Wastewater Chemistry – The 5 Stages of Chemical Analysis of Wastewater

An understanding of wastewater chemistry is necessary through titration to identify certain types of contaminants, especially metals and heavy metals. Such chemical analysis of wastewater is carried out in stages. In the following paragraphs I have explained what these stages are.

1. titrant

Ethylenediaminetetraacetic acid (N-CH2-CH2N), commonly known by its abbreviated form, EDTA is the most common titrant used during titration to understand wastewater chemistry. Alternate ethylenediaminetetraacetic acid (H4y) when used as EDTA is less soluble, while tetraneutralized solution (Na4Y) offers a highly alkaline solution as it is easily hydrolyzed. Therefore, a hexadentate (Na2H2y), which is a salt of EDTA, is used as titrant. It is obtained as a dehydrate of tetraacetic acid. It is a disodium salt commonly known as Ethylenedinitrile represented by the formula C10H14O8N2Na2.2H2O.

The chemical composition of EDTA is HOOC.H2C -> CH2COOH with either Mg or Ca divalent. This is a chelated bond.

2. Indicator

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

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

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

3. Defined End Point

To bring the test to a definitive endpoint, Mg ions must exist in the buffer. Therefore, to achieve this, a small amount 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 increase in pH is the precipitate which is Mg(OH)2 or calcium carbonate (CaCO3). If the pH continues to rise further, a yellowish-orange tint is obtained.

4. CaCO3 precipitation

If the titration process is allowed to continue for a long time, CaCO3 precipitation will occur. Therefore, all steps must be completed within five minutes. Diluting the solution can weaken the CaCO3 concentration, so it must be diluted before adding reagents to complete the titration.

5. Degree

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