Icons Used In Illustrations; Chemical And Biological Safety; Intended Use; Theory Of Operation - Hach CODmax III User Manual

2.2.2 Icons used in illustrations

Manufacturer User supplied
supplied parts

2.2.3 Chemical and biological safety

Chemical or biological hazards. If this instrument is used to monitor a treatment process
and/or chemical feed system for which there are regulatory limits and monitoring
requirements related to public health, public safety, food or beverage manufacture or
processing, it is the responsibility of the user of this instrument to know and abide by any
applicable regulation and to have sufficient and appropriate mechanisms in place for
compliance with applicable regulations in the event of malfunction of the instrument.

2.3 Intended use

The CODmax III is an online COD analyzer that is intended for use by industrial and municipal
professionals to automatically monitor the chemical oxygen demand (COD) in domestic wastewater,
industrial wastewater and surface water applications. COD monitoring is used to measure the
content of organic substances in water and is a primary parameter in all water quality procedures.
The analyzer is applicable for a wide range of water analysis with a chemical oxygen demand
concentration between 10 mg/L and 5000 mg/L. Homogenization of samples that contains
suspended solids is necessary to get reproducible results.
CODmax III can measure samples with a chloride concentration of less than 5000 mg/L.

2.4 Theory of operation

The analyzer uses the dichromate method of chemical analysis. The system adds a mixture of the
sample, a potassium dichromate digestion solution, a silver sulfate solution, and a strong sulfuric acid
to the digestion unit. Straight-chain aliphatic compounds are oxidized more effectively when silver
sulfate is added as a catalyst. The analyzer increases the temperature of the mixture to 175 °C
(347 °F). Chromium ions change color when the oxidizing agent is reduced from VI to III. The
analyzer measures the color change. The quantity of consumed dichromate ions is equivalent to the
oxidizable organic compounds.
Inorganic reducing species, such as nitrite, sulfide and ferrous iron will increase the measured result.
The inclusion of the oxygen demand of the reducing species in the COD value of the water sample is
accepted.
The main interference is chloride. Mercury sulfate is added to form undissociated mercury chloride,
which removes the chloride interference.
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