[thermoman]

I am having a problems with the %w/w determination of zinc oxide, I'm using the USP method in this link:  http://www.pharmacopeia.cn/v29240/usp29nf24s0_m89680.html
For some reason the assay result is 105% when the upper limit is 100.5%.

[Arkcon]

Do you get the out of spec assay result with a known traceable standard, or with an unknown sample you're trying to assay?  There are a number of things that could be causing the back titration to fail.  Maybe you can send a sample out for a quick ICP assay to see if your sample is really impure in some way.

[thermoman]

I'm testing an unknown sample from a manufacturer that send me some samples in the past with no problems in the assay, and i have no option to send the sample to ICP.
 

[marquis]

There are only a few vendors selling USP grade zinc oxide.  One wanted to only go to the start of the color change with the methyl orange, instead of the normal end point.  This would increase the reported percentage of zinc oxide.  Instead, I did a potentiometric titration (I used a pH meter), which took the indicator out of the equation entirely.  No more arguing about if it was the correct end point. 

You might try this and see if it changes the results.

The vendor we dealt with was no longer doing the USP titration, themselves.  They had switched to a method unapproved by the USP.  You might check and see how the vendor is doing the test.  The USP is pretty specific.  You can use alternate methods.  But when in doubt, the USP method will prevail.

Good luck.