171 



8. Remove stoppers and add 1 ml of the gum acacia solution. Dilute 

 to volume (25 ml) with deraineralized water. Stopper again and 

 invert 3 times. 



9. Allow samples to stand for 1.5 hours. 



10. Invert 10 times and read absorbance or transmittance on spectro- 

 photometer set on a wavelength of 440 nm. The wavelength is not 

 critical since only light blockage by the BaSO, precipitate is 

 measured and not absorbance. The standard curve is usually linear 

 from 10 to SO yg S. 



