[Golden_4_Life]

Using EDTA as the Titrant and eriochrome black as indicator, I titrated a diaper rash cream that had label claim of "20.5% ZnO".  Using 1.25 g of the cream, it was dissolved in IPA and ammon.chloride reagent, water to facilitate dispersal and complexation.  The method followed is the same as cited in the USP-NF (2008). 
I observed that the volume of titrant necessary to cause color change from "purple" to "pale aqua blue" endpoint was 93.5 mL.  This volume equated to ~30% ZnO, which contradicted the label claim of 20.5% ZnO.
Ostensibly the label claim is wrong or the assay does not work for such high levels of Zinc. Does this assay have an upper lim. of linearity? or Is there some other factor that would cause the ZnO determination to be over-estimated? Or is the label just plain wrong? 

Thanks in advance for any assistance provided.

[Borek]

No limit, it is a simple stoichiometry.

[Golden_4_Life]

Well Borek: thanks for the kind reply, and my first thought was the same as yours in  that it was simple stochiometric rel'ship therefore the titrant volume and ZnO conc would be linear to infinity (but shorely from 1% to 50% to be conservative). Thus, I am beset with the present problem: why do I get 30% and label claim says 20.5%?
At this point I can only conclude that the label claim is wrong--which if I go saying this I'm bound to piss off the manufacturer.

[Stepan]

Can you try independent method like AA or ICP? ZnO of pharmaceutical grade is too expensive. It would be wrong from a business perspective to put 30% if you should use 20% only.

Another thing, in many industries It is normal to report Metal compounds as Elemental Metal. Maybe they mean 20.5% of Zinc Oxide as Zinc?   

[Golden_4_Life]

Yes STEPAN, I' ve thought of sending out the sample to a contract lab to test via atomic absorption--but we don't want to have to pay for the extra cost---instead what I'm doing is to make up a "lab control sample" that is spiked to both 15%, 20%, and 30% ZnO.  The titration results should at minimum be "different" for each control.
As far as I can tell "ZnO" is Zn+2 + O2 -1 ---> ZnO. So, no other stochiometric adjustment factor is required--it should be just straight forward no?

[Stepan]

Golden, I will try to explain on my own example:
I work in Health and Safety chemistry. Our samples are not well defined: it can be ZnO or it can adsorb CO2 and become ZnCO3 or react with Silica dust and for a silicate. To avoid confusion how to report Zinc content, we report Zinc as element only, regardless what kind of salt or oxide it represents. 

If I would analyse your sample for my clients, I would report: Sample contains 24% of ZnO which is reported as elemental Zinc (which is equivalent to 30% of actual ZnO - the difference is due to oxygen). For my industry - this is normal. If it happens to work for your industry, this would partially explain difference in lab results and formulation.

[Golden_4_Life]

STEPAN:  
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.

Second:

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 in ZnO" 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!

[Stepan]

Golden, I am sure that you are 100% correct , and I have never thought otherwise.

I was only suggesting other possibility to explain such a dramatic difference. My experience with industrial chemistry (not pharmaceutical), tells me that there is a huge gap between what is on the paper and what is in reality. 20% difference is a norm.

My suggestion came from my own experience when I discovered that concentration is often reported in non-standard units: "per dry weight", "as elemental", "as recommended daily consumption", "per serving", "relative to....", and so on. I believe all those pseudo units are invented by industry either as a marketing tool or as a way to serve inter industry interest.

Answering the old question about AA: cost will be $30-50 (in USA/Canada) sounds reasonable  

[Golden_4_Life]

STEPAN:

  Although, a ~20% swing from the product label claim and the real amount of an API may be the "norm" in industrial chemistry/manufacturing, I must say that it is a bit unsettling to accept.  Nevertheless, I am happy that I do not have to piss off this manufacturer by asserting that their label claim is wrong--when it was due to a difference in how the ZnO level was stated.  It would be nice to allay my concerns by sending the sample out to have an AA confirmatory quant done, but the "thought-experiment" we just did (i.e., per the  %wt. Zinc in ZnO consideration via your Forum sharing) has satisfactorily served my interests in the matter.

     I understand what you're saying about "industry jargon" and how the product label statements of "amount"  oft contain non-IUPAC units of measure ---ostensibly to serve among other things "marketing purposes".  Thanks for the learning experience!