Oxidizable Organic Substances

In order to evaluate water, it is important to know the amount of oxidizable organic substances it contains. This is determined either via the total organic carbon (TOC) or via the dissolved organic carbon (DOC) or, indirectly, via the quantity of oxygen which is needed to oxidize these chemically oxidizable organic substances as completely as possible. The total amount of needed to do this is termed the chemical oxygen demand (COD) . 

The amount of potassium permanganate consumed in the chemical oxidation of organic substances contained in water, this amount being quoted in mg/1 KMnOzjL as well as in mg/1 as the corresponding oxygen equivalent. 

Oxidizability and chemical oxygen demand, like biological oxygen demand, do not provide an exact figure for the total amount of organic substances in water. However, they are used as characteristic values, whose usefulness has been proved in practice, for the evaluation of the total organic substances contained in a sample of water. 

The oxidizability or the chemical oxygen demand is determined on untreated water, water which has been allowed to settle out or water which has been filtered, depending on the purpose of the investigation. The results should state under which of the above conditions the investigation was carried out. 

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It is well known that anhydrous perchloric acid must never be allowed to come into contact with oxidizable organic substances such as alcohols, wood, cotton, paper, cork and most plastics otherwise ignition or explosion will result. 

Since the purpose is to record only the oxidizable organic substances, the influence of the oxidizable inorganic substances has to be excluded as far as possible or the content of the latter must be determined separately. 

Calcium nitrate is a strong oxidizing agent. Mixing with organic substances such as fuel oil or hydrocarbons or other oxidizable compounds can cause explosion. 

One may think of the reduction of an organic substance to a less-stable product which is subsequently protonated or rearranged to form a non-oxidizable species B [21, 22], or the formation of metallic compounds in the mercury phase. 

Much more important is the polarographic behaviour of organic substances. This is based on the presence of electroactive groups, reducible or oxidizable at the dropping mercury electrode. The electroactive groups with examples from pharmaceutical chemistry are shown in Table 5J.lt should be emphasized that the aldehydic and ketonic groups. 

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