Periphyton ash weight estimation from residue not digested after hach method of kjeldahl protein nitrogen analysis

The portion of a periphyton sample not digested after acid/peroxide digestion during Hach method of the Kjeldahl procedure for protein nitrogen determination was examined as an estimator of ash weight. This method, used on periphyton biofilms grown in artificial streams and having ash weights in the 200–300 mg/g range, was found to yield results similar but statistically higher and less precise than the ashing technique traditionally employed for ash weight determinations. Student's t comparison of paired analyses by the two methods showed no difference at a significance level of α = 0.05 provided a constant of 7.6 mg/g is subtracted from results of the digestion method. It is suggested that the technique may be useful where there would otherwise be insufficient sample for ash weight determination.