Titrimetry Methods, precipitation titrations

Titrimetry Methods
Precipitation Titrations

Precipitation reactions in aqueous solution range from rapid to slow, depending on the identity of the precipitant. Many precipitations are sufficiently rapid and complete to form the basis of quantitation by titration. Precipitation titrimetry has several advantages over precipitation gravimetry, including speed, sensitivity, and convenience.

Common Titrants

Argentometric Titrations

Most precipitation reactions involve the silver cation, Ag+. Silver precipitations are rapid and
quantitative, and silver nitrate, AgNO3, is used for the direct titration of a number of anions that
precipitate silver: all the halides except F–; SCN–, CNO–, AsO43–, PO43–, CN–, C2O42–, CO32–, S2–, CrO42–. See table 7-1 on page 167 in Harris for more detail. Titrations using AgNO3 as titrant are termed argentometric titrations. Sodium chloride is suitable as a primary standard, and is most often used for standardization of the titrant in argentometric titrations. Solid silver nitrate is also available in high enough purity to serve as a primary standard, but it is more expensive. Silver nitrate solutions are stable in the dark, and amber bottles are used for storage. Exposure to light can cause photoreduction of the silver cations, particular in the presence of trace impurities that may catalyze the reaction.
Sulfate Analysis The sulfate content of an aqueous solution may be determined by titration with aqueous barium chloride, BaCl2. The titrant is usually standardized using sodium sulfate.
Fluoride Analysis Fluoride cannot be analyzed by argentometric titration (AgF is soluble); instead, the sample may be titrated with lanthanum nitrate, La(NO3)3, or lead nitrate, Pb(NO3)2, since both LaF3 (pKsp = 16.2) and PbF2 (pKsp = 7.57) are insoluble. Sodium fluoride is a suitable primary standard.

Endpoint Detection

A variety of chemical indicators are used to indicate the endpoint of argentometric titrations: the
Fajans, Volhard, and Mohr methods are discussed in some detail in the lab handout Titrimetric
Analysis of Chloride, and in your textbook (Harris chapter 7). A silver wire or ring is a sufficient indicator electrode for potentiometric titrations using AgNO3, while a fluoride ISE is suitable for potentiometric endpoint detection for fluoride analysis using La3+
or Pb2+ titrant solutions.

Example Application: Analysis of Chloride in Surface Waters

Chloride is frequently a major anion in surface and groundwater; certainly is a major constituent of seawater. Although chloride in freshwater is usually of geological origin, runoff from roads salted during the winter may significantly increase the chloride content of surrounding streams, rivers and lakes. A high chloride concentration may impart a noticeably salty taste to potable water, and can also damage metallic pipes and growing plants. Argentometric titration of water samples is a standard method for chloride determination;
concentrations in the low ppm range may be detected using potentiometric titration.

Literature

Arthur I. Vogel "A Text-Book of Quantitative Inorganic Analysis", 2nd edition.
HA Laitinen, WE Harris,"Chemical Analysis", 2nd edition.