Reagent-Free Measuring Methods

  • © Givaga / Jane Lane - Fotolia© Givaga / Jane Lane - Fotolia
  • © Givaga / Jane Lane - Fotolia
  • Potassium dichromate © BXXXD at German Wikipedia

The functional principle of spectral photometry  is based on the absorption of electromagnetic radiation by substances. Besides the methods that use a chemical reaction to create a concentration dependant colour change, which can then be measured, there are many substances that can be measured without a colour reaction provided they absorb light within the wave length  range of the photometer.

Photometric measuring of RNA, DNA and protein concentrations are examples of such methods. To date, used exclusively in water treatment plants for the online measuring of processes, a measurement was taken that recorded several parameters simultaneously while directly determining the cumulative parameters of the “Chemical Oxygen Demand (COD)” in addition.

This technology is now available for lab photometers and, under certain conditions, makes it possible to measure COD, nitrate and nitrite content quickly, directly, without any reagents or an extraction process, therefore making it environmentally friendly.

The matrix, i.e. the specific composition of the sample does have to be appropriate for the method though. Due to the many possible matrices in water treatment plants, the method can be specially adapted to the matrix of a specific water treatment plant or sample matrix. In contrast to spectral sensors for online measuring, the laboratory process is currently only feasible at the discharge point of the water treatment plant. The device used can determine whether the sample is appropriate for the process, which means that incorrect measuring or misinterpretation cannot occur.



With regards to online sensors used in waste water treatment plants, the parameters for NO2, NO3, COD, TOC, BOD, DOC, UVT and SAC are determined by spectral devices with the most advanced technology. The measurements take place between 200 and 390 nm, which means close to the UV range. Many carbon compounds absorb in this range. Nitrate absorbs at a maximum of 200nm and Nitrite at 354nm. The cumulative parameters of total organic carbon (TOC) and dissolved organic carbon (DOC) indicate the level of pollution by carbon compounds which make up the largest proportion of oxygen demand.

The spectral absorption co-efficient (SAC) summarily records carbonyl groups, aromatic compounds, heterocycles and unsaturated aliphatic compounds within the range of 230 to 280nm. The UV Transmission value (UVT) records organic and inorganic compounds at 254nm. However, other substances absorb in this range of the spectrum too. A complicated calculation of the measured values in different wave lengths, using complex algorithms makes it possible to determine all values to a level of accuracy that are comparable to the classic photometric methods. The main advantage of this method is a continual and fast measurement. The time consuming extraction process and the use of poisonous substances such as potassium dichromate are not necessary either.



The method only works well if disturbing influences from the sample matrix and for example turbidity are not too big. These difficulties have already been overcome by the online sensors. The economic and environmental advantages of the reagent free method are so great that the optimisation of the process which will lead to its routine use can be expected to be carried out quickly. Then a measuring method would be available which would make it possible to take comprehensive and regular or even continual measurements of these important parameters in environmental and water monitoring.



A method like this makes it possible to detect organic impurities and their causes, quickly, clearly and inexpensively. Pollutants that get into surface, ground and drinking water from damaged water pipes, through accidents or over-fertilization in agriculture can be identified.      

This year, Germany will not manage to implement the EU water framework directive. There are too many different sources of pollution in our waters and they cannot be comprehensively and effectively identified with the technology that is currently available.

In order to achieve the objectives of the directive, all surface waters need to be brought into a good or very good status of water quality. At the moment, as stated by the European Commission’s report on the implementation of the water framework directive, not even 10% of Germany’s waters fulfil this criteria. It is for this reason that the European Commission has recommended that Germany improve the current Nitrate Action Plan and the regulations regarding the use of pesticides. The agricultural industry are to be given clear guidelines regarding the protection of waters and their implementation closely monitored. Non-compliance with the agreed environmental goals are to be more precisely explained. Particularly when a heavy financial burden is given as the reason, because subsidies are available from the EU but Germany is not making use of them.


Do you have any questions regarding photometry? Please contact WTW!




Xylem Analytics Germany Sales GmbH & Co., WTW
Dr.-Karl-Slevogt-Str. 1
82362 Weilheim
Phone: +49 881 183 0
Telefax: +49 881 183 420

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