Total organic carbon

The total organic carbon or TOC (English: total organic carbon) is a sum parameter for water and wastewater analysis and outputs the sum of the total organic carbon in a water sample. It is a measure of the organic contamination of the sample.

To determine the TOC content, the concentration of total organic carbon is determined in water and in most cases determined by automated measurement method.

Clean spring waters have a TOC content of 1-2 mg / l. Slightly polluted rivers and streams show values ​​around 2-5 mg / l In mesotrophic lakes values ​​already be achieved by 5-10 mg / l, in productive carp ponds typically 15-25 mg / l In highly polluted waters, the value can rise to over 100 mg / l. The TOC serves, among other sum parameters for the estimation of water quality.

The method is based on the oxidation of carbon compounds contained in the water and the subsequent determination of the developed thereby CO2 ( carbon dioxide ). The oxidation is usually carried out by the thermal combustion or alternatively via a UV Persulfataufschluss the water sample, the subsequent detection of CO2 using Infrarotphotometrie.

The determination of TOC in water chemistry is in a competitive relationship for the determination of chemical oxygen demand ( COD ). The advantages of the TOC determination lie in the increased accuracy, the lower required sample volume and improved automation. Another advantage over the COD determination is that here no sulfuric acid and incurred high with heavy metals (Hg, Ag, Cr) contaminated wastewater. Disadvantage is the much greater expenditure on equipment. The essential difference between the two methods is but in the object of measurement: For COD determination, the average oxidation state of the currently available carbon compounds plays a decisive role. All other oxidizable water constituents will be masked with detected or need (such as chloride, Cl - ) through special effort. TOC determination, however, is selective, it is only carbon compounds detected, regardless of the oxidation state of the carbon.

The measurement of TOC has prevailed especially in the analysis of drinking and surface waters, since in these cases a COD determination would often be inaccurate. In wastewater analysis is, however, especially in relation to the load and / or performance of wastewater treatment plants, the COD value in combination with the BOD value (biochemical oxygen demand) spread as a measure of the organic load.

Meanwhile, the TOC value is allowed as a measure of the organic pollution and the waste-water discharge. The conversion to CSB-/BSB-Angaben is problematic and only reasonably reliable when a constant oxidation state in most municipal wastewater is assumed. In contrast, applicable to industrial effluents from different industries also very different factors used to convert between the two variables.

• 2.1 difference method ( TOC = TC - TIC)
• 2.2 Direct method (TOC as NPOC )
• 2.3 Addition method

Analysis methods

In the standardization (eg DIN EN1484 ) Three different methods are described to determine the TOC content. This takes account of the fact that on the one hand all relevant connections are to be detected, to prevent disruption of the measurement is to be avoided by matrix effects as possible. Modern analyzers can be operated by all three methods. The selection of the appropriate method may vary depending upon the composition and concentration of the carbon compounds and for the expected contaminants.

NDIR detection

To determine the Kohlenstoffdioxidgehalts in the carrier gas ( blown with TIC / TOC ) is usually non-dispersive infrared sensors (NDIR ) are used, which are flowed through constant and reproducible by the carrier gas. Measured by the NDIR detector concentration is detected in dependence on time. The resultant integral of CO2 concentration over time (often referred to as a peak area ) is a measure of the carbon released from the sample C.

Combustion method ( combustion)

In the combustion method, the sample is completely combusted in a heated reactor and oxidize the carbon contained in the sample to carbon dioxide CO2.

Solid samples are oxidized at about 900 ° C or higher. For liquid samples, the combustion temperature is usually from about 700 to 1000 ° C, with higher temperatures favoring the sample digestion of solid samples such as liquid principle. The combustion and full implementation of the carbon contained in the sample in the gas phase CO2 is supported by the use of catalysts (eg copper oxide, cerium dioxide or platinum-containing catalysts).

The combustion gases are transported by a carrier gas ( usually synthetic air or pure oxygen ), dehumidified / dried and passed to the detector. The determination of the carbon dioxide takes place in a non-dispersive infrared detector ( NDIR).

Since June 2009, the draft standard DIN EN 15936 describes the test method for determining the TOC content in solids. The draft is the norm DIN EN 13137 and replace describes the combustion in a separate Feststoffanalysator and a new suspension method to determine solids in Flüssiganalysatoren.

Parallel to the TC or NPOC can determine the TNb (total bound nitrogen ) with a separate, nitrogen - specific detector (for example, chemical sensor, or ECD, chemiluminescence detector or IR detector ).

To get the TIC in the combustion devices, the sample ( usually in a separate, unheated reactor ) acidified and expelled the liberated carbon dioxide. The determination of the carbon dioxide is again in the NDIR detector.

A common problem with the combustion method are the salts contained in many samples, acids or caustic ingredients which (usually quartz ) can cause deactivation of the catalyst and to premature wear mainly of the combustion tube. On the part of the device manufacturers there are different approaches to circumvent this problem to some extent quite effective (matrix deposition ).

Wet chemical method " UV Persulfatmethode " (Wet - Chemical)

The sample is introduced into a heated digestion vessel. Acid is added and the inorganic carbon ( TIC ) is converted into carbon dioxide. The carbon dioxide is expelled with nitrogen and the NDIR - measured detector as TIC. The sample is added persulfate, it is irradiated with UV light, and the organic carbon (TOC ) is converted to carbon dioxide in the heated reactor. The carbon dioxide is expelled with nitrogen and the NDIR - measured as TOC detector. Fixed sample components (including suspended particles ) can not be completely digested. Therefore, this method is unsuitable for samples containing particles. Even salt- containing samples are problematic because with increasing salinity ( especially chloride) the persulfate reinforced enters into side reactions and no longer available for the oxidation of carbon available. There is a risk of false results.

Area of ​​application of the different methods

The wet chemical method " UV Persulfatmethode " is used because of the achievable high sensitivity preferably for the detection of minute TOC contents, for example, in pharmaceutical waters ( high-purity water, WFI, ...), since a large amount of sample can be used (up to about 20 ml). The disadvantage of this method is in addition to the unsuitability for samples containing particles also that some very stable organic compounds are not completely digested (eg, barbituric acid) under circumstances and there is the risk of false results.

In the field of environmental analysis ( wastewater, drinking water and surface water ) combustion method (injection amount to about 2 ml) is preferably used. The high temperature digestion is a method that unlocks certainly difficult oxidizable compounds and particles. Modern devices also reach the combustion method a high sensitivity and can therefore as an alternative to UV - method, among others, used for the analysis of pharmaceutical waters.

Calculation methods

Difference method ( TOC = TC - TIC)

In the region of high TOC concentrations (eg urban waste water ) is frequently performed by the " difference method ". Here, in the first step, the totality of all carbon compounds ( "TC" = total carbon) determined, and then in a second measurement of the proportion of inorganic carbon compounds (eg carbonates) ( " TIC " = Total Inorganic Carbon ) determined. By subtracting the TIC from the TC then you get the TOC value.

Direct method (TOC as NPOC )

Since both the TC and the TIC are subject to a measurement inaccuracy, the difference method often results in samples with a small compared to the TIC TOC proportion to inaccurate results. In the field of water analysis, therefore, the so-called " direct method " used. For this, the sample is acidified prior to the measurement in order to convert the inorganic carbon content " TIC " carbonic acid into carbon dioxide (CO2). With a stream of inert gas, the CO2 formed is then blown out of the sample. However, it also go volatile acids ( formic acid from acidified formate, acetic acid from acidified acetate, etc. ), and all volatile organic compounds lost ( eg fuel components) for the measurement. Therefore, the thus obtained TOC is referred to as " NPOC " (Non Purgeable organic carbon not blow-out of organic carbon). Nevertheless, this process is relatively TIC - rich ( = hard ) Water the procedure of choice that provides the best accuracy and reproducibility.

Addition method

Volatile organic compounds are not reported by the direct method. Must not be neglected their share of the TOC, set the " addition method " field. For this, the blown gas flow from CO2 is released and the volatile compounds present then oxidized and determined. The resulting " POC " ( Purgeable Organic Carbon) is then added to the NPOC to the TOC.

TOC concentration for analysis

Located in the sample to be determined only an organic substance, the TOC method can also be used to determine the concentration of the substance. The sample concentration in the substance can be determined as follows:

This equation determines the content of a substance by the measured concentration of organic carbon on the ratio of the molar masses of carbon and the substance, and the number of carbon atoms in a molecule of the substance, in relation to each other are placed. This is possible because a CO2 molecule produced from any carbon atom in a molecule of the substance in the analysis.