Chemistry of acid digestions
Table of Contents
Acid digestions are carried out with a wide variety of reagents. In addition to the various mineral acids, reagents such as hydrogen peroxide, potassium peroxydisulfate, boric acid and many others are used. The choice of reagents or the composition of a reagent mixture depends on the sample to be decomposed.
Sample materials can generally be differentiated as follows:
Organic substances
Organic substances are substances formed from carbon in combination with hydrogen. The substances are mostly of natural origin. These mainly include environmental samples, samples from the food or cosmetics/pharmaceutical industry, agricultural sciences and samples from clinical analysis. In addition, polymers, or pesticides are also considered organic samples. In general, oxidizing substances or mixtures are used. These include nitric acid, also mixed with hydrogen peroxide or sulfuric acid.
Inorganic substances
Compared to organic samples, inorganic samples usually do not contain carbon. Due to the complex composition, all samples from material science, geology or the metal industry are considered inorganic.
Depending on the sample composition, different acid mixtures can be used here.
- Pure metals: HCl, aqua regia, also in combination with HF
- Oxides: H2SO4/HCl, H3PO4/HCl, H2SO4/H3PO4 also in combination with HF
- Samples difficult to digest at high digestion temperatures: addition of H2SO4 or H3PO4 necessary to reduce vapor pressure of total acid mixture
- Oxides (especially aluminates): Digestion conditions dependent on modification. α-Al2O3 by H2SO4/HCl or H3PO4/HCl also in combination with HF using high digestion temperatures and digestion times. In this case, the use of a conventional pressure digestion system is recommended (similar as for carbon).
When selecting a suitable method, the type of sample and its composition is crucial. In addition to organic and inorganic samples, a distinction can be made between solid and liquid or water-soluble and insoluble samples. Basically, as much information as possible should be gathered before digestion (especially if working with unknown samples) and the sample should be analyzed on the basis of its composition.
Organic sample materials are generally decomposed to carbon dioxide and completely mineralized with the help of oxidizing acids (especially nitric acid) and reagents (especially hydrogen peroxide). Plastics are often dehydrated in situ by addition of sulfuric acid prior to oxidative attack. When digesting reactive samples, i.e. materials (fats, oils, etc.) that decompose exothermically under these conditions one must be very careful. By slow heating rates or heating at different temperature levels, even those types of samples can be decomposed safely.
Control of exothermic reactions
Reactions, that release energy in the form of heat into the environment, are called exothermic reactions. For example, fats and oils react strongly exothermically when they are digested. The reactivity differs according to the type and number of saturated or unsaturated fatty acids. Unsaturated fatty acids containing multiple bonds usually require somewhat higher digestion temperatures. Stronger exothermic reactions are also to be expected. The release of energy is noticeable by a spontaneous rise in temperature. In order to control this type of reactions and to ensure safe reaction control, temperature monitoring with suitable temperature sensors is necessary. By selecting the appropriate temperature program and monitoring it, excessive material stress can be prevented.
In contrast, the spectrum of inorganic sample materials is significantly more diverse. Even conditions of sample pretreatment, e.g. whether a ceramic was fired at a lower or higher temperature, can have an influence on whether a sample is easy or difficult to dissolve. In general, however, inorganic samples should also be completely mineralized and dissolved. Acid mixtures with hydrochloric acid and/or hydrofluoric acid components are used for this purpose. To ensure that the solutions remain stable over a longer period of time, the solubility of the salts formed must be taken into account.