Isolation and measurement of humic substances

Over 30 % of dissolved organic carbon (DOC) in surface water is classified under the category of humic substances (HUS). These are relatively high-molecular compounds with a complex structure which still remains to be clarified accurately. For this reason it is especially important to characterize HUS by determining their physico-chemical properties. 

In order to characterize and quantify aquatic HUS specifically, they must first of all be isolated. The method most widely used at present isr sorption with synthetic resins at a low pH followed by elution with alkaline solution (i.e. at a high pH). Mantoura and Riley used amberlite on a polystyrene base (XAD-2) whilst Thurman and Malcolm used polyacrylamide (XAD-8). The method used by Frimmel and Niedermann on numerous bodies of water is shown in the chart below. 

A further division into humic acids (HA) and fulvic acids (FA) can be made. According to the working definition, HAs are only soluble in an alkaline environment, whereas FAs are soluble in both acidic and alkaline media. To differentiate between the two, add HCl to the alkaline eluate in the XAD column until pH 2.0 0.1 is reached. Filter off the precipitated HA 

Characterization of the HUS, HA or FA should be conducted following the criteria given in Fig. 127. 

The values obtained are especially informative if they are made comparable by being expressed in relation to 1 mg Coj-g. The resulting quantities A (254 nm)/mg DOC and A (436 nm)/mg DOC are also called specific UV absorption or specific yellowing. 

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