NITROPRUSSIDE AND METHYLENE-BLUE METHODS FOR SILICONE MEMBRANE DIFFERENTIATED FLOW-INJECTION DETERMINATION OF SULFIDE IN WATER AND WASTE-WATER

Hydrogen sulfide evolved from an acidified sample is preconcentrated by permeation in a stationary alkaline acceptor solution enclosed in a silicone rubber sample loop. Depending on the sample volume preconcentrated, the applicable analytical range spans low mu-g/L to tens of mg/L for both methods. The methylene blue method is more sensitive by a factor of approximately 30 and actually permits practical determinations in the sub-mu-g/L levels. The limit of detection (LOD) for the nitroprusside method ranges from 20-mu-g/L for a 20-mu-L sample by conventional flow-injection determination (no membrane, throughput 30 samples/h) to < 2-mu-g/L for 12 mL sample preconcentrated in the membrane system (throughput 5 samples/h). The membrane is highly resistant to fouling and permits analysis of untreated wastewater samples bearing suspended solids, oil, grease, etc. without any pretreatment. No significant interference is observed with either chemistry, Although the nitroprusside chemistry is less sensitive, it does not involve the use of concentrated aggressive reagents and is recommended unless ultratrace determinations are essential. Viable reaction mechanisms are proposed for both of these chemistries.