The Precision of Product Consistency Tests with Reference Glasses ARM-1 and SRM 623
An interlaboratory study (ILS) was conducted in 2014 to measure the precision with which the ASTM-International product consistency test method A (PCT-A) can be performed using properly prepared samples of reference glass ARM-1 or SRM 623 and test parameter values near the centers of ranges specified in procedure ASTM C1285. The ILS did not evaluate the full range of test parameter values allowed in the PCT-A method and the results do not represent the precision that should be expected if extreme values of the ranges are used. However, the precision measured in the ILS quantified the precision of tests conducted with mean parameters that were used to estimate the effects of individual parameters. Follow-on tests were conducted to assess the effects of several individual test parameter values on the test response and uncertainties in the derived glass dissolution rates. The effects of particle size distributions in glass samples prepared by wet- and dry-sieving methods, methods to remove fines, and the limiting parameter values for the water-to-glass (w/g) mass ratio and test duration allowed in PCT-A were measured. Test responses for all variations except test duration exceeded the precision measured in the ILS for conducting triplicate PCT-A tests within a single laboratory. The accuracy of glass dissolution rates derived from PCT results benefits from using a tighter control of these test variables than required in ASTM C1285. Comparison with previous analyses of the method shows how use of the relative standard deviation to represent the test precision can be misleading, and that the mean response and standard deviation should both be considered.