Electrometric

Water (content) NFT 60-154 ISO/DIS 6296 ASTM D 1744 Karl Fischer Method (electrometric, alter addition of KF reagent)... 

Two methods are used to measure pH electrometric and chemical indicator (1 7). The most common is electrometric and uses the commercial pH meter with a glass electrode. This procedure is based on the measurement of the difference between the pH of an unknown or test solution and that of a standard solution. The instmment measures the emf developed between the glass electrode and a reference electrode of constant potential. The difference in emf when the electrodes are removed from the standard solution and placed in the test solution is converted to a difference in pH. Electrodes based on metal—metal oxides, eg, antimony—antimony oxide (see Antimony AND ANTIMONY ALLOYS Antimony COMPOUNDS), have also found use as pH sensors (8), especially for industrial appHcations where superior mechanical stabiUty is needed (see Sensors). However, because of the presence of the metallic element, these electrodes suffer from interferences by oxidation—reduction systems in the test solution. 

Titration methods using adsorption indicators, based on the precipitation of insoluble iodides, have also been proposed (81—84). The sensitivity of these methods is less than that for the thiosulfate titration. Electrometric titration of the reaction between iodine and thiosulfate (85) was not found to be practicable for routine deterrninations of minute quantities of iodine. 

Ellipsometry (kinetics) Electrometric reduction (kinetics thickness) Interference colours and spectrophotometry (kinetics thickness)... 

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. 

At this stage it should be pointed out that the original definition of pH = —log cH (due to Sorensen, 1909 and which may be written as pcH) is not exact, and cannot be determined exactly by electrometric methods. It is realised that the activity rather than the concentration of an ion determines the e.m.f. of a galvanic cell of the type commonly used to measure pH, and hence pH may be defined as... 

The end point of the reaction is conveniently determined electrometrically using the dead-stop end point procedure. If a small e.m.f. is applied across two platinum electrodes immersed in the reaction mixture a current will flow as long as free iodine is present, to remove hydrogen and depolarise the cathode. When the last trace of iodine has reacted the current will decrease to zero or very close to zero. Conversely, the technique may be combined with a direct titration of the sample with the Karl Fischer reagent here the current in the electrode circuit suddenly increases at the first appearance of unused iodine in the solution. 

The Karl Fischer procedure was applied to the determination of water present in hydrated salts or adsorbed on the surface of solids. The procedure, where applicable, was more rapid and direct than the commonly used drying process. A sample of the finely powdered solid, containing 5-10 millimoles (90-180 mg) of water, was dissolved or suspended in 25 mL of dry methanol in a 250-mL glass-stoppered graduated flask. The mixture was titrated with standard Karl Fischer reagent to the usual electrometric end point. A blank titration was also carried out on a 25 mL sample of the methanol used to determine what correction (if any) needed to be applied to the titre obtained with the salt. 

Electrometric titrations with two polarizable electrodes involving chelating agents. K. Stulik and F. Vydra, Chelates Anal. Chem. 1969, 2, 93-115 (70). 

Electrometric titration shows the purity to be at least 95%. The product does not melt below 300°. 

Batl Bates, R.G. Electrometric pH Determinations, New York Wiley, 1954. 

Seligson, S. McCormick. G. J. and Sleeman, R. Electrometric method for the determination of chloride in serum and other biological fluids. Clin. Chem. (1958), 4, 159 -169. 

It is certainly clear that a coulometric titration, like any other type of titration, needs an end-point detection system in principle any detection method that chemically fits in can be used, be it electrometric, colorimetric, photoabsorptionmetric, etc. for instance, in a few cases the colour change of the reagent generated (e.g., I2) may be observed visually, or after the addition of a redox, metal or pH indicator the titration end-point can be detected photoabsorptiometrically by means of a light source and photocell combination. Concerning the aforementioned coulometric titration of Fe(II), it is... 

Fig. 3.87. Disturbance of electrometric indicator function by generating current.

For end-point detection, any method usual in acid-base titration can be used with electrometric indication the precautions for protection against the... 

Electrometric methods have also been used for the final measurement of carbon dioxide Szekielda and Krey [146] devised a conductometric method. By far the most popular methods, however, have used commercial CHN analysers for the final measurement [147-150]. The actual measurement of carbon... 

Quantitative measurements of the strength of an acid are best based on electrometric measurements of the pH of the partly neutralized solution. Conductivity measurements are likely to give ionization constants that are too high due to the presence of conducting impurities. Estimates of acid strength from rate data are also somewhat unreliable... 

One area of analytical chemistry which is currently developing rapidly is the automation of methods. Some degree of automation has been used for a number of years in instruments such as automatic burettes coupled to absorptiometric or electrometric end-point detectors, and in data output devices which provide continuous pen recording or signal integration facilities. The major features of recent developments include the scope for instrumental improvements provided by solid-state electronic circuits and the increasing application of digital computers (Chapter 13). 

Three pKa values have been reported for leucovorin (free acid) they are 3.1, 4.8, and 10.4, as determined by electrometric titration.6 The first two values are attributed to the glutamyl carboxyls, and 10.4 is assigned to the hydroxyl group at the 4 position,8 by comparison to model compounds. 

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