Several Problems of Nitric Acid System in Microwave Digestion

Microwave digestion generally uses nitric acid to stabilize the elements in the solution, and the concentration is slightly higher. For beginners, this conclusion is very useful. However, several issues with the nitric acid system have not been clearly explained, as mentioned in previous articles. This article will provide a brief overview and explanation.

The above figure is from the publicly available information of a foreign element standard solution manufacturer, which is summarized in a professional and detailed manner, and is consistent with my actual experience. The following will analyze several problems of elements in nitric acid system through a detailed explanation of this figure.

Firstly, red represents good solubility, indicating that most elements are easily soluble in dilute nitric acid aqueous solutions. I lack experience with the insoluble Mo element here and have not found any instability. For now, I think it may be unstable in the nitric acid system. Insoluble elements can be divided into several categories:

The first category: F represents elements that can be dissolved in nitric acid by complexing with F, which means only a small amount is needed (I think less than 0.1%). These elements include Ti, Zr, Hf, Nb, Si, Ge, Sn, Sb (T represents tartaric acid can also stabilize Sb).

The second type: elements that require a large amount (percentage content) of hydrofluoric acid to stabilize: Ta, W.

The third category: Ru, Rh, Ir, Pt, Au require a percentage of HCl to dissolve in nitric acid.

In addition, many elements also have certain personalities that need to be emphasized: Os should never come into contact with nitric acid, as it can easily form OsO4 and then evaporate. However, in general, it is still recommended to add a percentage of hydrochloric acid and use it immediately or as soon as possible. This approach is not a big problem, as a loss of 10% is acceptable for most applications.

The determination of Cl cannot be carried out by adding acid, as Cl ions will be oxidized to chlorine gas, which is prone to volatilization and residue; Both Br and I are easily oxidized to elemental form, resulting in severe memory effects.

Hg and Au below 100ppm need to be stored in borosilicate glass containers because Hg divalent ions are easily adsorbed in general plastic containers, and this effect is significant for low concentrations of Hg. So, Hg and Au should be stored in hydrochloric acid as much as possible.

Te element is no longer soluble in nitric acid system in the range of over 1000ppm; The element Ag requires a percentage of HCl to stabilize, otherwise it will turn into AgCl when encountering trace amounts of Cl, and then photodissociate into elemental Ag.

The above are several problems that exist in the nitric acid system, which can lead to poor linearity, abnormal recovery rates, unstable data, low results, and poor solution stability in experiments. However, if implemented in a current manner, these issues can be alleviated to a certain extent.

This is a fundamental issue. If there are problems at this stage, the results cannot be accurate and may be mixed with elements such as pollution, matrix effects, and mass spectrometry interference, causing serious obstacles for analysts.

The article was reprinted from the official account "ICPMS enthusiast".