USE OF THE BLANK TEST FOR DETECTING THRESHOLD EFFECTS IN THE INDIRECTSPECTROPHOTOMETRIC DETERMINATION OF SULFATE-IONS IN TAP WATER

Four different types of blank measurements, two of which were obtained directly (placebo and method blanks) and the other two (total Youden and system blanks) by extrapolation, are examined. The latter are usua lly more suitable for analytical work in dynamic ranges far above the detection limit, whereas the former two are only pertinent in dealing with detection limits. However extrapolated blanks can indicate when t here is a risk of threshold effects emerging as the analyte concentrat ion approaches zero, because neither the fetal Youden nor the system b lank are affected by such effects. In addition, the presence of thresh old effects can impair both the measurement of detection limits and th e application of the method, obscuring the one-to-one correspondence b eween analytical signal and analyte amount. One case of a significant threshold effect is demonstrated in the indirect spectrophotometric de termination of the sulfate ion in tap water. Also, a general approach is put forward for establishing the detection limit as the intercept o f the lower confidance band with the analyte-amount axis (x-axis) from a calibration plot which includes the detection limit in its dynamic range, once the absence of threshold effects has been confirmed. Final ly, the absence of direct interference from the matrix in the results of the spectrophotometric method is demonstrated by comparison with th e results of the gravimetric determination of barium sulfate.