Analytical applications of triethylenetetraminehexa-acetic acid: Part 4. amperometric titrations of metal ions

The behaviour of metal ions during titration with triethylenetetraminehexaacetic acid (TTHA) in several supporting electrolytes was investigated by d.c. and square-wave polarography. The composition of the complexes (M₂L or ML) formed during titration is reported as well as the [M] : [L] ratio corresponding to the end-point of amperometric titration. The optimal conditions (by d.c. and s.w. detection) are given for titrations of Cu(II), Ni(II), Cd(II), Pb(II) and Zn(II) based on the reduction current of metal ions, as well as for titrations of Cu(II), Cd(II), Pb(II), Zn(II), Al(III), Ca(II) and Mg(II) based on the anodic current of TTHA. Application of the anodic current of TTHA permits determinations of polarographically inactive metals and is valuable in analysis of mixtures.     

Titration curves in complexometric titrations with the redox system Fe(III)/Fe(II)

Potentiometric titrations of metal ions with EDTA have been carried out with a platinum or graphite electrode and the Fe(III)/Fe(II) redox system. In the absence of oxygen and for pH < 2 the titration curves may be described by an equation similar to that given previously for titrations with silver and mercury electrodes. Titration curves for bismuth and indium, which are more strongly complexed than iron, are asymmetrical and useful for analytical purposes. When the titrated ions are complexed less strongly than iron(III) ions the kinetics of metal complexation have a pronounced effect. The titration curves of thorium and copper, which react more rapidly than iron, are analytically useful. The curves recorded rapidly after titrant additions have a better end-point break than those which correspond to thermodynamic equilibrium. When a metal, e.g., nickel, is weakly bound by EDTA, and reacts more slowly than iron, a very small end-point break or none at all is observed.     

A note on the determination of thorium in the presence of some bivalent metal ions

A new method for complexometric determination of thorium in the presence of bivalent metals without their masking is presented. Two aliquots of metal solution are necessary. In one the sum of the thorium and other metals is determined by DTPA titration. The second aliquot is treated with Zn(2)TTHA complex. Only thorium, forming a 1:1 complex with TTHA, displaces twice its molar amount of zinc from the complex, while other elements displace only an equimolar amount of zinc. The amount of thorium corresponds to the difference of consumption in the two titrations.     

~1H and ~(13)C NMR studies of TTHA complexes with diamagnetic rare earth ions

TTHA complexes with diamagnetic rare earth ions (La3+, Y3+ and Lu3+) were studied by 1H and 13C NMR spectroscopy. A symmetric structural model was suggested for La(TTHA) complex and an asymmetric model for Y(TTHA) and Lu(TTHA) complexes. The complex formation was dependent on the pH value of the solution. The interactions of La(TTHA) with the additional metal ions (La3+, Y3+ and Ca2+) were relatively weak, but relatively strong for that of Lu(TTHA) with the additional Lu3+.     

Polarographic studies of iron complexes as potential contrast as agents in magnetic resonance imaging

Iron(III) complexes of polyaminocarboxylic acids of potential use as contrast agents in magnetic resonance imaging wre investigated by differential pulse polarography. The complexes with diethylenetrinitrilopentaacetic acid, trans-1,2-cyclohexylenedinitrilotetraacetic acid and triethylenetetranitrilohexaacetic acid (TTHA) wre found to decompose slowly at pH 7.2. With TTHA, a mixture of 1:1 and 2:1 complexes was obtained, and the transformation between the two complexes was slow. Ethylenediimonobis[(2-hydroxyphenyl)acetic acid] (EHPG) was found to be the most suitable ligand. The complex formation is kinetically slow, and a special procedure for the preparation of the complex is required; the complex is then stable for one week at pH 7.2. The polarographic measuremeents are preferably made at pH 9.2, where a well-defined reduction peak is obtained. The complex is stable for 2 days at pH 9.2. At pH values below 9, a double peak is obtained, except for low concentrations of the complex. Both Fe(III)EHPG and Fe(II)EHPG adsorb at the mercury electrode. The polarographic determination can be done in the presence of 10% of urine or serum without interference. The detection limit is in the low μM range.     

The computerized determination of double-layer capacitance with the use of kalousek-type waveforms and its application in titrimetry

A method for the rapid determination of double-layer capacitance—potential curves of electrodes is described. An on-line computer is used to apply Kalousek-type waveforms to the electrochemical cell and to measure the accompanying current response. The capacitances are determined from the slope of the plots of log current against time. For 0.1 M KCl, the computerized method agrees well with the bridge method, except for the potential range of 0 to –0.15 V. The method is very useful for automating titrations with tensammetric detection of the end-point. The method is applied to the titration of barium with a macrocydic compound (kryptofix 222) and the titration of cetyl-trimethyl-ammonium bromide with bromocresol purple. The accuracy of the titrations is ±2%.     

Binding of Cu(II) to poly(methacrylic acid)

The binding of Cu(II) ions to partly neutralized poly(methacrylic acid) (PMA) has been investigated by potentiometric titration and dialysis to determine the stoichiometry the Cu–PMA complexes formed. Partly ionized PMA was titrated with solutions of the metal ion to enable a large range of metal ion/polymer ratios to be studied. Combination of the results from these two techniques at ionic strength 0.1 indicates that at very low Cu(II)/polymer ratios, a 4:1 complex exists, but at higher ratios the complex breaks down to give a mainly 2:1 coordination with some 1:1 binding. Conductance titrations support these results. Viscometric titrations show strong interactions between the metal and polymer, preventing the full extension of the polyion at high degrees of ionization, and spectrophotometric titrations support the existence of at least two types of complexes in the solution.     

示波滴定

本文综述南京大学化学系电化学分析小组几年来在示波滴定方面的研究工作。文中提出了一系列新的电滴定方法,这些方法兼有指示剂法和物理化学滴定法的优点,即终点直观、仪器便宜,操作简便、快速准确,适用于药物分析及冶金分析,受到国内分析工作者的重视。文中提出了示波滴定的定义、分类和每一方法的特点及发展前途的预测。     

Solution Equilibria. A Unified Mathematical Treatment of Experimental Polarographic and Potentiometric Data from Acid?Base and Ligand Titrations. A Polarographic and Ion Selective Electrode Study of CdII?(N‐(2‐hydroxyethyl)iminodiacetic acid)?OH System

It has been demonstrated that potentiometric and polarographic data coming from either acid? base or ligand titrations can be evaluated by the same set of equations and mathematical procedures involving mass‐balance equations written for any metal? ligand model, including polynuclear species. It is shown that the concept of the complex formation curves, used previously in modeling and refinement of stability constants in acid‐base titration, is of general nature and can be successfully used in polarographic and potentiometric experiments conducted as a function of pH or an excess of a ligand. It appears that the linear relationship ΔE vs. log [M], typical for potentiometric studies with ISE, holds also in the case of the study of kinetically mixed metal‐ligand system studied by polarography when the corrected shift in the polarographic signal is used. The relationship ΔE vs. log [M] applies equally to the acid‐base and ligand titration for both experimental techniques employed (potentiometry and polarography). The significance of the corrected shift is discussed and its meaning in the study of kinetically fast or slow metal? ligand systems is elaborated. Advantages of the acid‐base titration over the ligand titration are discussed. The generalized mathematical data treatment was successfully employed in the study of the Cd^II? (N‐(2‐hydroxyethyl)iminodiacetic acid) system. Results obtained from both analytical techniques (potentiometry with the use of an ion selective electrode, and two polarographic techniques) and analytical procedures (acid? base and ligand titrations) compare well with each other and with the literature data (the formation and stability constants of ML and ML₂). In addition, a new complex M(HL) was identified and its stability constant is reported.     

非配偶型铂电极上的电位滴定

配偶型铂电极上的电位滴定是Nernst型滴定,其“电极/溶液”界面上总有一对决定电位的电对。非配偶型电极的界面上不存在明显的电对,通常终点很难判断。本文采用示波器代替电位测量仪指示电位变化。由于荧光点的移动十分迅速又无惯性,所以在滴定终点时能敏锐地反映瞬时电位峰,指示终点,使一大类原来不能进行的滴定得以进行。     

金属离子Bi(Ⅲ)络合物体系的极谱法研究

The B（i Ⅲ）-Ligand（pyridine-2-carboxylic acid）system was studied by the differential pulse polarography（DPP）at the fixed total-ligand to total-metal concentration ratio［LT］:［MT］and varied pH. An appropriate model for the metal complex system is designed according to polarographic peak parameters,that is,the observed shift in the peak potential and the change in the peak current with pH changes in the sample solution. The polarographic experimental complex formation curve（ECFC）and theoretical complex formation curve（TCFC）were used for checking the designed metal complex model as well as optimizing stability constants. The ECFC,in which experimental parameters（a shift in a peak potential and a variation in a peak current）are included,appears to be a characteristic function for a particular metal-ligand system. The TCFC is a theoretical curve calculated from massbalance equations for the designed metal-ligand model. Using the computer analytical program,stability constants of metal complexes and species distribution diagrams are obtained by solving the mass-balance equations written for the metal complex system and fitting the complex formation curve. Five bismuth complexes MHL,ML,ML2,ML3,ML3 （ OH）,and their stability constants as logβ 7.52±0.15,7.48±0.06,13.94±0.02,18.12±0.03 and 26.78±0.03,respectively,are finally reported while the TCFC fits best the ECFC.     

Stalagmometric titrations

End-point detection in some precipitation titrations is achieved by measurement of the surface tension between mercury and the solution ("stalagmometric" titration). The drop-time of a polarographic dropping mercury electrode in open circuit is plotted against volume of surface-active titrant added, and shows a break or a peak at the end-point. The stalagmometric titration of sodium tetraphenylborate with Zephiramine (tetradecyldimethylbenzylammonium chloride) was satisfactory and made possible the determination of potassium by back-titration. Potassium was also titrated directly with tetraphenylborate and with calcium dipicrylaminate. Sodium dodecylbenzenesulphonate was titrated directly with Zephiramine and the results were compared with those obtained by the p-toluidine method and Epton's method.     

Coulometric titration by means of a controlled-potential-pulsed-current potentiometric technique-I Metal ions by electro-deposition

A new method for electrochemical titrations is proposed, based on the redox transformation of the test species by means of a controlled-potential pulsed current, followed by measurement of the potential in the intervals between the current pulses: the end-point is found by means of Sörensen's linearization technique. Investigations on various metal ions (Ag⁺, Cu²⁺, Cd²⁺, Pb²⁺) have shown that the accuracy and sensitivity, which depend on the nature of the species titrated, are comparable with those of other titration techniques. The method permits analytical separations and determinations of metal ions in mixtures. No particularly elaborate instrumentation is required and the apparatus described is simple to use, reliable and inexpensive.     

Simple instrument for titrimetry without visual indicators. Gasometric titrations: Dichromatometry

Even ions which do not release gases with a certain titrant can be determined by direct gasometric titration. To achive this end, a substance which liberates a gas with the titrant, must be added to the sample solution. The end-point of the main reaction can be read from the gasometric titration curve of this indicator substance.This principle is applied in the present paper to the titration of ferrous and stannous ions by potassium dichromate. Hydrazine sulfate serves as indicator. The results are comparable in precision and accuracy to the values obtained by the conventional visual end-point titrations.     

Zn2TTHA等双核金属配合物的NMR研究

报道了Ｚｎ、Ｍｇ、Ｐｂ和Ｈｇ与ＴＴＨＡ形成的２：１双核配合物的动态ＮＭＲ研究，在所研究的温度范围内观测到Ｚｎ２ＴＴＨＡ分子内重排过程，并通过模拟谱求出分子内重排过程的速率常数，提出Ｚｎ２ＴＴＨＡ在溶液中的构型由２个对称的八面体单元组成，而不是由２个四面体构成的；用二位交换的Ｂｌｏｃｈ方程估计Ｍｇ、Ｐｂ和Ｈｇ双核配合物的活化能Ｅａ值；发现分子内重排活化能大小的顺序与中心金属离子的离子势呈线性关系。     

Analytical applications of triethylenetetraminehexaacetic acid : Part I. the influence of TTHA on the polarographic reduction of some metal ions

The influence of triethylenetetraminehexaacetic acid (TTHA) on the d.c. and square-wave polarographic curves of Cu(II), Pb(II), Cd(II), Ni(II), Co(II), Zn(II), Fe(III), Bi(III), Sb(III), As(III) and In(III) has been investigated in several supporting electrolytes over the pH range 1–13. The E_{$\text{1}\text{2}$} and E_p values with and without a 10-fold excess of TTHA are compared. Some analytical applications are suggested.     

Polarographic study of the substitution reactions of the chelates of triethyl-enetetraminehexa-acetic acid-I Reaction of the zinc chelate with calcium ions

The mechanism of the substitution reaction between the binuclear zinc-triethylenetetraminehexa-acetic acid (TTHA) chelate (Zn(2)X(2-)) and calcium ions was studied by the polarographic technique. The results obtained under equilibrium and kinetic conditions permitted formulation of the reaction mechanisms as depending on the ratio Zn:Ca in the system studied. The reaction studied proceeds only in the presence of ammonia. When calcium is replaced with a metal which has a TTHA chelate more stable than Zn(2)X(2-), the mixed binuclear chelate is not formed.     

Die voltametrische Indikation komplexometrischer Titrationen

Summary A survey of the principles of complexometric titration is undertaken, paying particular regard to the methods of indication employed. The fundamentals of voltametric indication are also discussed, with emphasis being placed on the interpretation given to the measured indication curves by various workers. It is then shown how voltametric indication may be successfully applied to complexometric titrations.Complexometric titrations in acid solutions, can be exactly indicated with the aid of a Pt-double-electrode of 10 to 50 sq. mm. The electrode is polarized by means of a constant current of 1 to 10 A, and its potential difference is measured throughout the course of the titration. If the electrode is electrolytically pretreated in a HNO₃-Bi-solution for 10–20 min with a current of a few A, Bi deposits on both the anode and the cathode, and the resulting activation of the electrode causes a markedly improved indication at the end point.The untreated Pt-double-electrode is only in certain cases suitable for the indication of titrations in alkaline solutions. If, however, the electrode is electrolytically pretreated in an ammoniacal solution for approx. 15 min with a current of a few A, Tl-oxide deposits on the anode, and the performance of the resulting indicator is extremely good.Indication curves resulting from the titration of numerous elements in acid and ammoniacal solutions prove the high suitability of the above described electrodes for the purpose under review. The end point, which is indicated by a change in potential of several 100 mV, is found to be identical with the equivalence point of the reaction. This method of indication also lends itself unreservedly to indirect or substitution titrations, as well as to sequence titrations, when, for instance, one component is complexometrically titrated in an acid solution and the other in an ammoniacal solution. The electrodes described are not only suited for titrations with EDTA, but also with more recent complexing agents, of which DTPA, EGTA and DCTA have been examined. As the electrodes may be used for titrations with solutions having a concentration of as low as 0.0001 M, they enable the titrimetric determination of amounts of as little as about 1 g in a volume of about 50 ml. This has been clearly shown by the titration of magnesium.The reproducibility of the indications, and thus the accuracy of the titrations, has been found to be ± 0.01 ml when working with 0.1 M and 0.01 M titration solutions. In the case of 0.001 M solutions, the results are only slightly less accurate. A few g of material may be titrated with an accuracy of at least ± 10%.As a general rule, the voltametric indication curves of complexometric titrations are dependent only on the depolarising effect of the free complexing agent upon the anode. If, however, ions such as Cu, Tl or Pb, which may themselves give cathodic and/or anodic electrode processes, are present in the solution under investigation, their action interferes with the basic run of the indication curve. A number of different curve shapes thus occur, all of which can be evaluated very accurately. The metal complexes themselves, do not have any significant influence on the electrochemical process, in so far as this method is concerned.This method of indication is not applicable, if the solution under investigation contains substances which depolarize the anode more than the free complexing agent, e.g. reducing agents.The investigations are extended to cover polarized Au, Ag and Cu electrodes, with discussion of their differences in properties as compared with platinum. The study is rounded off by considering the slope of the indication curves as a function of various parameters, among others the amount of substance to be titrated, and also the question of the evaluation of the curves.     

Coulometric trace determination of chloride

1-2000 nmole of chloride was determined by coulometric titration with silver ions. The error was 0.1-5 %, depending on sample size. Chloride in water could be determined down to 0.01 ppm. The titrations were made in 75% acetic acid, 25% water. The displacement of the electrode potential on dilution of the solvent with water was determined. Two alternative procedures to compensate the end-point displacement are described. The titration times were 2-4 min for pretitration of the solvent and 2-4 min for titration of the sample. In some cases another 4 min were required for equilibrium to be reached. Various solvent compositions were used in order to find the optimum conditions.     

Determination of dissociation constants for coordination compounds of Cr(III) and Co(III) using potentiometric and spectrophotometric methods

The acid–base properties of analogous complex ions of chromium(III) and cobalt(III) in aqueous solution have been studied. The equilibrium constants for all metal complexes were determined by using potentiometric and spectrophotometric titration methods. First, dissociation constants for the studied complexes of Cr(III) and Co(III) were determined by means of the potentiometric titration method and using the STOICHIO computer programme. Then, pH-spectrophotometric titrations were performed and the OriginPro 7.5 computer programme was used to calculate the same constants. The measurements using both methods were carried out under the same conditions of temperature, T = 298.15 K, and over the same pH range 2.00–10.00, respectively. It turned out that the two methods used enabled us to obtain acidity constants in very good agreement.