Multicommutation in flow analysis. Part 5: Binary sampling for sequential spectrophotometric determination of ammonium and phosphate in plant digests

A flow set up for the spectrophotometric determination of ammonium and orthophosphate in acidic digests of plant materials is described. The flow network comprises solenoid valves assembled so that multicommutation and binary sampling can be implemented, providing facilities to determine sequentially these two analytes. Determinations are based on the molybdenum blue and indophenol blue methods. The flow network presents an active hardware, controlled by a microcomputer running a software written in Quick BASIC 4.5, and affords facilities to determine the analytes employing a single pumping channel. The volumes of sample and reagents, and the flow rates are settled previously as entry data. Profitable features such as a throughput of 80 determinations per hour, and low reagents consumption (180 μl salicylic acid, 120 μl sodium hypochloride, 120 μl ammonium molybdate and 80 μl ascorbic acid per determination) were achieved. A 2.0% relative standard deviation for ammonium and 1.5% for orthophosphate (n = 8), using a typical sample with 73.1 mg/l nitrogen and 8.3 mg/l phosphorus were estimated. Accuracy was ascertained by comparing data obtained for phosphorus by inductively coupled plasma atomic emission spectrometry and for nitrogen by titration as ammonium with sodium hydroxide. On applying the paired t-test between results, no significant difference at the 95% confidence level was observed.