Chemistry of Wastewater – The 5 Stages of Chemical Analysis of Wastewater

Understanding the chemistry of wastewater through titration is necessary to identify certain types of pollutants, especially metals and heavy metals. Such chemical analysis of wastewater is done in stages. In the paragraphs below I have explained what these stages are.

1. Titrant

EthyleneDiamineTetraAcetic acid (N-CH2-CH2N) commonly known by its short form – EDTA is the most common titrant used during titration to understand the chemistry of wastewater. The alternative Ethylenediaminetetraacetic acid (H4y) when used like an EDTA proves less soluble while the Tetraneutralized solution (Na4Y) offers a highly alkaline solution as it is easily hydrolyzed. Hence a Hexadentate (Na2H2y), which is a salt of the EDTA, is used as a titrant. It is obtained as a dehydrate of the Tetraacetic acid. It is a disodium salt commonly known as Ethylenedinitrilo represented by the formula C10H14O8N2Na2.2H2O.

The chemical composition of the EDTA is HOOC.H2C -> CH2COOH with any one of the divalents Mg or Ca. This is a chelated bonding.

2. Indicator

It is a substance that changes color to indicate completion of chelation with EDTA. When using the afore mentioned process, at a pH of 10.0 + 0.1, either Mg or Ca reacts with the indicator. This reaction causes the solution to change to a wine red color. If the solution already contains both Mg and Ca, and if the EDTA is added, then it forms a EDTA chelate compound which is highly stable. During this reaction the EDTA consumes all of Mg and Ca, resulting in the solution turning blue, as free indicators are released giving a good demonstration of the chemistry of wastewater. Given below are the two reactions:

H2R + Ca^++ (blue) -> CaR + 2H^+ (wine red)

CaR + Na2H2y (EDTA salt) -> CaNa2y + H2R (Chelate – Blue)

3. Definite End Point

To bring the test to a definite end point, Mg ions should exist in the buffer. Hence to accomplish this, a little quantity of Mg salt (EDTA) is added. This salt is complexometrically neutral. As the pH of this buffer solution increases, it leads to a sharp end point. But a byproduct of this increasing pH is the precipitate that is either Mg(OH)2 or Calcium Carbonate (CaCO3). If the pH continues to increase further then a dye of yellowish orange color is obtained.

4. CaCO3 Precipitation
If the titration process is allowed to continue long then it will result in CaCO3 being precipitated. Hence all steps should be completed within a span of five minutes. Diluting the solution can weaken the concentration of CaCO3, therefore it has to be diluted before the reagents are added to complete the titration.

5. Titration
Titrations are done to understand the chemistry of wastewater. But the indicators used in the titration are affected by hot water. Hence it should always be done at room temperature.

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