APPENDIX C 

 PROCEDURE FOR THE TURBIDIMETRIC DETERMINATION OF 

 SULFATES IN SOIL EXTRACTS, DIGESTED SOIL 

 EXTRACTS, AND DIGESTED PLANT TISSUE 



REAGENTS 



Acetic/phosphoric acid : Mix 900 ml concentrated acetic acid with 



300 ml concentrated H.PO, . 



3 4 



Gum acacia solution : Dissolve 5 g gum acacia in 500 ml hot water. 

 Filter hot through a Whatman no. 42 filter paper on a Buchner funnel 

 using suction. Cool and dilute to 1 liter with acetic acid. 



Barium sulfate seed suspension : Dissolve 18 g BaCl2 - 2H20 in 44 ml hot 

 water. Add 0.5 ml of the 2,000 ppm S standard. Bring to a boil, and 

 cool quickly. Add 4 ml of the gum acacia solution and mix well. Pre- 

 pare fresh daily. 



Barium chloride solution : Add 200 g BaCl2'2H20 to a 1-liter volumetric 

 flask. Add enough hot water to dissolve. Cool and dilute to volume. 



Standard sulfate solution (2,000 ppm S) : Dissolve 1.0875 g of oven- 

 dried K2SO4 in 0.1 N HC1 and dilute to 100 ml. Prepare a 10 ppm S 

 working standard daily by diluting 1 ml of the 2,000 ppm S standard 

 to 200 ml with demineralized water. 



PROCEDURE 



1. Prepare standards containing to 100 ug S by pipetting appropriate 

 volumes of the 10 ppm S standard into test tubes calibrated at 



25 ml. Standards of 0, 20, 40, 60, 80, and 100 ug S are usually 

 adequate to establish a curve. 



2. Pipette an aliquot of sample containing 20 to 80 ug S into a test 

 tube. Do not run more than 30 samples with a single set of 

 standards. 



3. Add 2 ml of the acetic/phosphoric acid. 



4. Add water to a volume of approximately 18 ml; stir on a vortex 

 mixer. 



5. Add exactly 0.5 mi of the barium sulfate seed suspension. 



6. Add I ml of the barium chloride solution. 



7. Stopper each tube with a plastic stopper and invert three times. 

 Wait 10 minutes and invert three more times. Wait 5 minutes and 

 invert three more times. 



170 



