1.9 Total carbon analysis  (Page 3/5)

The solution will bubble away CO ₂ . Because the only source of carbon in the sample is in theory the organic forms of carbon (assuming adequate pre-treatment of the sample to remove the inorganic forms of carbon), the evolved CO ₂ comes from organic sources of carbon.

Elemental forms of carbon in this method present problems for oxidation of elemental carbon to CO ₂ , meaning that not all of the carbon will be converted to CO ₂ , which will lead to an underestimation of total organic carbon content in the quantification steps. In order to facilitate the oxidation of elemental carbon, the digestive solution of dichromate and H ₂ SO ₄ is heated at 150 °C for some time (~30 min, depending on total carbon content in the sample and the amount of dichromate added). It is important that the solution not be heated above 150 ^o C, as decomposition of the dichromate solution.

Other shortcomings, in addition to incomplete digestion, exist with this method. Fe ²⁺ and Cl ^- in the sample can interfere with the chromate solution, Fe ²⁺ can be oxidized to Fe ³⁺ and Cl ^- can form CrO ₂ Cl ₂ leading to systematic error towards higher organic carbon content. Conversely MnO ₂ , like dichromate, will oxidize organic carbon, thereby leading to a negative bias and an underestimation of TOC content in samples.

In order to counteract these biases, several additives can be used in the pre-treatment process. Fe ²⁺ can be oxidized with mild oxidant phosphoric acid, which will not oxidize organic carbon. Treatment of the digestive solution with AgSO ₂ can precipitate silver chloride. MnO ₂ interferences can be dealt with using FeSO ₄ , where the oxidation power of the manganese is dealt with by taking the iron(II) sulfate to the +3 oxidation state. Any excess iron(II) can be dealt with using phosphoric acid.

Quantification of toc

What follows sample treatment, where all of the organic carbon has been digested, is a titration to oxidize the excess dichromate in the sample. Comparing the excess that is titrated to the amount that was originally added to the original solution, one can do stoichiometric calculations according to [link] and calculate the amount of dichromate that oxidized the organic carbon in the sample, thereby allowing the determination of TOC in the sample.

How this titration is run is up to the user. Manual, potentiometric, titrations are all available to the investigator doing the TOC measurement, as well as some others.

• Manual titrations are similar to any other type of manual titration method. An indicator must be used in manual titrations, and in the case of this wet method, commercially available “ferroin” is used. Titrant is typically ferrous ammonium sulfate. Titrant is added until equivalence is reached. Indicative of reaching equivalence is color change catalyzed by the indicator. Depending on the sample measured color change may be difficult to notice.
• Insertion of platinum electrodes to the sample can be used to measure conductance of sample using potentiometric tirtration. When sample reached endpoint, conductance will essentially be 0 or whatever the endpoint of the solution was set to. This method presents several advantages over manual titration methods because titration can be automated to respond to feedback from platinum electrodes so equivalence point determination is not color dependent.
• Alternative to titration methods, capture of evolved CO ₂ presents another pheasable quantification method, as oxidized organic carbon will be evolved as CO ₂ . CO ₂ can be captured on absorbent material such as ascarite or other tared absorbent, whose mass change as a result of absorbed CO ₂ can be measured, or the absorbed CO ₂ could be desorbed and quantified via IR non-dispersive cell.