microgramme par titrage complexom&rique avec EDTA, EGTA et DTPA. Le point de fin de dosage est detect6 en suivant la vague anodique de l’agent chelatant sur lWctrode de mercure rotative. Tous les alcalino-terreux peuvent &e tids a Whelle du microgramme avec une precision raisonnable, et l’on peut titrer le calcium a I’EGTA en la presence d’un exc+s 1000 fois plus grand de magnesium. Zusamme&ssung-Erdalkalimetalle wurden in Mikrogrammengen durch komplexometrische Titration mit EDTA, EGTA und DTPA bestimmt. Der Endpunkt wurde durch Beobachtung der anodischen Stufe des Chelatbildners an der rotierenden Quecksilberelektrode ermittelt. Alle Erdalkalien lassen sich im Mikrogrammbereich nau bestimmen; Calcium kann in Gegenwart eines ausreichend lOOO-fachen&e rschusses von Magnesium mit EGTA titriert werden. REFERENCES r and G. den Boef, Talanta, 1970,17,945. 1. F. Freese. H. J. J 2. R. T. Campbell ana?e C. N. Reilley, ibid., 1%2,9,153.
Talanta,1970,Vol.17, pp. 1009 to
1013. Pcrxamon Press. Printed in Northern Ireland
A microwave-excited emissive detector for gas chromatography Further studies with sulphur compomals (Receiued 19 March 1970. Accepted l’2 April 1970)
Tms communication describes some improvements to a microwave-excited emissive detector for gas chromatographic detection of some sulphur compounds. The system previously described’ has been modified by restricting the volume occupied by the discharge, using a catalyst to aid the thermal degradation of the sulphur compounds, using all of the sample eluted from the gas chromatograph, and stabilizing the vacuum in the detector tube. These modifications have led to an overall improvement in this type of detection technique and have yielded superior detection limits, particularly for those sulphur compounds of relatively low volatility or high thermal stability.
1006
titration with silver in the amperometric methods:*’ in which positive errors were attributed to reactions with groups other than tbiol. Since the electrode used in the present study responds directly to the thiol level, it is apparent from the results in Table II that silver gives correct results. Department of Radiation BioIogy and Biophysics University of Rochester School of Medicine and Dentistry Rochester, New York 14620, U.S.A.
T~pr Y. TOIUBARA hRYSA KOVAL
Summary-A silver wire immersed in a thiol solution gives a potential responsive to the thiol concentration, and is a sensitive indicator electrode in the potentiometric titration of thiols with mercury(H) chloride, p-chloromercuryphenyl sulphonate, and silver nitrate at pH 4%95,7-9.5 and 95 respectively. Titrations of simple thiols such as cysteine or a protein such as albumin are equally successful, but the potential break was smaller for the protein. The end-point could be determined within an increment of titrant equal to 5 nmole of thiol. An inert atmosphere is needed for titration at pH > 7. Zusammenfassuug-Ein in eine Thiolliisung tauchender Silberdraht aibt em Potential. das ein Ma8 filr die Thiolkonzentration darstellt. ber Draht ist eine empfindliche Indikatorelektrode bei der potentiometrischen Titration von Thiolen mit Quecksilber(H)chlorid, pChlormercuriphenylsulfonat und Silbernitrat bei pH 4,5-9,5: 7-9,5 bnv. 9.5. Titrationen einfacher Thiole wie Cvstein oder elnes Proteins wie Albumin sind gleichermahen moglich; her Potentialsprung war iedoch beim Protein klemer. Der Endnunkt lien sich hmerhalb einer iugabe von Titrant feststellen, die $ nMol Thiol entspricht. Bei Titrationen bei pH > 7 benatigt man eine inerte Atmosphere. R&mm&Un fll d’argent immergd dans une solution de thiol donne un potentiel sensible B a concentration en thiol, et est une electrode indicatrice sensible dans le titrage potentiometrique des thiols par le chlorure de mercure(II), le p-chloromercuriphenyl sulfonate et le nitrate d’argent il pH 4.5-9.5, 7-9,5 et 9,5 respectivement. Les titrages de thiols simples tels que la cyst&me ou d’une proteine telle que l’albumine sont egalement couronnes de suc&, mais la cassure de potentiel est plus petite pour la proteine. Le point de tin de dosage peut &tre d&ermine avec un increment d’agent de titrage &gal ii 5 nmole de thiol. Une atmosphere merte est necessaire pour le titrage a pH > 7. REFERENCES 1. 2. 3. 4.
R. Benesch and R. E. Benesch, Arch. Biochem., 1948,19,35. I. M. Kolthoff, W. S. Shore, B. H. Tan and M. Matsuoka, Anal. Biochem., 1965,12,497. R. Cecil and J. R. McPhee, Biochem. J., 1955,59,234. A. C. Allison and R. Cecil, ibid., 1958,69,27.
Talmta, 1970, Vol. 17, pp. 1006 to 1009. Perpamon Press. Printed in Northern Ireland
Amperometric
complex-formation
titration
of traces of alkaline earths
(Received 17 February 1970. Accepted 17 April 1970) IN A PREVIOUS communication1 the principles of the amperometric complex-formation titration of metal ions with indication by following the anodic wave of the excess of ligand, have been presented. The work of Campbell and Reilley,* who used a dropping mercury electrode, could be extended to a higher sensitivity by using a rotating mercury electrode as the indicator electrode. The criterion for a sharp end-point of a complexometric titration curve appeared to be
Short
1007
communications
where K’ is the conditional stability constant of the metal complex involved and c is the original concemration of the metal ion. When errors of 1% or slightly more are tolerable values of K’c of l(r or even 10” can be used. In the present paper the determination of the alkaline earth metals with EDTA, EGTA and DTPA as chelating agents* is described. A prediction of the limit of determination can easily be made by means of the sharpness criterion mentioned above. In Table I the conditional stability constants TABM I.--vALuBs Metal
EDTA
OF log
K&=
EGTA
AT
pH 10 DTPA
ca Mg Sr Ra of the various complexes are given for a pH-value of 19. The Table shows that the determination of the alkaline earths should be possible at the 10-5M-concentration level, when the appropriate chelatin agent is used, Le., any of the three for calcium, EDTA or DTPA for magnesium, DTPA or EGTA Bor strontium or barium. When errors of about 1% are admissible determinations at the lO-a&f-level should be possible. As the titration cell is constructed for determinations in a volume of 10 ml the minimum amount of these metals that can be determined should be about 10 nmole, which corresponds to about @2-14 pg, depending on the metal. EXPERIMENTAL Apparatus The titration cell has a volume of 10 ml. It is connected by means of fritted glass discs on the one
side with a saturated calomel electrode and on the other side with a compartment containing an auxiliary tungsten electrode. All three compartments of the titration cell are fIlled with the same supporting electrolyte solution. The rotating mercury electrode (600 rpm) used as the indicator electrode was described in the previous paper. The Reckmann Electroscan 30 supplied the voltage for the cell and also served for the measurement of the current. Titrations were carried out by adding the titrant solution from a l-ml syringe microburette. Selection of the potential
The selection of the potential to be applied to the indicator electrode is very important. The most favourable potential depends on the concentration of the metal ion to be titrated and on the condition of the electrode. The correct selection of the potential can be made by recording polarograms with the indicator electrode of solutions containing the supporting electrolyte solution and small but different amounts of the titrant. Reagents
Potassium nitrate (10 g) and ammonium nitrate (8 g) were dissolved in 1 litre of distilled water. The pH was adjusted to 10 by addition of potassium hydroxide to give a O*lM buffer solution. A O+OlM buffer solution was prepared in a similar way. Stock solutions of the alkaline-earth metals were prepared from the chemically pure nitrates or sulphates and were standardized complexometrically. Solutions of EDTA, DTPA, and EGTA were standardized against calcium carbonate. Procedure The sample, containing l-100 pg of the alkaline earth metal, is dissolved in 10 ml of 04OlM0.1 M buffer solution. The solution is put in the titration cell and titrated with a lo-“-l O-aM solution
of the appropriate complexing agent; the titration graph is recorded on the Electroscan. In order to avoid too large an i&drop across the cell, the titration is stopped when a 20% excess of the titrant has been added. The end-point is obtained as the point of intersection of the two straight parts of the curve. l
EDTA = ethylenediaminetetra-acetic acid. EGTA = ethyleneglycol bis(&rnhroethyl ether)-iV,N,N’,N’-tetra-acetic acid. DTPA = diethylenetriaminepenta-acetic acid.
Short communications
1008
RESULTS In Table II some typical results are given for determinations in the NO-,q range, and in Table III for the l+g range. It can be seen from Table III that 1O-o&fsolutionsof the alkaline earth metals can be titrated with fair accuracy. The time required for a titration is between 3 and 5 min. TABLEIL-D
I~TBWINA~ONS OF ALKALWS EARTH MSTALS IN THE lOI%@ RANGE AT pH 10
taken
found
487 Mg 48-7 Mg 59.3 Ca 79.0 Ca 128.8 Sr 128.8 Sr 128.8 Sr 2697 Ba 202.4 Ba
47.7 47.8 58-8 77.1 128.4 1270 128.8 261.3 201.3
TN=
Error, %
Std. devn., % (no. of detns.)
1;
0.6 (4) 0.5
-O+I -2.2 -0.3 -1.4 -3
:;. g; 200 2.2 (5) 5.2 1.6 (4) (5)
-05
37 (S)
Titrant EDTA DTPA EGTA DTPA EDTA EGTA DTPA EGTA DTPA
TION OF THE ALKALINE EARTH METALS IN THE 1-,Ug RANGE AT PH 10
~.-~
Met& iug
Errof,
Std. de-m., % (5 variates)
taken
found
%
@365 Mg t-J=; ES
&371 0.374 0.416 0.830 160 1.29 l-37 2.04 2.05
+1.6 +24 $5 -t-7 -!-l-2 +6
;:; 1.3 3.5 20
::.5
;:;
0.790 ca 1.58 ca 1.29 Sr 1.29 Sr 2.02 Ba 202 Ba
4.1 2.3
T&rant DTPA EDTA EDTA EDTA EDTA EGTA DTPA EGTA DTPA
Determittath of calcium in the presence of magnesium The investigations have been extended to incIude the determination of calcium with EGTA in the
presence of magnesinm. The maximum admissible concentration of magnesium can be found from the sensitivity condition mentioned in the previous paper [equation (W)]:’ &Qf,
< (x - 1)
where n is the factor by which the sensitivity may be reduced. The determination of 1.5 x lo-@M calcium has been carried out in the presence of a lOO-foldand a lOOO-foldamount of magneshrm. A considerable decrease in the slope of the titration curve after the equivalence point and thus a considerable decrease in sensitivity was to be expected in these cases. The experimental results agree reasonably with the predictions, but as the decrease of linearity observed was smaher than predicted, TABLBN.-DETERMINATIONS
OF CAL~M wrm PRESENCE OF MACmESIUM
Calcium, fig present
found
Magnesium W
0.596 0,596
0.581 0.589
37.6 364
EGTA
IN
THE
Error, %
Std. devn., % (no. of detns.)
-2.5 -1-2
58 (6) 49 (9)
1009
Short communications
the experiments turned out to be better than expected. An exact agreement could not be expected as the stability constants used in the calculations were determined under conditions other than the actual experimental conditions. The analytical results are given in Table IV. Although a great number of complexometric titrations of the alkaline earth metals have been described, the amperometric indication of the excess of ligand with a rotating mercury electrode would seem to offer improvements in sensitivity and speed. Only a few minutes are required for each titration. L&oratory for Analytical Chemistry University of Amsterdam Nieuwe Achtergracht 125 Amsterdam-C
G.
DEN Bow
F. Fna~sa M. M. P. F. KRAMBR H. POPPE
Summary-Alkaline earth metals were determined in microgram ouantities bv comulexometric titration with EDTA. EGTA and DTPA. ?he~end@int w’s_ detected by following the anodic wave of the chelating agent at the rotating mercury electrode. All the alkaline earths can be titrated at the microgram level with reasonable accuracy, and calcium may be titrated with EGTA in the presence of a lOOO-fold excess of magnesium. R&m&-Gn a dose les m&aux alcalino-terreux en quantites de l’ordre du microgramme par titrage complexom&rique avec EDTA, EGTA et DTPA. Le point de fin de dosage est detect6 en suivant la vague anodique de l’agent chelatant sur lWctrode de mercure rotative. Tous les alcalino-terreux peuvent &e tids a Whelle du microgramme avec une precision raisonnable, et l’on peut titrer le calcium a I’EGTA en la presence d’un exc+s 1000 fois plus grand de magnesium. Zusamme&ssung-Erdalkalimetalle wurden in Mikrogrammengen durch komplexometrische Titration mit EDTA, EGTA und DTPA bestimmt. Der Endpunkt wurde durch Beobachtung der anodischen Stufe des Chelatbildners an der rotierenden Quecksilberelektrode ermittelt. Alle Erdalkalien lassen sich im Mikrogrammbereich nau bestimmen; Calcium kann in Gegenwart eines ausreichend lOOO-fachen&e rschusses von Magnesium mit EGTA titriert werden. REFERENCES r and G. den Boef, Talanta, 1970,17,945. 1. F. Freese. H. J. J 2. R. T. Campbell ana?e C. N. Reilley, ibid., 1%2,9,153.
Talanta,1970,Vol.17, pp. 1009 to
1013. Pcrxamon Press. Printed in Northern Ireland
A microwave-excited emissive detector for gas chromatography Further studies with sulphur compomals (Receiued 19 March 1970. Accepted l’2 April 1970)
Tms communication describes some improvements to a microwave-excited emissive detector for gas chromatographic detection of some sulphur compounds. The system previously described’ has been modified by restricting the volume occupied by the discharge, using a catalyst to aid the thermal degradation of the sulphur compounds, using all of the sample eluted from the gas chromatograph, and stabilizing the vacuum in the detector tube. These modifications have led to an overall improvement in this type of detection technique and have yielded superior detection limits, particularly for those sulphur compounds of relatively low volatility or high thermal stability.