Amperometric titrations procedures

A selection of coulometric titrations of different types is collected in Table 14.2. It may be noted that the Karl Fischer method for determining water was first developed as an amperometric titration procedure (Section 16.35), but modern instrumentation treats it as a coulometric procedure with electrolytic generation of I2. The reagents referred to in the table are generated at a platinum cathode unless otherwise indicated in the Notes. 

Sulfate ion can be determined by an amperometric titration procedure using Pb " " as the titrant. If the potential of a Hg electrode is adjusted to -1.00 V vs. SCE, the current can be used to monitor the Pb " concentration during the titration. In a calibration experiment, the limiting current, after correction for background and residual currents, was found to be related to the Pb " concentration by / / = lOcpt, -, where j) is the limiting current in xA and Cpb is the Pb-" concentration in mM. The titration reaction is... 

The stability constant of a complex of pyrophosphate with Ni (presumably NiPjO though this is not specified by the authors) was measured by an amperometric titration procedure. Small amounts of the nickel nitrate salt were titrated with the pyrophosphate in an ammonium nitrate buffer solution (0.1 M), adjusted to a pH value of 8 with aqueous ammonia. A Pb02 indicating electrode was used with a platinum foil coimter electrode and a saturated calomel reference electrode. The procedure was found to be very sensitive to pyrophosphate even in the presence of phosphate. 

Miscellaneous Procedures. Most of the other procedures for the determination of —SH groups are variants of the processes indicated above. Of particular interest are the several amperometric titration procedures which, in effect, depend on mercaptide formation. Polarographic methods have also been used. An interesting submicro method for cystine by a Cartesian diver technique has recently been developed the method depends on the catalytic effect of —SS— groups on the decomposition of azide ion to nitrogen (78,94). The method cannot be applied to the direct determination of —SH groups. 

In cases where it proves impossible to find a suitable indicator (and this will occur when dealing with strongly coloured solutions) then titration may be possible by an electrometric method such as conductimetric, potentiometric or amperometric titration see Chapters 13-16. In some instances, spectrophotometric titration (Chapter 17) may be feasible. It should also be noted that ifit is possible to work in a non-aqueous solution rather than in water, then acidic and basic properties may be altered according to the solvent chosen, and titrations which are difficult in aqueous solution may then become easy to perform. This procedure is widely used for the analysis of organic materials but is of very limited application with inorganic substances and is discussed in Sections 10.19-10.21. 

Amperometry refers to measurement of current under a constant applied voltage and under these conditions it is the concentration of the analyte which determines the magnitude of the current. Such measurements may be used to follow the change in concentration of a given ion during a titration, and thus to fix the end point this procedure is referred to as amperometric titration. 

Procedure The amperometric titration may be carried out in a 100 ml beaker. A saturated KN03 salt bridge is employed to provide contact between the saturated calomel electrode and the analyte solution. The various steps involved are as follows ... 

Water/waste water Measure initial temperature and pH and protect sample from light throughout the procedure. Phenylarsine oxide is used as Amperometric titration 0.5 mg/L No data APHA 1998 (Method 2350-C) (Method 4500-CL02-C) (Method 4500-CL02-E)... 

To determine the amylose content of starch, the iodine reaction has been most commonly used because amylose and amylopectin have different abilities to bind iodine. The methods such as blue value (absorbance at 680 nm for starch-iodine complex using amylose and amylopectin standards), and potentiometric and amperometric titration have been used for more than 50 years. These procedures are based on the capacity of amylose to form helical inclusion complexes with iodine, which display a blue color characterized by a maximum absorption wavelength (kmax) above 620 nm. During the titration of starch with iodine solution, the amount (mg) of iodine bound to 100 mg of starch is determined. The value is defined as iodine-binding capacity or iodine affinity (lA). The amylose content is based on the iodine affinity of starch vs. purified linear fraction from the standard 100 mg pure linear amylose fraction has an iodine affinity of 19.5-21.0mg depending on amylose source. Amylopectin binds 0-1.2mg iodine per 100mg (Banks and Greenwood, 1975). The amylose content determined by potentiometric titration is considered an absolute amylose content if the sample is defatted before analysis. 

Amperometric Titration s(Polarometric Titrations). In a strict sense, the term "ampero-metric should be applied to titrations in which a polagraphic diffusion-controlled limiting current is measured, according to the procedures described in references 3 to 8. This method has to be differentiated from galvanometric titration bf E.Salomon... 

In the iodimetric titration procedure, the combustion gases are bubbled through a diluent solution containing pyridine, methanol, and water. This solution is titrated with a titrant containing iodine in a pyridine, methanol, and water solution. In automated systems, the titrant is delivered automatically from a calibrated burette syringe and the endpoint detected amperometrically. The method is empirical, and standard reference materials with sulfur percentages in the range of the samples to be analyzed should be used to calibrate the instrument before use. Alternative formulations for the diluent and titrant may be used in this method to the extent that they can be demonstrated to yield equivalent results. 

Iodine in water may be analyzed by amperometric titration or by a colorimetric procedure using leucocrystal violet. In the latter method, mercuric chloride is added to the potable water sample, hydrolyzing iodine to hypoiodous acid. The latter reacts instantaneously with leucocrystal violet, forming a crystal violet dye. The absorbance is measured at a wavelength of 592 nm. 

Various analytical methods now employ amperometric measurements as part of their procedures. In particular, amperometric titrations have been widely used for the analysis of various substances in samples ranging from water to radioactive materials. Also, amperometric sensors, such as the dissolved oxygen probe and various amperometric biosensors, are widely used for clinical, environmental, and industrial monitoring. Furthermore, amperometric detectors have gained considerable use since the 1970s in high-performance liquid chromatographic determination of various substances and in flow injection analysis. 

Modern versions of the Winkler method improve the sensitivity and accuracy of the method by computer control of the titration procedure and the endpoint detection. Instead of visual observation of the decolouration of the blue starch-iodine complex, either the starch-iodine complex colour or the iodine colour itself is measured photometrically in the visible to ultraviolet (UV) spectral range. The spectral absorbance of an I3- solution (oxygen sample before titration) is depicted in Fig. 4-1. Grasshoff (1981) described a dead-stop titration of iodine with thiosulphate using amperometric endpoint detection. Bradburg and Hambly (1952) have compared various endpoint detections for iodine-thiosulphate titrations in low concentration ranges and stated relative sensitivities for visual-starch, colouri-metric-starch, amperometric, UV absorption as 1 0.2 0.002 0.0015. 

The relative change of conductance of the solution during the reaction and upon the addition of an excess of reagent largely determines the accuracy of the titration under optimum conditions this is about 0.5 per cent. Large amounts of foreign electrolytes, which do not take part in the reaction, must be absent, since these have a considerable effect upon the accuracy. In consequence, the conductimetric method has much more limited application than visual, potentiometric, or amperometric procedures. 

In addition to the titrations described in detail in Sections 16.25-16.27, 16.29 and 16.30, many other titrations may be performed amperometrically. An indication of the scope of this method is given by the procedures listed in Table 16.1. 

Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the amperometric end-point with the Karl Fischer reagent. Quickly add 0.2 g of prednisolone sodium phosphate sample, stir for 1 minute and again titrate to the amperometric end-point with the Karl Fischer reagent. The difference between the two titrations gives the volume (v) of Karl Fischer reagent consumed by the sample. 

Procedure Dissolve 0.25 g of procainamide hydrochloride in 50 ml of 2 M hydrochloric acid, add 3 g of potassium bromide, cool in ice and titrate slowly with 0.1 M sodium nitrite Vs, stirring constantly and determining the end-point amperometrically. Each ml of 0.1 M sodium nitrite Vs is equivalent of 27.18 mg of... 

In a limited number of experiments, in which six different hydrogen peroxide concentrations were examined with four different pH values, the validity of the procedure used was checked by comparing the results obtained with the sensor electrode to those obtained by means of titration. The results obtained with the sensor electrode have a maximum divergence of 3% compared with the concentrations obtained by titration. The final aspect of the amperometrical detection method that was examined at laboratory scale is the stability in time of a calibrated sensor electrode. 

The endpoint may be detected by addition of colored indicators, provided the indicator itself is not electroactive. Potentiometric and spectrophotometric indication is used in acid-base and oxidation-reduction titrations. Amperometric procedures are applicable to oxidation-reduction and ion-combination reactions especially for dilute solutions. 

A coulometric titration, like a more conventional volumetric procedure, requires some means of detecting the point of chemical equivalence. Most of the end-point detection methods applicable to volumetric analysis are equally saiisfactory here. Visual observations of color changes of indicators, as well as poicn-tionieiric, amperometric, and photometric measurements have all been used successfully. 

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