The formulation of the electron and proton balance equations for solving complicated equilibrium problems in redox titrations

 A plain, didactically convenient formulation of the electron balance and of the proton balance equations, suitable for complicated redox titration systems with redox, acid-base, precipitation and complexation side-reactions is discussed. Two typical examples, the standardisation of a permanganate solution by titration of iron(II), and the standardisation of a iodine solution by titration of sodium arsenite, are presented. Received: 30 January 1996/Accepted: 9 February 1996     

复杂氧化还原滴定曲线方程的Origin解析和曲线绘制

本文首先利用物料平衡和林邦副反应思想分别推导出n1=n2和n1≠n2两种情况时的氧化还原滴定曲线方程,然后应用Origin软件对所推导的不同曲线方程进行解析,建立起以溶液电势E为因变量,滴定分数a为自变量的函数关系,并绘制其滴定曲线。     

Potentiometric titration of anionic polyelectrolytes using a cationic surfactant solution as a titrant and a titrant-sensitive plasticized poly(vinyl chloride) membrane electrode

A potentiometric titration method for the direct determination of anionic polyelectrolytes using a plasticized poly(vinyl chloride) membrane electrode is described. A cationic surfactant solution is used as titrant. The method is based on ion association between the anionic polyelectrolyte in the sample and the cationic surfactant in the titrant. The end-point is detected as a sharp potential change due to an increase in the concentration of the free cationic surfactant at the equivalence point. The sharpness of the titration curve at the end-point is compared for several cationic surfactants; a solution of stearyltrimethylammonium ion has been found to be superior. Received: 8 October 1996 / Revised: 17 December 1996 / Accepted: 18 December 1996     

Potentiometric titration of anionic polyelectrolytes using a cationic surfactant solution as a titrant and a titrant-sensitive plasticized poly(vinyl chloride) membrane electrode

A potentiometric titration method for the direct determination of anionic polyelectrolytes using a plasticized poly(vinyl chloride) membrane electrode is described. A cationic surfactant solution is used as titrant. The method is based on ion association between the anionic polyelectrolyte in the sample and the cationic surfactant in the titrant. The end-point is detected as a sharp potential change due to an increase in the concentration of the free cationic surfactant at the equivalence point. The sharpness of the titration curve at the end-point is compared for several cationic surfactants; a solution of stearyltrimethylammonium ion has been found to be superior. Received: 8 October 1996 / Revised: 17 December 1996 / Accepted: 18 December 1996     

Trypsin inhibitors analysis: direct chromatographic titration

A titrimetric method for the standardisation of three human urinary trypsin inhibitors (UTIs) has been developed. The same substances are also present in human plasma, and their amounts in such biological fluids are of bioclinical interest. The method is based on the direct titration of each UTI with a solution of bovine trypsin of known molarity, and monitoring the decreasing residual amounts of the inhibitor by reversed-phase high-performance liquid chromatography. The proposed method provides a suitable alternative to the traditional enzymic - spectrophotometric method. Mean within-day and between-day coefficients of variation and the limit of determination for the standardisation of each UTI are reported.     

Precise coulometric titration of sodium thiosulfate and development of potassium iodate as a redox standard

In this paper, we determine the effective purity of potassium iodate as a redox standard with a certified value linked to the international system of units (SI units). Concentration measurement of sodium thiosulfate solution was performed by precise coulometric titration with electrogenerated iodine, and an assay of potassium iodate was carried out by gravimetric titration based on the reductometric factor of sodium thiosulfate assigned by coulometry. The accuracy of the coulometric titration method was evaluated by examining the current efficiency of iodine electrogeneration, stability of sodium thiosulfate solutions and dependence on the amount of sodium thiosulfate solution used. The measurement procedure for gravimetric titration of potassium iodate with sodium thiosulfate was validated based on determination of a reference material of known purity (potassium dichromate determined by coulometry with electrogenerated ferrous ions) using the same gravimetric method. Solutions of 0.2 and 0.5 mol/L sodium thiosulfate were stable over 17 days without stabilizer. Investigation of the dependency of titration results on the amount of sodium thiosulfate solution used showed no significant effects, no evidence of diffusion of the sample, and no effect of contamination appearing during the experiment. Precise coulometric titration of sodium thiosulfate achieved a relative standard deviation of less than 0.005% under repeating conditions (six measurements). For gravimetric titration, the results obtained for the effective purity of potassium dichromate were sufficiently close to its certified value to allow confirmation of the validity of the gravimetric titration was confirmed. The relative standard deviation of gravimetric titration for potassium iodate was less than 0.011% (nine measurements), and a redox standard with a certified value linked to SI units was developed.     

Effect of excess AgO and EDTA on the redox titration involved in potentiometric determination of plutonium

Attempts to carry out potentiometric determination of plutonium (by AgO-oxidation method) following the determination of thorium (by complexometric EDTA titration) gave positively biased irreproducible values of plutonium. In order to understand the factors leading to the erroneous values of plutonium, the effect of varying amounts of AgO and EDTA on redox [Fe(II)/K₂Cr₂O₇] titration in the medium consisting of 1M H₂SO₄ and 0,4M HNO₃ was studied. When AgO (up to 200 mg) was added in the titration medium and destroyed by sulfamic acid prior to the redox titration, the dichromate tittre value (amount of standard K₂Cr₂O₇ solution equivalent to 1 g of Fe(II) solution) showed negative bias. The bias was found to decrease with increase in time interval between AgO destruction and the redox titration. The presence of EDTA (up to 15 mg) gave positive bias in the titre value. Results on the titre values obtained under different conditions of the aqueous medium indicated the possibility of formation of unstable Ag(II)-sulfamic acid compled, which is probably responsible for the irreproducible values of plutonium.     

A titrimetric method for the sequential determination of thorium and plutonium

A method for the sequential determination of thorium and plutonium has been developed. In the sample solution containing thorium and plutonium, thorium is first determined by complexometric titration with EDTA and then in the same solution plutonium is determined by redox titration employing potentiometry. Prior to the determination of plutonium, EDTA is destroyed by fuming with concentrated HClO₄. Thorium is determined at 10 mg level and plutonium at 1 mg level with precision and accuracy of better than ±0.5%.     

Thermometric titration of uranium(iv)with potassium dichromate

A thermometric method is presented for the redox titration of uranium(IV) with dichromate. The investigation was made primarily to determine the applicability of the thermometric method to redox titrations in solutions that contain ions deleterious to electrodes used in electrometric methods. A relative standard error of ±1% attainable in the titration of quantities of uranium(lV) of the order of 5 mg. Less than l 5 min is required to complete a titration. Fluoride ion can be tolerated in the solution provided that sufficient aluminium(111) is added to complex all the fluoride.     

Investigation of iodine liberation process in redox titration of potassium iodate with sodium thiosulfate

Potassium iodate is often used as a reference material to standardize a sodium thiosulfate solution which is a familiar titrant for redox titrations. In the standardization, iodine (triiodide) liberated by potassium iodate in an acidic potassium iodide solution is titrated with a sodium thiosulfate solution. The iodine liberation process is significantly affected by the amount of acid, that of potassium iodide added, the waiting time for the liberation, and light; therefore, the process plays a key role for the accuracy of the titration results. Constant-voltage biamperometry with a modified dual platinum-chip electrode was utilized to monitor the amount of liberated iodine under several liberation conditions. Coulometric titration was utilized to determine the concentration of a sodium thiosulfate solution on an absolute basis. Potassium iodate was assayed by gravimetric titration with the sodium thiosulfate solution under several iodine liberation conditions. The liberation process was discussed from the changes in the apparent assay of potassium iodate. The information of the appropriate titration procedure obtained in the present study is useful for any analysts utilizing potassium iodate to standardize a thiosulfate solution.     

A model for the propagation of a redox reaction through microcrystals

Chronoamperometry of reversible redox reactions with the insertion of cations into solid particles immobilised at an electrode surface is analysed theoretically using a semiinfinite planar diffusion model. A coupled diffusion of electrons and ions within the crystal lattice is separated in two differential equations. The redox reaction is initiated by the polarisation of the three-phase boundary, where the crystal is in contact with both the electrode and the solution. From this contact line the redox reaction advances on the surface and into the crystal body by the diffusion of ions and conductance of electrons. The effects of the geometry and conductivity of the particles on the current are discussed. Received: 28 December 1996 / Accepted: 17 February 1997     

An Interfacial Voltaic Cell for End Point Detection in Complexometric Titration of Metal Ions

An interfacial voltaic cell was firstly described by B. Waligra et al for the potentiometric titration of alkaloids in non-conducting solution.Our group had developed an improved one for the potentiometric titration of various alkaloids and their salts as well as pharmaceutical preparations,organic acids inorganic acids and bases. It had also been used as an end-point indicator for the potentiometric redox titration, and precipimetric titration.     

A titrimetric method for the sequential determination of thorium and uranium

A method for the sequential determination of thorium and uranium has been developed. In the sample solution containing thorium and uranium, thorium is first determined by complexometric titration with ethylenediaminetetraacetic acid (EDTA) and then in the same solution uranium is determined by redox titration employing potentiometry. As EDTA interferes in uranium determination giving positive bias, it is destroyed by fuming with HClO₄ prior to the determination of uranium. A precision and accuracy of better than ±0.15% is obtained for thorium at 10mg level and uranium ranging from 5 mg to 20 mg in the aliquot.     

A generalized approach for the calculation and automation of potentiometric titrations Part 1. Acid-Base Titrations

Fast and accurate calculation procedures for pH and redox potentials are required for optimum control of automatic titrations. The procedure suggested is based on a three-dimensional titration curve V = f(pH, redox potential). All possible interactions between species in the solution, e.g., changes in activity coefficients and influences of redox potential on pH variations, are taken into account. The number of titrant additions can be reduced considerably without loss of precision, by using the fact that the pH of a protolyte or mixture of protolytes at some fraction titrated does not depend strongly on the actual concentration.     

Potentiometric titration of diprotic systems

 The approximate nature of Gran’s method of linearization of potentiometric titration curves applied to diprotic systems has been thoroughly investigated. A function involving pH and titrant volume V (which is linearly dependent on V and becomes zero at the first equivalent point of titration of a weak diprotic base with a strong monoprotic acid) has been critically examined and validated by means of widely extended simulated experiments. Accurate experimental confirmation of the theoretical expressions has been obtained by performing many real titrations of the primary standard Na₂CO₃ with HCl. This particular application, analytically significant itself, can be a prototype of analogous applications. Received: 25 March 1996/Revised: 15 November 1996/Accepted: 19 November 1996     

Potentiometric titration of diprotic systems

 The approximate nature of Gran’s method of linearization of potentiometric titration curves applied to diprotic systems has been thoroughly investigated. A function involving pH and titrant volume V (which is linearly dependent on V and becomes zero at the first equivalent point of titration of a weak diprotic base with a strong monoprotic acid) has been critically examined and validated by means of widely extended simulated experiments. Accurate experimental confirmation of the theoretical expressions has been obtained by performing many real titrations of the primary standard Na₂CO₃ with HCl. This particular application, analytically significant itself, can be a prototype of analogous applications. Received: 25 March 1996/Revised: 15 November 1996/Accepted: 19 November 1996     

Determination of iron, nickel, chromium and vanadium by means of redox and ion-exchange columns

Redox and anion-exchange columns have been used to separate and determine iron, nickel, chromium and vanadium in solution. The anion-exchange columns provide some of the separations, and the redox columns are used for the determination of the iron, chromium and vanadium. The chromium and vanadium may be determined in the presence of the iron by adjustment of the acidity in the redox column. By using a column "memory" technique, titration of the actual metal solution has been avoided. The method shows some advantages over conventional methods.     

Method for avoiding the interference of formamide with the Karl Fischer titration

 The Karl Fischer titration is based on a specific chemical reaction. Several measures exist to make all the water of insoluble samples accessible for the chemical reactants. The most efficient are the titration at elevated temperatures, the use of a homogenizer in the titration vessel and the modification of the polarity of the working medium (essentially methanol) by the addition of appropriate solvents like chloroform or formamide. It is known however that formamide interferes with the Karl Fischer reaction and so causes more or less false results. This effect increases with higher temperatures. A method is therefore presented to avoid this interference, even when working at the boiling point of the working medium. It takes advantage of the fact that the side reaction has a practically constant velocity, at least as long as usual titrations last. Thus, a constant additional consumption of Karl Fischer reagent is observed. This can be accounted for by measuring this effect before the start of the determination and by deducting the additional reagent consumption, which is proportional to the duration of the titration, from the totally added volume. With certain modern titrators this can even be carried out automatically. They can continuously measure the so-called drift, the titration rate necessary to keep the titration cell dry, and have the capability to use this drift as stop criterion for the titration. This means that the analysis is terminated when the drift existing before the titration is reached again. The additional consumption of reagent, to be deducted from the total volume, can (automatically) be calculated from the drift rate and the titration time. The proposed procedure allows the use of formamide as additional solvent, even at high temperatures, in order to shorten determination times considerably. It avoids false results due to the interference, which has so far prevented its use when exact results were desired and when the duration of the analysis was long. Received: 30 May 1996 / Revised: 26 July 1996/Accepted: 30 July 1996     

Polymerization of methyl methacrylate with ceric ion-methanol redox system

The polymerization of methyl methacrylate initiated by Ce⁴⁺ methanol redox system was studied in aqueous solution of nitric acid at 15°C. The polymerization was initiated by primary radicals formed from Ce⁴⁺/alcohol complex. Poly(methyl methacrylate) chains containing the alcohol residue were obtained. Variations in the temperatuare and concentration of the components of the redox system allowed the control of the rate of polymerization and molecular weight of the polymer. The concentration of the hydroxyl end groups in the poly(methyl methacrylate) of low molecular weight was determined by titration and by spectrometric method.     

Stannous ion determination in99mTc radiopharmaceutical kits

Two simple and selective methods for determination of stannous ion in radiopharmaceutical kits are proposed. One of this permits the estimation of stannic ion. The first method unsed is a potentiometric titration of Sn²⁺ in HCl medium, using KIO₃ solution under nitrogen gas and a platinum redox electrode. The second method consists of a complexometric titration of tin (Sn²⁺ and Sn⁴⁺) using an EDTA standard solution at pH 5.5–5.6 without the use of nitrogen gas. The procedures employed indicate that both methods can be used for routine quantitative determination of tin in most labeled radiopharmaceuticals.