Titration precipitation

In most precipitation titrations, silver nitrate solution is used and a silver salt, AgA, precipitates from solutions of an anion A. At the end-point, [Ag ]=[A ] and the solubility product S of AgA is given by  

Adapted from Vogel s Textbook of Quantitative Inorganic Analysis, 4th Edn, J. Bassett, R.C. Denney, G.H. Jeffery and J. Mendham, Longman Scientific and Technical, Harlow (1978), p. 280. 

Precipitation is the term used to describe the process whereby a substance leaves solution rapidly, forming either a crystalline solid or amorphous solid (the precipitate). In the case of a precipitation titration, this process occurs when the analyte forms a precipitate with the titrant. The most common types of precipitation titrations use silver nitrate as the titrant. They are often referred to as argentimetric titrations. 

Titration curves used in precipitation reactions usually use a concentration-dependent variable called the p function rather than the concentration itself The p function for a species X is defined as follows  

When 25 mL of 0.1 mol L AgN03 has been added, 75 mL of NaCl remains in a total volume of 125 mL. Therefore, the concentration of the chloride ion is given by 

Beyond the equivalence point the situation changes. For 100.1 mL AgNOs solution  

Argentimetric is derived from the Latin argentum, which means silver. 

To be exact the pfunction should be defined in terms Of activities instead of concentrations. 

Titration curves - plots of concentration of titrand against volume of titrant used. 

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Table 11.30 lists standard solutions for precipitation titrations and Table 11.31 lists specific reagents as indicators, adsorption indicators, and protective colloids for precipitation titrations. 

The list given below includes the substances that are most used and most useful for the standardization of solutions for precipitation titrations. Primary standard solutions are denoted by the letter (P) in Column 1. 

Examples of titration curves for (a) a complexation titration, (b) a redox titration, and (c) a precipitation titration. 

Thus far we have examined titrimetric methods based on acid-base, complexation, and redox reactions. A reaction in which the analyte and titrant form an insoluble precipitate also can form the basis for a titration. We call this type of titration a precipitation titration. 

The titration curve for a precipitation titration follows the change in either the analyte s or titrant s concentration as a function of the volume of titrant. For example, in an analysis for V using Ag+ as a titrant... 

Sketching the Titration Curve As we have done for acid-base, complexometric titrations, and redox titrations, we now show how to quickly sketch a precipitation titration curve using a minimum number of calculations. 

How to sketch a precipitation titration curve see text for explanation. 

Initial attempts at developing precipitation titration methods were limited by a poor end point signal. Finding the end point by looking for the first addition of titrant that does not yield additional precipitate is cumbersome at best. The feasibility of precipitation titrimetry improved with the development of visual indicators and potentiometric ion-selective electrodes. 

Finding the End Point Potcntiomctrically Another method for locating the end point of a precipitation titration is to monitor the change in concentration for the analyte or titrant using an ion-selective electrode. The end point can then be found from a visual inspection of the titration curve. A further discussion of potentiome-try is found in Chapter 11. 

Precipitation titrimetry is rarely listed as a standard method of analysis, but may still be useful as a secondary analytical method for verifying results obtained by other methods. Most precipitation titrations involve Ag+ as either an analyte or... 

The scale of operations, accuracy, precision, sensitivity, time, and cost of methods involving precipitation titrations are similar to those described earlier in the chapter for other titrimetric methods. Precipitation titrations also can be extended to the analysis of mixtures, provided that there is a significant difference in the solubilities of the precipitates. Figure 9.43 shows an example of the titration curve for a mixture of % and Ch using Ag+ as a titrant. 

Einally, the last experiment describes an unustml precipitation titration. 

The reaction of a positively charged polyelectrolyte with a negatively charged polyelectrolyte produces a precipitate, forming the basis for a precipitation titration. This paper provides an overview of colloid titrations, discussing... 

Potcntiomctric Titrations In Chapter 9 we noted that one method for determining the equivalence point of an acid-base titration is to follow the change in pH with a pH electrode. The potentiometric determination of equivalence points is feasible for acid-base, complexation, redox, and precipitation titrations, as well as for titrations in aqueous and nonaqueous solvents. Acid-base, complexation, and precipitation potentiometric titrations are usually monitored with an ion-selective electrode that is selective for the analyte, although an electrode that is selective for the titrant or a reaction product also can be used. A redox electrode, such as a Pt wire, and a reference electrode are used for potentiometric redox titrations. More details about potentiometric titrations are found in Chapter 9. 

Procedure B. Pipette 25 mL of the diluted solution into a 250 mL conical flask containing 5mL 6 M nitric acid. Add a slight excess of standard 0.1M silver nitrate (about 30 mL in all) from a burette. Then add 2-3 mL pure nitrobenzene and 1 mL of the iron(III) indicator, and shake vigorously to coagulate the precipitate. Titrate the residual silver nitrate with standard 0.1M thiocyanate until a permanent faint reddish-brown coloration appears. 

The indicator electrode employed in a potentiometric titration will, of course, be dependent upon the type of reaction which is under investigation. Thus, for an acid-base titration, the indicator electrode is usually a glass electrode (Section 15.6) for a precipitation titration (halide with silver nitrate, or silver with chloride) a silver electrode will be used, and for a redox titration [e.g. iron(II) with dichromate] a plain platinum wire is used as the redox electrode. 

Titrations can be carried out in cases in which the solubility relations are such that potentiometric or visual indicator methods are unsatisfactory for example, when the reaction product is markedly soluble (precipitation titration) or appreciably hydrolysed (acid-base titration). This is because the readings near the equivalence point have no special significance in amperometric titrations. Readings are recorded in regions where there is excess of titrant, or of reagent, at which points the solubility or hydrolysis is suppressed by the Mass Action effect the point of intersection of these lines gives the equivalence point. 

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