A simplified rubazoic acid method for the determination of ammonia on a semi-micro scale is described. The absorbance of a pink-violet aqueous solution of rubazoic acid is measured at 550 nm. The method can be directly applied to fresh waters with NH4+-N concentrations of between 20 and 1000 #x3BC;g. 1#x2212;1The semi-micro method was compared with the original extraction rubazoic acid method of Proch#xE1;zkov#xE1; (1964) and the results showed good agreement (R2= 0.98, n = 25). The direct nesslerization method gave significant overestimates
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