Ch. 13 Titrimetric methods Titrimetric method Volummetric titrimetry Gravimetric titrimetry Coulometric titrimetry

1. General of titrimetry Standard solution(Standard titrant) Titration Back titration Equivalent point and end point of titration Titration error : difference in volume or mass between the equivalent point and the end point Indicator

Primary standards High purity. Atmospheric stability Absence of hydrate water so that the composition of the solid does not change with variations in humidity. Modest cost. Reasonable solubility in the titration medium Reasonably large molar mass so that the relative error associated with weighting the standard is minimized. Standard solutions Be sufficiently stable so that it is necessary to determine its concentration only once. React rapidly with the analyte so that the time required between additions of reagent is minimized. React more or less completely with the analyte so that satisfactory end points are realized Undergo a selective reaction with the analyte that can be described by a balanced equation

The accuracy of a titrimetric method : no better than the accuracy of the concentration of the standard solution Determination of standard solution Direct method : 정확히 무게를 달아 용량 플라스크에 용해 Standardization 무게를 잰 일차 표준 물질 무게를 잰 일정한 양의 2 차 표준물질 측정된 부피의 다른 용액을 적정하기 위하여 표준화 될 적정 시약을 사용 Secondary standard solution 2 차 표준 물질 또는 다른 표준용액으로 표준화 된 적정 시약

Volumetric calculation Molarity Normality : equivalent Gravimetric titrimetry Advantages * Calibration of glassware and tedious cleaning to ensure proper drainage are completely eliminated. * Temperature corrections are unnecessary because weight molarity does not change with temperature, in control to volume molarity. This advantage is particularly important in nonaqueous titrations because of the high coefficients of expansion of most organic liquids(about ten times that of water). * Weight measurements can be made with considerably greater precision and accuracy than can volume measurements. For example, 50 gr or 100 gr of an aqueous solution can be readily measured to ± 1 mg, which corresponds to ± mL. This greater sensitivity makes it possible to choose sample sizes that lead to significantly smaller consumtion of standard reagents. * Weight titrations are more easily automated than are volumetric titrations.

Titration curves End point changes in color due to the reagent, the analyte, or an indicator change in potential of an electrode that respond to the concentration of the reagent or the analyte Types of titration curves Sigmoidal curve, Linear segment curve

Concentration changes during titration

Titration curve for the titration of 50 mL of 0.1M AgNO 3 with 0.1M KSCN

Precipitation titrimetry

Effect of concentration on titration curve A : 50 mL of 0.05M-NaCl with 0.1M-AgNO 3 B : 50 mL of 0.005M-NaCl with 0.01M-AgNO 3

Effect of reaction completeness on titration curve 50 mL of 0.05M-anion solution with 0.1M-AgNO 3

Titration curves for mixtures of anions

Indicators for Argentometric titrations End point Chemical : color and turbidity color change : in limited range in p-function within jump Potentiometric Amperometric Mohr, Fajans, Volhard method