Titration efficiency

It is generally desirable for bulk electrolytic processes to be carried out with high current efficiency. This requires that the working electrode potential and other conditions be chosen so that no side reactions occur (e.g., reduction or oxidation of solvent, supporting electrolyte, electrode material, or impurities). In electrogravimetric methods, 100% current efficiency is usually not necessary, as long as the side reactions do not produce insoluble products. In coulometric titrations at constant current, 100% titration efficiency (rather than current efficiency) is required the distinction is discussed in Section 11.4.2. 

Thus some is indirectly oxidized to Fe ", and the titration efficiency for the oxidation of Fe is maintained. This then resembles an ordinary titration of Fe " with Ce" , in that a true equivalence point is reached. For this reason this technique is usually called a coulometric titration (of Fe with the electrogenerated titrant, Ce ). Note that some end-point detection technique (as is also required in an ordinary titration) must be used to indicate when the oxidation of Fe is complete, since neither the current nor the potential of the working electrode is a good indicator of the course of the reaction. 

The next most easily reduced material is Fe , which is reduced to Fe . The Fe is stirred out into the bulk of the solution where it reacts with the remaining Ce. If a higher current (12) were originally selected, then the current divides from the beginning between the reduction of Ce " and Fe +. The net result is the same, however, since all of the Ce eventually is reduced, either directly at the electrode or indirectly by Fe . If the ferric ammonium sulfate were not added, the current-versus-potential curve would have the original plateau (A) but then would follow the dashed curve D. In this case, either at the start of the titration (level 12), or sometime during the titration (level ii), hydrogen would be produced and be lost from solution. Under these conditions the titration efficiency would be less than... 

Albery W J and Brett C M A 1983 The wall-]et ring disc electrode. 2. Collection efficiency, titration curves and anodic stripping voltammetry J. Electroanal. Chem. 148 201... 

The procedure is computationally efficient. For example, for the catalytic subunit of the mammalian cAMP-dependent protein kinase and its inhibitor, with 370 residues and 131 titratable groups, an entire calculation requires 10 hours on an SGI 02 workstation with a 175 MHz MIPS RIOOOO processor. The bulk of the computer time is spent on the FDPB calculations. The speed of the procedure is important, because it makes it possible to collect results on many systems and with many different sets of parameters in a reasonable amount of time. Thus, improvements to the method can be made based on a broad sampling of systems. 

Heat reagent-grade material for 1 hr at 255-265°C. Cool in an efficient desiccator. Titrate sample with acid to pH 4-5 (first green tint of bromocresol green), boil the solution to eliminate the carbon dioxide, cool, and again titrate to pH 4-5. Equivalent weight is one-half the formula weight. 

End Point Determination Adding a mediator solves the problem of maintaining 100% current efficiency, but does not solve the problem of determining when the analyte s electrolysis is complete. Using the same example, once all the Fe + has been oxidized current continues to flow as a result of the oxidation of Ce + and, eventually, the oxidation of 1T20. What is needed is a means of indicating when the oxidation of Fe + is complete. In this respect it is convenient to treat a controlled-current coulometric analysis as if electrolysis of the analyte occurs only as a result of its reaction with the mediator. A reaction between an analyte and a mediator, such as that shown in reaction 11.31, is identical to that encountered in a redox titration. Thus, the same end points that are used in redox titrimetry (see Chapter 9), such as visual indicators, and potentiometric and conductometric measurements, may be used to signal the end point of a controlled-current coulometric analysis. For example, ferroin may be used to provide a visual end point for the Ce -mediated coulometric analysis for Fe +. 

Any Fe + lost in this fashion must be replaced by the additional reduction of Fe +, reducing the current efficiency and increasing the time needed to reach the titration s end point. The net result is that the reported concentration of Cr207 is too large. 

Accuracy The accuracy of a controlled-current coulometric method of analysis is determined by the current efficiency, the accuracy with which current and time can be measured, and the accuracy of the end point. With modern instrumentation the maximum measurement error for current is about +0.01%, and that for time is approximately +0.1%. The maximum end point error for a coulometric titration is at least as good as that for conventional titrations and is often better when using small quantities of reagents. Taken together, these measurement errors suggest that accuracies of 0.1-0.3% are feasible. The limiting factor in many analyses, therefore, is current efficiency. Fortunately current efficiencies of greater than 99.5% are obtained routinely and often exceed 99.9%. 

For efficiency of desiccants in drying acetone see Burfield and Smithers [7 Org Chem 43 3966 1978]. The water content of acetone can be determined by a modified Karl Fischer titration [Koupparis and Malmstadt A/w/ Chem 54 1914 1982]. 

This reaction is subject to a number of errors (1) the hydriodic acid (from excess of iodide and acid) is readily oxidised by air, especially in the presence of chromium(III) salts, and (2) it is not instantaneous. It is accordingly best to pass a current of carbon dioxide through the reaction flask before and during the titration (a more convenient but less efficient method is to add some solid sodium hydrogencarbonate to the acid solution, and to keep the flask covered as much as possible), and to allow 5 minutes for its completion. 

Discussion. Iodine (or tri-iodide ion Ij" = I2 +1-) is readily generated with 100 per cent efficiency by the oxidation of iodide ion at a platinum anode, and can be used for the coulometric titration of antimony (III). The optimum pH is between 7.5 and 8.5, and a complexing agent (e.g. tartrate ion) must be present to prevent hydrolysis and precipitation of the antimony. In solutions more alkaline than pH of about 8.5, disproportionation of iodine to iodide and iodate(I) (hypoiodite) occurs. The reversible character of the iodine-iodide complex renders equivalence point detection easy by both potentiometric and amperometric techniques for macro titrations, the usual visual detection of the end point with starch is possible. 

Prepare an approximately 0.1 M silver nitrate solution. Place 0.1169 g of dry sodium chloride in the beaker, add 100 mL of water, and stir until dissolved. Use a silver wire electrode (or a silver-plated platinum wire), and a silver-silver chloride or a saturated calomel reference electrode separated from the solution by a potassium nitrate-agar bridge (see below). Titrate the sodium chloride solution with the silver nitrate solution following the general procedure described in Experiment 1 it is important to have efficient stirring and to wait long enough after each addition of titrant for the e.m.f. to become steady. Continue the titration 5 mL beyond the end point. Determine the end point and thence the molarity of the silver nitrate solution. 

The catalytic efficiency increases, under comparable conditions (pH, concentration of catalyst, etc.) in the sequence Cl < Br - S(CH3)2 < SCN < SC(NH2)2 < I . Titration with a calibrated solution of NaN02 (usually 0.05 to 0.10 m) is used for the analytical determination of aromatic amines, dissolved in aqueous H2S04 or HC1. Here nucleophilic catalysis is achieved by adding KBr. This allows a titration to be completed much faster than without that addition. 

This reaction serves as a known model reaction to characterize mass transfer efficiency in micro reactors [5]. As it is a very fast reaction, solely mass transfer can be analyzed. The analysis can be done simply by titration and the reactants are inexpensive and not toxic (although caustic). 

Since reproducibility of the flow system is critical to obtaining reproducibility, one approach has been to substitute lower-performance columns (50-to 100-p packings) operated at higher temperatures.1 Often, improvements in detection and data reduction can substitute for resolution. Chemometric principles are a way to sacrifice chromatographic efficiency but still obtain the desired chemical information. An example of how meaningful information can be derived indirectly from chromatographic separation is the use of system or vacancy peaks to monitor chemical reactions such as the titration of aniline and the hydrolysis of aspirin to salicylic acid.18... 

However, in a similar solvent with autoprotolysis such as m-cresol, 100% current efficiency could not be obtained in anodic oxidation with respect to base titrations. According to a further study158, the following reaction scheme seems most probable ... 

Infants and children older than 1 year of age are considered to be very efficient metabolizers of drugs and may actually require larger doses than those predicted by weight adjustment of adult doses or shorter dosing intervals [33], On the basis of metabolic activity, sustained-release formulations would appear to be ideal for children 1-10 years old, if bioavailability issues prove not to be problematic. The ability to clear drugs in critically ill children may be severely compromised therefore, dosing in this subgroup of patients requires careful titration [34]. 

---