Several Problems of Hydrofluoric Acid and Sulfuric Acid Systems in Microwave Digestion

Hydrofluoric acid matrix

The HF substrate must have a hydrofluoric acid resistant system, but due to insufficient funding, the atomizer must be made of PFA. Hydrofluoric acid resistant systems often have more severe memory effects and are expensive. However, as long as the sample contains Si related substances, HF is necessary, and certain elements can only be stabilized by the HF system. Of course, more elements cannot be stabilized in a percentage HF matrix at ppb level concentrations.

The difference with hydrochloric acid is significant, and there are not too many elements that can be stabilized in the HF matrix. The five elements of the first main group; The nine elements of the fourth, fifth, and sixth subgroups, plus Mn and Fe; the seven elements at the transition between metal and non-metal on the periodic table: B, Al, Si, Ge, Sn, Sb, Te.

Attention everyone, once a large amount of HF is present, it is difficult to stabilize precious metals and rare earth elements even with hydrochloric acid.

Here's a piece of information to share: the rapid digestion method, using aqua regia extraction, can easily accurately determine the Hg in soil. However, adding HF is inaccurate. Some teachers believe that Hg has run away, but in fact, I have conducted experiments and found that even when acidified to near dryness at 120 degrees Celsius, the loss of Hg is within 20%. Actually, I think it was the precipitation of fluoride that led to the lower result.

At this point, I must make an explanation: in the semiconductor field or similar applications, due to extremely low content (no absolutely insoluble substances), good container quality (close relationship between container walls and adsorption and precipitation), clean instruments (similar to containers), and on-site use (even precipitation formation takes time), many situations where the substrate is unstable can still be solved. However, once a certain factor is not met, there will definitely be issues with the stability of the solution.

Additionally, HF is necessary for the preparation of nitrides, such as Si3N4.

Hydrofluoric acid is very annoying, and it is much more harmful to humans than nitric acid and hydrochloric acid. Therefore, if you can't use it, don't use it. If you can use it less, never use it more.

Sulfuric acid matrix

Setting aside the fact that sulfuric acid itself requires the determination of elemental content, sulfuric acid is also an important acid for dissolving samples. For example, in the determination of rare earth elements in soil, due to the precipitation of fluoride, it is necessary to use the high boiling point of sulfuric acid to re dissolve the rare earth elements.