Introduction

Natural water courses enclose many dissolved minerals and gases that act together chemically by one another in complex and varied methods. The interaction might alternately aid or obstruct natural purification processes of natural water systems. Strictly speaking, most of the chemical interactions that play a part in self-purification of water courses are biologically mediated. Such chemical reactions aren't spontaneous, but require an external source of energy for initiation.

Biochemical Parameters Crucial to Water Chemistry and Analysis

Dissolved Oxygen (DO)

All the gases of the atmosphere are soluble in water to several degrees. Both nitrogen and oxygen are classified as poorly soluble. Since they don't react by water chemically, their solubility is directly proportional to their partial pressures. The solubility of oxygen in saline water is less than in fresh water. The solubility of atmospheric oxygen, in fresh waters ranges from 14.6 mg/L at 0⁰C to about 7 mg/L at 35⁰C under one atm of pressure. Dissolved oxygen is one of the most significant constituents of natural water system. A stream must have a minimum of about two mg/L of melted oxygen to continue higher life shapes these as fish and other aquatic animal species. At least, four mg/L of melted oxygen is needed for game fish and several species might need more.

The amount of oxygen originate via determination in a example of water at the time of collection is the dissolved oxygen.

Methods of DO Determination

The Winkler of iodometric method and its modifications are the standard volumetric procedures for determining dissolved oxygen. The test depends upon the fact that oxygen oxidizes Mn²⁺ to a higher state of valence under alkaline conditions and that Mn in higher states of valence is capable of oxidizing I^- to I₂ under acidic conditions. Therefore, the amount of I₂ liberated is equivalent to the dissolved oxygen originally present.

The iodine is measured with standard sodium thiosulphate solution and interpreted in terms of dissolved oxygen.

The Winkler Method (Unmodified)

The unchanged Winkler process is applicable only to comparatively pure waters because the process is subject to interference from a great many substances. Certain oxidizing agents such as nitrite (NO2-) and Fe3+ are capable of oxidizing I^- to I₂ and create consequences that are as well high. Reducing agents these as Fe²⁺, SO₃^2-, S^2- and polythionate, decrease I₂ to I^- and produce effects that are as well low.

The reactions involved are:

Mn²⁺ + 20H^-   Mn(OH)₂(s) (white precipitate)

Mn²⁺ + 20H- + ½O₂    MnO₂(s) + H₂O or

Mn (OH)₂  + ½O₂   MnO₂(s) + H₂O

If no oxygen is present, a white precipitate of Mn (OH)₂ forms when MnSO₄ and the alkali-iodide reagent (NaOH + KI) are added to the example. But if oxygen is present, several of the Mn²⁺ is then oxidised to Mn₄ and precipitates as a brown hydrated oxide.

After shaking the sample for a time sufficient (at least 20 seconds) to permit all oxygen to react, the floc is allowed to settle; then H₂SO₄ is added. Under the low pH conditions, the MnO₂ OXIDISEs I^- to I₂.

MnO₂ + 2I- + 4H⁺   Mn²⁺ + I₂ + 2H₂O

I₂ is rather insoluble in water, but forms a complex with the excess I^- present to reversibly form the more soluble tri-iodate, therefore preventing the escape of I₂ from solution

 I₂ + I^- I₃^-

Stopper the sample and shake for 10 seconds or more. Titrate the samples (200 mL) by 0.0125M or 0.025N thiosulphate to the end point. The mL of thiosulphate utilized is interpreted straight in terms of mg/L of dissolved oxygen.

The Azide Modification of the Winkler Method

The nitrite ion is one of the most frequent interferences encountered in the DO determination of wastewaters, river waters and incubated biochemical oxygen demand (BOD) samples. It does not oxidise Mn²⁺ but does oxidise I^- to I₂ under acidic conditions. It is chiefly troublesome because its reduced form, N₂O₂, is oxidised by oxygen that enters the sample during the titration procedure and is converted to NO^2- again, establishing a cyclic reaction that can lead to erroneously elevated consequences. The reactions engaged are:

2NO₂^- + 2I^- + 4H⁺       I₃ + N₂O₂ + 2H₂O

N₂O₂ + ½O₂ + H₂O 2NO₂^- + 2H⁺

When interference from nitrite is present, it is impossible to attain a permanent end point. As soon as the blue colour of the starch indicator is discharged, the nitrite formed will react through more I^- to create I₂ and the blue colour of the starch indicator will return. To overcome this interference, sodium azide (NaN₃) in alkali-KI reagent is utilized. When H₂SO₄ acid is added, the following reactions take place and the NO₂^- is demolished

NaN₃ + H⁺ HN₃ + Na⁺

HN₃ + NO^- + H⁺    N₂ + N₂O + H₂O

By this process, nitrite interference is abolished and the process of determination maintains the simplicity of the original Winkler procedure.

mg/L DO = 16000xMxV /V₂ (V₂* 2.0) V₁

where M = molarity of thiosulphate solution

V = Volume of thiosulphate (mL) utilized for titration

V₁ = Volume of the bottle with stopper in place

V₂ = Volume of aliquot taken for titration

If all the contents of the bottle are titrated, then mg/L DO = 16000 xMxV/ (V₁ *2.0)

Biochemical Oxygen Demand (BOD)

The amount of oxygen consumed during microbial utilisation of organics in a water sample is called the BOD. The greater the decomposable matter present, the greater the oxygen demand and the greater the BOD value.

Measuring the BOD

The BOD is computed through determining the oxygen consumed from a water sample situated in an air-tight container and kept in a controlled environment for a preselected period of time. In the standard test, a 300 mL BOD bottle is utilized and the example is incubated for five days at 20⁰C. Light must be eliminated from the incubator to prevent algal growth that might create oxygen in the bottle. Since the saturation concentration for oxygen in water at 20⁰C is approximately nine mg/L, dilution of the example through BOD-free, oxygen-saturated water is required to compute BOD values greater than just a few mg/L.

The BOD of a diluted sample is calculated by

BOD = DO_i * DO_f/P

where DO_i = the initial dissolved oxygen concentration (mg/L)

DO_f = the final dissolved oxygen concentration (mg/L).

P = the decimal fraction of the sample in the 300mL bottle.

Most natural waters and municipal wastewaters will have a population of microorganisms that will devour the organics. In sterile waters, microorganism must be added and the BOD of the substance enclosing the organisms must be computed and subtracted from the total BOD of the mixture. The presence of toxic substances (these as residual chlorine, chloramines and copper) will invalidate the BOD consequences.

Determining BOD₅

The BOD of a wastewater was suspected to range from 50 to 200 mg/L. Three dilutions (5, 10 and 20 mL) of the wastewater were arranged to cover this range of 50 to 200 mg/L. First, the examples (5, 10 and 20 mL wastewater) were situated in 3 corresponding 300 mL standard BOD bottles. Each was then diluted to 300 mL through organic free, oxygen- saturated water. The initial dissolved oxygen was computed and the bottles tightly stoppered and situated in the incubator at 20⁰C for 5 days, after that the dissolved oxygen was again computed. The values attained are as computed in the table below.

Since the final DO is less than 2.0 mg/L, the third BOD₅ value of 110 mg/L can be disregarded. Therefore, the average BOD₅ of the waste water is

½ (138+142) mg/L i.e. 140 mg/L.

Application of BOD Data

BOD is the main criterion utilized in stream pollution control where organic loading must be restricted to continue desired dissolved-oxygen levels. The determination is employed in studies to calculate the purification capacity of streams. It assists regulatory authorities in checking the quality of effluent discharged to streams. Information concerning the BOD of wastes is a significant consideration in the design of treatment facilities. After treatment plants are situated in operation, the test is utilized to estimate the efficiency of numerous processes.

Chemical oxygen demand

COD is a compute of the total amount of oxygen needed for whole oxidation to CO₂ and H₂O of organic matter present in a example of wastewater or effluent. COD is a speedily calculated parameter utilized to find out the pollution strength of domestic and industrial waste.

The determination is attained via using strong oxidizing agents under acidic conditions. Excess amount of the oxidizing agent is utilized. Oxygen is liberated, numerous are used to oxidise an equivalent amount of the waste to CO₂ and the unused is computed via titration through a reducing agent of known strength. The amount utilized for the oxidation of the wastes is identified via difference. Potassium dichromate, K₂Cr₂O₇, is the most appropriate oxidizing agent. The reaction is:

Cr₂O^2- + 14H⁺ + 6e^-    2Cr³⁺ + 7H₂O

or Cr₂O₇^2- + 8H⁺  2Cr³⁺ + 4H₂O + 3(O).

Ferrous ion is an excellent reducing agent for dichromate. The reaction is:

Cr₂O^2-₇+ 6Fe²⁺ + 14H⁺ 2Cr³⁺ + 6Fe³⁺ + 7H₂O.

Ferroin (110- Phenanthroline monohydrate + iron (II) sulphate heptahydrate) is an excellent indicator for this purpose. It provides an extremely sharp brown colour transform that is simply detected. Computation of COD is made from the formula:

COD (mg/L) = (V_b * _Vs)xMx16000/mLsample

or COD (mg/L) = (V_b *V_s )xNx16000 /mLsample

Where

V_b = mL ferrous ammonium sulphate used for blank

V_s = mL ferrous ammonium sulphate used for sample

M = molarity of ferrous ammonium sulphate

N = normality of ferrous ammonium sulphate

Inorganic interferences

Among the decreased inorganic ions, which can be oxidised under the conditions of COD test, Cl^- causes the most serious problems as it is generally present in elevated concentrations in most wastewaters.  Its presence can reason erroneously high consequences to be attained.

6Cl^- + Cr₂O₇^2- + 14H⁺    3Cl₂+ 2Cr₃+ + 7H₂O.

Though, this interference can be abolished via the addition of mercuric sulphate to the sample prior to the addition of other reagents. The mercuric ion joins through the chloride ion to form a poorly ionized mercuric chloride complex.

Hg²⁺ + 2Cl^-   HgCl₂

Nitrite ion, NO₂^-, is another source of interference capable of being oxidised to nitrate in the pressure of dichromate. This interference can be overcomes via the addition of sulfamic acid to the dichromate solution. Nitrite ion, by other ones these as ferrous and sulphide ions that can as well cause interference seldom take place to any significant amounts in waste or natural waters.

Applications of COD Data

In conjunction by the BOD test, the COD test is useful in indicating toxic conditions and the presence of biologically resistant organic materials. The test is extensively utilized in the operation of treatment facilities since of the speed by that consequences can be attained.

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