Acid-base titration accuracy

The accuracy of a standardization depends on the quality of the reagents and glassware used to prepare standards. For example, in an acid-base titration, the amount of analyte is related to the absolute amount of titrant used in the analysis by the stoichiometry of the chemical reaction between the analyte and the titrant. The amount of titrant used is the product of the signal (which is the volume of titrant) and the titrant s concentration. Thus, the accuracy of a titrimetric analysis can be no better than the accuracy to which the titrant s concentration is known. 

Accuracy When working with macro-major and macro-minor samples, acid-base titrations can be accomplished with relative errors of 0.1-0.2%. The principal limitation to accuracy is the difference between the end point and the equivalence point. 

The scale of operations, accuracy, precision, sensitivity, time, and cost of methods involving redox titrations are similar to those described earlier in the chapter for acid-base and complexometric titrimetric methods. As with acid-base titrations, redox titrations can be extended to the analysis of mixtures if there is a significant difference in the ease with which the analytes can be oxidized or reduced. Figure 9.40 shows an example of the titration curve for a mixture of Fe + and Sn +, using Ce + as the titrant. The titration of a mixture of analytes whose standard-state potentials or formal potentials differ by at least 200 mV will result in a separate equivalence point for each analyte. 

A double end point, acid—base titration can be used to determine both sodium hydrosulfide and sodium sulfide content. Standardized hydrochloric acid is the titrant thymolphthalein and bromophenol blue are the indicators. Other bases having ionization constants in the ranges of the indicators used interfere with the analysis. Sodium thiosulfate and sodium thiocarbonate interfere quantitatively with the accuracy of the results. Detailed procedures to analyze sodium sulfide, sodium hydro sulfide, and sodium tetrasulfide are available (1). 

Emphasis was therefore put on analytical procedures able to determine many elements in parallel and/or requiring almost no previous separation. procedures preferred were X-ray fluorescence using a Am source and Si(Li)-detector, atomic absorption spectrophotometry, gamma spectrometry using tracer isotopes and Ge(Li)-detector and acid-base titrations with recording of the pH-volume derivative. Table 2 summarises the use of these methods for the different elements, and it also gives a rough indication of interferences, sensitivity and accuracy obtained. 

More than brief discussion of the numerous ways in which end points can be taken other than by visual methods is beyond our scope. For example, end-point techniques may involve photometry, potentiometry, amperometry, conductometry, and thermal methods. In principle, many physical properties can be used to follow the course of a titration in acid-base titrations, use of the pH meter is common. In terms of speed and cost, visual indicators are usually preferred to instrumental methods when they give adequate precision and accuracy for the purposes at hand. Selected instrumental methods may be used when a suitable indicator is not available, when higher accuracy under unfavorable equilibrium conditions is required, or for the routine analysis of large numbers of samples. 

An indicator used for any acid-base titration should ideally change colour at the pH corresponding to the mid-point of the almost vertically straight portion of the titration curve. (However, there is little loss in accuracy if the indicator changes colour anywhere within the range of the almost vertically straight portion of the curve, since the pH change is relatively... 

What are some of the advantages of nonaqueous acid-base titrations First of all, they are cheap. Secondly, they are simple in many cases you don t need a trained chemist to do them. A third advantage is that nonaqueous acid-base titrations are extremely accurate. For the analysis of organic compounds where you are assaying a major constituent, nonaqueous acid-base titrations are unmatched in their accuracy. I think it is well to reme-ber that some of the very excellent instrumental methods are, in many cases somewhat inaccurate. Finally, why use some smelly solvent such as acetic acid or pyridine, as is frequently necessary, instead of water or perhaps water mixed with alcohol The reason for this is that nonaqueous titrations are much broader in their application than are acid-base titrations carried out in water or in water mixtures with a waterlike solvent such as alcohol. 

Potentiometric acid-base titration is one of the most accurate and most widely applicable methods. In the absence of interferences, the accuracy which can be achieved is usually limited only by volumetric errors, preparation of titrants of known strength, and factors related to equilibrium constants of the titration reaction Problems involved in the availability and selection of an indicatoi are avoided. Also, since the property measured is a change ir potential rather than a change in the absorption of light, colorec and turbid solutions do not impose difficulties. 

The precision and accuracy with which the end point can be detected is a vital factor in all titrations. Because of its simplicity and versatility, chemical indication is quite common, especially in acid-base titrimetry. 

Analytical chemistry has found great utility in conductimetric measurements in spite of its apparent nonspecificity. Rapid quantitative accuracy of a few tenths of a percent may be quickly accomplished by direct conductimetric determination of binary electrolytic solutions such as aqueous acids, bases, or salts. A nearly linear increase in conductivity is observed for solutions containing as much as 20% of solute. The concentration of strong solutions, such as the salinity of seawater, may be determined from conductance measurements traces of electrolyte impurities, such as the impurity in ultrapure water, may be reported at the pgl level. Conductimetric titrations may increase the accuracy of endpoint detection and permit titrimet-ric analysis of weak electrolytes, such as boric acid, which is not feasible by potentiometric or colorimetric... 

Titrimetric methods with potentiometric end point location can be applied when an electrode with the needed selectivity is not available. The precision and accuracy of potentiometric titrations are superior comparing it with the properties of direct potentiometry. However, the concentration range where potentiometric titration can be used effectively is narrower. A solution with analyte concentration below 1 mM seldom is determined by potentiometric titrations. Potentiometric end point location is most often employed in the case of acid-, base-, precipitate-, redox-, or complexometric titrations. 

Table I shows results of repeated titrations with automatic stop using 2- /xa difference current for this acid—base system. From the stoichiometry of this neutralization, 1 ml of titrate is equivalent to 0.122 ml of titrant. The accuracy was approximately Zio. The standard deviation from the arithmetical mean, about 0.5, was calculated using inefficient statistics [3].

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