TOC Determination in Wastewater

TOC DETERMINATION IN DIFFICULT MATRICES
The TOC content is a measure of the concentration of organically bound carbon and is therefore a direct indication of the pollution levels of organic compounds in different matrices. The application field for TOC analysis is quite broad, being used mainly in ultra pure water, surface water, drinking water applications and outlets of clarification plants. But nowadays, demand is increasing to measure TOC in more problematic matrices such as acids, bases and samples highly concentrated with salts. The question is which influence the matrix may have on TOC determination. This article indicates the diverse influences of matrix on the different TOC techniques and gives examples of how to avoid these influences.

TOC MEASURING METHOD
Two measuring methods are being applied successfully in TOC analysis:

• Catalytic combustion, which converts carbon compounds into CO2 using high temperatures and a catalyst, with subsequent detection of the CO2 formed using an NDIR detector
• Wet chemical oxidation, which uses a combination of UV irradiation and persulfate for oxidation. The CO2 formed is detected either via NDIR or conductivity detector. In environmental applications, TOC determination using catalytic combustion has been applied successfully – based on the higher oxidation potentials, especially for particles.

The German Wastewater Ordinance, for instance, contains the following instruction on the TOC method: ‘a TOC instrument with thermal-catalytic combustion (minimum temperature 670°C) must be used’.

An advantage of the wet-chemical oxidation is the use of a higher injection volume. This leads to lower detection limits and improved reproducibility in the ppb range, which is a basic requirement for ultrapure water analysis in the pharmaceutical and semiconductor industry.