Spectrophotometric Determination of Total Nitrogen in Soils with On-line Gas-diffusion Separation

1 Spectrophotometric Determination of Total Nitrogen in Soils with On-line Gas-diffusion Separation [57] 

Mixed acid-base indicator 0.02 g cresoi red, 0.04 g bromothymol blue, and 0.08 g bromocresol purple dissolved in 3 ml 0.1 M NaOH, and diluted to 100 ml with 

Acceptor solution 10 ml buffer solution, 4 ml mixed indicator diluted to 500 ml with distilled water and adjusted to wine red colour by adding drop-wise dilute acid or base. The absorbance of the solution at 590 nm should be within 0.25-0.3 with water as reference. 

1 FI manifold for the determination of total nitrogen in soil digests by spectrophotometry with gas-diffusion separation. S, sample R, 5 M sodium hydroxide SR membrane gas-diffusion separator A, buffer acceptor stream with acid-base indicator, D, detector, and W, waste outlets for donor and acceptor streams [57]. 

The FI manifold and operational parameters are shown in Fig. 8.1. A 10-s loading period with 30-s injection is recommended. 100 p sample is injected into a water carrier after being filled into the sample loop, and merged downstream with the sodium hydroxide to generate ammonia which is transferred through the gas diffusion membrane into the acceptor stream. The absorbance change of the acceptor is recorded and peak heights taken for NH4-N concentration evaluation. 

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