Exer 9.docx

Chemical indicators produce a color change or occasionally the appearance or disappearance of turbidity in the solution being titrated. The requirements for an indicator for a precipitation titration are that (1) the color change should occur over a limited range in p-function of the titrant or the analyte and (2) the color change should take place within the steep portion of the titration curve for the analyte. For example, in Figure 17-3, we see that the titration of iodide with any indicator providing a signal in the pAg range of about 4.0 to 12.0 should give a satisfactory end point. Note that, in contrast, the end-point signal for the titration of chloride would be limited to a pAg of about 4.0 to 6.0. (Skoog)

Precipitation titrations are based on reactions that yield ionic compounds of limited solubility. Precipitation titrimetry is one of the oldest analytical techniques, dating back to the mid-1800s. The slow rate at which most precipitates form, however, limits the number of precipitating agents that can be used in titrations to a handful. (skoog, page 449) Precipitation titrations has it foundations in the formation of precipitates, an ionic compound that is limited of solubility. The solubility of a compound is defined by its Ksp value, or the solubility constant. There are various factors considered in choosing a precipitating reaction. First, there should be a complete formation of precipitate. Ksp is accountable for this. When Ksp is of higher value, meaning, there is a complete reaction between the compounds, then it’s possible that little to no amount of precipitate will be formed (Skoog, 2013). Second, a precipitation reaction should be stoichiometric. There should be a reasonable ratio in the reaction. Lastly, there should be an ideal indicator, which are few in precipitation titrations. Because of these reasons, only few compounds are found to be suitable in precipitation titration. Titration curves for precipitation reactions are calculated in a completely analogous way to the methods described in Section 14B for titrations involving strong acids and strong bases.Sta

Standardization of KSCN In the standardization, the primary standard used is In standardization of KSCN, addition of Fe3+ is necessary as it serves to be the indicator. When adding AgNO3 in the solution, it is expected to react with the SCN. When the amount of SCN is exhausted with its reaction the AgNO3, it will react with the Fe3+ and will form a blood-red solution with AgNO3. The reaction is: ======== add reaction ========================= Note that at equivalence point, when the solution turned to blood-red, meq AgNO3 = meq KSCN.

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