Analytical Chem istry - DePauw University

640 Analytical Chemistry 2.0The best way to appreciate the theoreticaland practical details discussed in this sectionis to carefully examine a typical analyticalmethod. Although each method isunique, the following description of thedetermination of sulfate in water providesan instructive example of a typicalprocedure. The description here is basedon Method 4500–SO 4 2– –C in StandardMethods for the Analysis of Water andWastewater, American Public Health Association:Washington, D. C. 20 th Ed.,1998.Table 10.15 Examples of Analytes Determined byTurbidimetry or Nephelometryanalyte precipitant precipitateAg + NaCl AgClCa 2+ Na 2 C 2 O 4 CaC 2 O 4Cl – AgNO 3 AgClCN – AgNO 3 AgCN2–CO 3 BaCl 2 BaCO 3F – CaCl 2 CaF 22–SO 4 BaCl 2 BaSO 4mittance or scattering, as given by equation 10.32 or equation 10.33 isproportional to the concentration of the scattering particles, which, in turn,is related by the stoichiometry of the precipitation reaction to the analyte’sconcentration. Examples of analytes that have been determined in this wayare listed in Table 10.15.Representative Method 10.5Turbidimetric Determination of Sulfate in WaterDescription o f Me t h o dAdding BaCl 2 to an acidified sample precipitates SO 4 2– as BaSO 4 . Theconcentration of SO 4 2– may be determined either by turbidimetry or bynephelometry using an incident source of radiation of 420 nm. Externalstandards containing know concentrations of SO 4 2– are used to standardizethe method.Pr o c e d u r eTransfer a 100-mL sample to a 250-mL Erlenmeyer flask along with 20.00mL of a buffer. For samples containing >10 mg SO 4 2– /L, the buffer contains30 g of MgCl 2. 6H2 O, 5 g of CH 3 COONa.3H 2 O, 1.0 g of KNO 3 ,and 20 mL of glacial CH 3 COOH per liter. The buffer for samples containing