84 



6.1 TURBIDIMETRY 



Most of the S analyses reported in this dissertation were done by 

 turbidimetry . The procedure used in this study is given in Appendix C. 

 The turbidimetric method is rapid and sensitive, but there are a num- 

 ber of problems and limitations with this technique which should be 

 mentioned. Turbidimetry is subject to many interferences, and the 

 formation of reproducible BaSO, suspensions under uniform precipitating 

 conditions is extremely difficult. High concentrations of Na, K, Ca , 

 Mg, N0_, PO , , and SiO can all interfere with the turbidimetric sulfate 

 determination, but only large amounts of Ca are likely to cause sig- 

 nificant interference in practice (Butters and Chenery, 1959) . Col- 

 loidal organic matter can interfere with the precipitation of BaSO, 

 by acting as a protective colloid in solutions low in S and by copre- 

 cipitating with BaSO, at high concentrations of S. 



Uniform precipitating conditions are essential in order to obtain 

 reproducible results. The use of concentrated acetic and phosphoric 

 acids buffers the pH and reduces variability due to acid concentration. 

 A seed suspension of small and uniformly sized BaSO, crystals acts as 

 nuclei or seed crystals and insures more rapid precipitation and uni- 

 form results at low concentrations. Uniform tubes, stoppered and 

 carefully inverted a given number of times, also help to insure uni- 

 form precipitating conditions. Gum acacia added to the solution acts 

 as a stabilizer and provides for greater reproducibility of the sus- 

 pensions . 



6.2 INDIRECT METHODS 



Sulfate S in soils analyzed in the study of S distribution in 

 selected Florida soils was- determined bv the indirect method of Hue 



