Turbidimetric flow-injection determination of total nitrogen and potassium in vegetables

A turbidimetric flow-injection system was developed for the determination of total nitrogen and potassium in vegetable samples using a single spectrophotometer as detector. As a precipitating agent, 3.0% (w/v) sodium tetraphenylboron solution prepared in 2.0% (w/v) poly(vinyl alcohol) was used. A gas diffusion process was included in the manifold to separate ammonium ions from the rest of the sample and to allow paired analysis. Total nitrogen and potassium determinations were carried out on the solutions remaining in the acceptor and donor streams, respectively. Results obtained were precise (relative standard deviations <2.1 and 1.6% for total N (<25 mg g^-1) and K (<55 mg g^-1) determinations, respectively) and in agreement with those of reference methods. Analysis can be carried out at a rate of up to 35 samples per hour (corresponding to 70 determinations per hour) within concentration range 87-430 mg N-NH₄⁺ l^-1 and 78-390 mg K⁺ l^-1 for the total nitrogen and potassium determinations, respectively.