Accuracy titration method

Calibration solutions of cadmium and lead have been prepared from high purity (higher than 98%) cadmium and lead nitrate dissolved in nitric acid. A high accuracy titration method (EDTA complexometry reaction with photometric detection) was used to determine the final... 

A titration method for primary and secondary nitroparaftins uskig hypochlorite gives good accuracy ( 0.1%) (84). It is based on the foUowkig equation ... 

Even in the case of the more exact determination of endpoints by spectrophotometry, doubts still remain regarding the accuracy of the indicator titration method. For example, there is always the possibility that certain... 

This review updates a classic and influential review by Halevi.3 What makes the current review timely is the recent development of a nuclear magnetic resonance (NMR) titration method capable of exquisite accuracy and not subject to the systematic error associated with possible impurities in one of the samples and not in the other. New values can now be compared with previous ones. 

This NMR titration method was subsequently applied to equilibrium IEs on acidity.30 33 Like the previous methods, it too benefits from the high sensitivity of 13C and 19F chemical shifts, and even 111 chemical shifts, to both isotopic substitution and state of protonation. Figure 1 shows the NMR titration of a mixture of tri(methyl-d)amine and tri(methyl-t/2)amine in D20, plotted according to Equation (19). The slope is 1.1618 0.0004. The intercept is -0.0061 0.0046, properly zero. The correlation coefficient is an impressive 0.999999, which is an indication of the accuracy achievable. Another remarkable result was the measurement of the relative basicity of the two exceedingly similar isotopomers of 1 -benzyl-4-methylpiperidine-2,2,6-t/3 (6). These are truly isotopomers (here stereoisomers), which bear the same number of isotopic substitutions and differ only in the position of the isotope, which is either axial or equatorial. 

Determination of the amount of substance is thus in direct relation to basic units of the SI system and does not need a RM for comparison. The Faraday constant is one of the fundamental constants (it can be expressed as the product of the electron charge and the Avogadro constant). It enables the attainment of high precision and accuracy and is independent of the atomic weights of the elements in the sample. Its drawback is lower selectivity, a feature common to titration methods. This makes coulometry especially suitable for determination of relatively pure substances used as standards by other (relative) methods. The Faraday constant has been proposed as an ultimate standard in chemistry [3],... 

Instead, a wide variety of spectroscopic and electrochemical titration methods are often employed to determine the equilibrium constants for a molecular recognition process at several different temperatures, which are then analyzed by the van t Hoff equation to give the thermodynamic parameters for the process. However, there is a critical tradeoff between the accuracy of the value obtained and the convenience of the measurement since the thermodynamic parameters, evaluated through the van t Hoff treatment, do not take into account the possible temperature dependence of the enthalpy change, i.e. heat capacity, and are less accurate in principle. In fact, it has been demonstrated with some supramolecular systems that the van t Hoff treatment leads to a curved plot and therefore the thermodynamic parameters deviated considerably from those determined by calorimetry.3132 Hence one should be cautious in handling thermodynamic parameters determined by spectroscopic titration and particularly in comparing the values for distinct systems determined by different methods. 

For maximum accuracy, the batch method (separate solutions prepared in volumetric flasks) is preferred, but a titration method (aliquots of ligand solution added to a single solution in a spectrophotometer cell) may be necessary in some situations. Spectrophotometry permits the use of much lower concentrations of metal ion and ligand than are feasible in the pH titration method, if the molar absorptivity of the complex is high. 

The current-time measurements required for a coulometric titration are inherently as accurate as or more accurate than the comparable volume/molarity measurements of a conventional volumetric method, particularly when small quantities of reagent are involved. When the accuracy of a titration is limited by the sensitivity of the end point, the two titration methods have comparable accuracies. 

Diphenylurea has a solubility of 5.5 mg in 100 cm 3 of saturated aniline solution [1560], and for improved accuracy in phosgene determinations it is recommended that phosgene is passed into the aniline solution until a small amount of precipitate is formed. This is then filtered off, and the clear filtrate (now saturated with diphenylurea) is subsequently used for the phosgene determination [1560]. Although traces of phosgene can be readily detected by the appearance of slight turbidity in diphenylurea-saturated aniline solutions, the filtration and weighing of sueh precipitates is impractical, and the method is not as accurate as the iodine titration method (Section 3.2.1.2) [1560]. 

Titration method The antiserum to be tested is diluted serially to establish the limit at which enzymic reaction can no longer be detected. Visual inspection is possible (about similar detectability as absorptiometry). The concept is simple and the test is quantitative for the antibody activity. The definition of the titre, however, may be confusing since with one method the titre may be much larger than with another for the same serum. Titrations generally are workintensive and costly. It is difficult to establish by visual inspection the dilution (e.g. 1 512, or reciprocals, e.g., 512) at which the real end-point occurs. Thus, reproducibility may easily be one two-fold dilution difference from the accurate value. Therefore, a titre of, e.g. 1 4096 implies a false sense of accuracy (Section 1.3). Titration by visual inspection yields a discontinuous scale of results. 

It must be remarked, however, that the curves shown in Fig. 3 are hardly typical. Two definite points of inflection are observed for a dibasic acid only if the two ionization constants are of quite different orders of magnitude, Auerbach and Smolczyk7 have shown, theoretically, that the two points of inflection will not appear unless the ratio Ki/E2 is greater than 16. However, it is safe to say that the ratio must be considerably greater than that if the inflection points are to be obtained experimentally with any accuracy. Also, if the two ionization constants are not quite different the values of the constants determined by the potentiometric titration method described above may be somewhat in error, as is shown by the authors just mentioned and by Simms. ... 

In Figure 3.21 the second intermetallic compound LijSb also looks like a stoichiometric compound. The high accuracy of the coulometric titration method reveals that this compound has a very small region of variable stoichiometry of Lij+ Sb with = 10". This is shown in Figure 3.22. 

Water-soluble potassium (expressed as K2O) can be determined by manual or automated flame photometric and tetraphenylboron titration methods for all types of fertilizer samples. Atomic absorption can be used for samples below 5% K2O to maintain the highest degree of precision and accuracy. (Note Tetraphenylboron method for potassium is being used less often because of safety issues associated with formaldehyde.) Methods using ICP-OES for high concentration of potassium are currently being developed. Note Potentiometric measurements can be used to determine potassium in fertilizers but are not official approved methods and do not meet the accuracy and precision requirements as current methods. 

A variant of the technique devised by Ishizawa et al examines the transmission pattern of an He-Ne laser beam to study pressure dependence of LCST with high accuracy . Coupled with titration methods (that is varying composition by addition of solvent or solution), turbidimetry has been used to good effect to study cloud point curves in polymeri-polymerj-solvent and ionic polymer systems and to obtain solubility parameters for copolymers in mixed solvents. ... 

Almgren et aL (1982) have reported a precision of 0.2 % for this method, but differences as high as 1 % between repeat determinations. As with the potentimnetric titration methods, accuracy will depend to a large degree on the accurate of the volume measurements and of the hydrochloric acid concentration. 

Calcium may be determined by atomic absorption spectrophotometry. This technique has been used for the estimation of calcium in biological fluids and agricultural materials. It is a speedy method, superior to the more tedious chemical determination by oxalate precipitation and more specific than the EDTA titration method. The accuracy is of the order of 2 per cent and sensitivity limits have been reported at 0 08 to 1 p.p.m. of calcium in solution. The interference problems are very similar to those experienced with the emission method although not quite so formidable. 

For samples in which the sulphur concentration is below 50 Mg/g, the combustion - borate titration method has known limitations the volume of titrant, even with a very dilute (0.005 N) sodium borate solution, is small, and the colour change of the indicator at the end-points is difficult to establish with reasonable accuracy. 

The reactivity of the epoxides towards certain nucleophiles, and optionally in combination with electrophilic catalysis [173], is also used to characterize epoxidized PBDs. The epoxide content in PBDs is usually determined by titration methods using acids that add to epoxides. Hydrogen bromide [174], perchloric acid [175], and hydrochloric acid [166,176] are typical examples they can expand the accuracy of measurement in combination with spectroscopic techniques [177]. The polymeric products are hydrins. 

In this technique, a titration of the sample (enclosed in a Dewar container) is performed with an automatic motor-driven buret at the same time as the sample solution temperature measurement. Temperature changes are very small (0.001-0.2°C), and therefore temperature measurements must be made with considerable accuracy. The method yield heats of reaction for a particular chemical system. Applications of the method include determination of the concentration of an unknown, reaction stoichiometry, and thermodynamic parameters. These can be performed in nonaqueous solvents and fused salts. 

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