Crystal seeding in flow-injection turbidimetry: determination of total sulfur in plants

Reproducible addition of on-line produced suspensions is proposed to improve supersaturation conditions in flow turbidimetry. This crystal seeding leads to a simplification in system design, and an improvement in sampling rate and/or sensitivity in procedures usually limited by rate of turbidity formation. The feasibility of the approach is demonstrated in developing a turbidimetric flow-injection procedure for determination of total sulphur in plant materials based on lead sulphate precipitation after adding a confluent stream with lead phosphate nucleant. Effects of reagent concentrations, acidity, temperature and timing were investigated, and timing and ethanol content manifested themselves as the most relevant parameters. 400 samples may be run per hour, special procedures for system washing are not needed, surfactant is not required and baseline drift is not observed after operation periods of 8 h. Results are precise (R.S.D. < 0.005 for 5–25 mg S l−1) and in agreement with those obtained by the usual flow-injection procedure based on barium sulphate.