Performance characteristics of methods of analysis used for regulatory purposes. I. Drug dosage forms. F. Gravimetric and titrimetric methods

The original gravimetric and titrimetric methods approved by AOAC for the analysis of pharmaceutical preparations, particularly during the period 1915-1950, show precision, recovery, and outlier parameters approximately the same as those exhibited by the previously reviewed instrumental methods that are currently used. Fifty-nine published collaborative studies utilized gravimetric methods and 85 used titrimetric. The studies of the gravimetric methods encompassed 47 analytes, 95 dosage forms, and 136 assays; the corresponding figures for the titrimetric studies are 72, 112, and 152. An average of approximately 7 laboratories participated per study. The line of best fit of the relative standard deviation between-laboratories (RSDR) plotted against the negative logarithm of the fractional concentration, C, extends from 1.2 and 1.0% for the gravimetric and titrimetric methods, respectively, at 100% concentration to 2.2 and 2.8% at 1.0% concentration. Below this concentration the precision of the titrimetric methods degenerates faster than that of the gravimetric methods. Above about 0.1% concentration the gravimetric and titrimetric methods are somewhat more precise than the instrumental methods in current use for drug analysis. The difference, however, is not statistically significant and the general equation, RSDR = 2 exp(1-0.5 log C), is also applicable to gravimetric and titrimetric methods above a concentration level of about C = 0.001 (0.1%).