Philippe,

A high temperature cycle after addition of boric acid is necessary to be sure that any insoluble fluorides, like alkaline earth and lanthanides that may have precipitated during the original digestion are dissolved.  The higher the temperature that is used in the original dissolution, the more refractory are the precipitated fluorides, or at least that's been my experience.  Because fluoride is a weak acid, you may be able to dissolve precipitated fluorides by adding a lot of nitric acid, ~10% by volume of concentrated nitric acid in the final solution.  Of course that requires a fluoride resistant sample introduction system and all plastic vessels for the samples.  If you want to do quantitative silicon analysis at low levels, you're still going to need to use plastic ware and an HF resistant sample introduction system.  Oh, and if you use boric acid to complex the fluoride, you'll never be able to do trace boron analysis.  The stuff never goes
 away.


I've also had better luck with organic containing samples like coal with a two step digestion, nitric acid only at high temperature in the first step than HF addition and a lower temperature , ~120C, step to dissolve silica and other oxides like TiO2 that aren't soluble in nitric acid only.  The original sample needs to be finely ground too.

DeWitt Payne

Retired
Kingsport, TN 37663


________________________________
From: Ph. De Raeve <[log in to unmask]>
To: [log in to unmask]
Sent: Thursday, September 15, 2011 3:32 AM
Subject: HF neutralization with H3BO3

Dear all,

Very often, Silicon containing samples are digested in a micro-wave oven with acid mixtures containing HF.
In order to allow injection in a ICP with quartz nebulizer and torch, remaining HF may be eliminated by addition of Boric acid.
In most papers, a second micro-wave cycle at about 200°C is required to complete this HF "neutralization".

Is high temperature necessary for this neutralization ? 
Some propose a simple mixing with an excess of a saturated Boric acid solution...
Is there a way to evaluate the remaining HF after H3BO3 treatment ?

Philippe De Raeve
Quality Assistance SA
Technoparc de thudinie, 1
6536 Donstiennes
Belgium
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