1.9 Total carbon analysis  (Page 2/5)

Dissolution of total inorganic carbon

In order to measure the organic carbon content in a sample, the inorganic sources of carbon, which exist in the form of carbonate and bicarbonate salts and minerals, must be removed from the sample. This is typically done by treating the sample with non-oxidative acids such as H ₂ SO ₄ and HCl, releasing CO ₂ and H ₂ O, as shown in [link] and [link] .

Non oxidative acids are chosen such that minimal amounts of organic carbon are affected. Although the selection of acid chosen to remove the inorganic sources of carbon is important; depending on your measurement technique, acids may interfere with the measurement. For example, in the wet measurement technique that will be discussed later, the counter ion Cl ^- will add systematic error to the measurement.

Treatment of a sample with acid is intended to dissolve all inorganic forms of carbon in the sample. In selectively digesting and dissolving inorganic forms of carbon, be it aqueous carbonates or bicarbonates or trapped CO ₂ , one can selectively remove inorganic sources of carbon from organic ones; thereby leaving behind, in theory, only organic carbon in the sample.

It becomes apparent, in this treatment, the importance of sample homogenization. Using the rock example again. If a rock is treated with acid without homogenizing, the inorganic carbon at the surface of the sample may be dissolved. Only with homogenization can the acid dissolve in inorganic carbon on the inside of the rock. Otherwise this inorganic carbon may be interpreted as organic carbon, leading to gross errors in total organic carbon determination.

Shortcomings in the dissolution of inorganic carbon

A large problem and a potential source of error in technique measurement are the assumptions that have to be made, particularly in the case of TOC measurement, that all of the inorganic carbon has been washed away and separated from the sample. There is no way to distinguish TOC or TIC spectroscopically, the experimenter is forced to assume that they are looking at is all organic carbon or all inorganic carbon, when in reality there may be some of both still on the sample.

Quantitative measurement of toc

Most TOC quantification methods are destructive in nature. The destructive nature of the methods means that none of the sample may be recovered. Of the methods, there are two destructive techniques that will be discussed in this module. The first is the wet method to measure TOC of solid sediment samples, and the second is a the dry combustion.

Wet methods

Sample preparation

Following sample pre-treatment with inorganic acids to dissolve away any inorganic material from the sample, a known amount of potassium dichromate (K ₂ Cr ₂ O ₇ ) in concentrated sulfuric acid are added to the sample as per the Walkey-Black procedure, a well known wet technique. The amount of dichromate and H ₂ SO ₄ added can vary depending on the expected organic carbon content of the sample, typically enough H ₂ SO ₄ is added such that the solid potassium dichromate dissolves in solution.The mixture of potassium dichromate with H ₂ SO ₄ is an exothermic one, meaning that heat is evolved from the solution. As the dichromate reacts according to [link] .