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Chemical Stability and Compatibility - Tungsten (W) ICP-OES working standard stability

 
We are using Tungsten (1000 ppm) standard diluted to 100 ppm in 5% HNO3 and 25% HCl. In both cases the element is unstable after a few days and precipitation occurs. How to remedy this? We are using ICP-OES with a standard glass set-up and are hesitant to introduce HF that will corrode glass. Also, we are using yttrium as internal standard (both flowing and in multi-metal make-ups). Will the Y be affected if your suggestion is to raise the HF level?
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We are assuming the 1000 ppm stock tungsten standard you are using is in dilute HNO3/trace HF. If that is the case, then it is possible that lowering the concentration of the working standard from 100 ppm to 10-20 ppm could extend the time-frame of stability, and we would expect the HCl version to offer the greatest chance of success. Adding additional HF should remedy the issue, but you are correct that unstable Y-fluoride complexes could result. A general recommendation would be to create a 10 ppm W solution in 25% HCl, and if that does not work, repeat and add 0.1% v/v HF. The addition of Y to this solution should occur last when you are very close to the final volume and after thorough mixing. If you are still observing issues, then complexation of the tungsten with EDTA might offer an alternative HF-free approach, and we’d be happy to discuss that option with you.

Posted: 09/02/18 22:44:10

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