Two issues here--and thanks for the Feedback reply
First, I have analyzed the 15%, 20%, and 30% ZnO cream samples made up as a test control batch, and the USP Titration procedure resulted in excellent linearity for this range (R-square = 0.9897, in a plot of ZnO conc. vs. Titrant Volume (mL 0.1 N EDTA). So, I am convinced that my reagents and assay are working just fine. So, I am confident that my data are both valid and correct.
After reading (and re-reading) thru your posted comment, I must concede that although it is unconventional to report the ZnO fortification value on a label as the "amount of Zinc
" because the formulators add the raw material Zinc Oxide directly into the mixing vats; nevertheless, your reasoning to explain the discrepancy between the manufacturer's label claim and my empirical data indeed bears merit.
Please correct me if I am mis-interpreting what you're disserting:
1. Zinc Oxide (ZnO) is composed of both Zinc and Oxygen---OK I agree.
2. You computed the "fractional mass of Zinc IN Zincoxide" by accounting for:
Mol wt. ZnO = 81.38; Mol wt Zn = 65.38; Mol wt. O = 16. So, fractional % of Zinc in ZnO = 65.38/81.38 = 0.8034. OK I agree.
3. So, by taking 80.34% of the ZnO value obtaind via the USP titration assay (which I observed to be 30.57% ZnO), then the relative amount of Zinc in the diaper rash creme becomes 30.57 x 0.8034 = 24.55% wt. of Zinc. OK.
4. By comparing the label claim of 20.62% ZnO to the empirical result of 24.55% wt. Zinc; then the values are apparantly justified.
5. But there is still a ~16% swing in percent agreement--yet closer than before--source of this error still unknown?
Hmmmm, just gave you a mole snak!