Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods639titative analysis, therefore, it is necessary to maintain a uniform distributionof particle sizes throughout the sample and between samples and standards.Most turbidimetric and nephelometric methods rely on precipitationto form the scattering particles. As we learned in Chapter 8, the propertiesof a precipitate are determined by the conditions under which it forms. Tomaintain a reproducible distribution of particle sizes between samples andstandards, it is necessary to control parameters such as the concentrationof reagents, the order of adding reagents, the pH and temperature, theagitation or stirring rate, the ionic strength, and the time between theprecipitate’s initial formation and the measurement of transmittance orscattering. In many cases a surface-active agent—such as glycerol, gelatin,or dextrin—is added to stabilize the precipitate in a colloidal state and toprevent the coagulation of the particles.Ap p l i c a t i o n sTurbidimetry and nephelometry are widely used to determine the clarityof water, beverages, and food products. For example, a nephelometric determinationof the turbidity of water compares the sample’s scattering tothe scattering of a set of standards. The primary standard for measuringturbidity is formazin, which is an easily prepared, stable polymer suspension(Figure 10.65). 20 Formazin prepared by mixing a 1 g/100 mL solutionof hydrazine sulfate, N 2 H 4. H2 SO 4 , with a 10 g/100 mL solution of hexamethylenetetramineproduces a suspension that is defined as 4000 nephelometricturbidity units (NTU). A set of standards with NTUs between 0and 40 is prepared and used to construct a calibration curve. This methodis readily adapted to the analysis of the clarity of orange juice, beer, andmaple syrup.A number of inorganic cations and anions can be determined by precipitatingthem under well-defined conditions and measuring the transmittanceor scattering of radiation from the precipitated particles. The trans-20 Hach, C. C.; Bryant, M. “Turbidity Standards,” Technical Information Series, Booklet No. 12,Hach Company: Loveland, CO, 1995.NNNNhexamethylenetetramine+ 6H 2 O + 2H 2 SO 4 6H 2 CO + 2(NH 4 )SO 4nH 2 CO + (n/2)H 2 NNH 2NNNNformazinn/4+ nH 2 OFigure 10.65 Scheme for preparing formazinfor use as a turbidity standard.