The excess potassium dichromate is titrated with ferrous ammonium sulphate (FAS) until all of the excess oxidizing agent has been reduced to Cr3+. The oxidation-reduction indicator ferroin is also added during this titration as it changes its color from red to pale blue when oxidized.
Why do we use ferroin indicator?
It is a popular redox indicator for visualizing oscillatory Belousov–Zhabotinsky reactions. Ferroin is suitable as a redox indicator, as the color change is reversible, very pronounced and rapid, and the ferroin solution is stable up to 60 °C. It is the main indicator used in cerimetry.
Which is the indicator used for the determination of COD?
Using potassium dichromate
. Most commonly, a 0.25 N solution of potassium dichromate is used for COD determination, although for samples with COD below 50 mg/L, a lower concentration of potassium dichromate is preferred.
Which chemical is used as an indicator for COD titration?
ferroin
2.1 Organic and oxidizable inorganic substances in an aqueous sample are oxidized by potassium dichromate solution in 50 percent (by volume) sulfuric acid solution. The excess dichromate is titrated with standard ferrous ammonium sulfate using orthophenanthroline ferrous complex (ferroin) as an indicator.
Why is h2so4 used in COD?
Chemical Oxygen Demand (COD) 1% Silver Sulphate in Sulphuric Acid (conc.) provides the primary and secondary digestion catalysts for oxidation of carbon during chemical oxidation.
Which instrument is used for determination of COD test?
COD Test Procedures
A COD reactor/heating (150°C) block and a colorimeter are turned on so that both instruments are allowed to stabilize. Pre-prepared low-range (3-50 ppm) or high-range (20-1500 ppm) vials are selected for the COD test based on expected results.
What is the Colour of ferroin indicator?
Ferroin Indicator Solution Properties (Theoretical)
| Compound Formula | C36H24FeN6O4S |
|---|---|
| Molecular Weight | 692.52 |
| Appearance | Red liquid |
| Melting Point | N/A |
| Boiling Point | N/A |
What is the principle of COD test?
The principle behind the COD testing method is that, under acidic conditions, a strong oxidising agent will oxidise almost any organic compound to carbon dioxide. COD analysis will measure the equivalent amount of oxygen that is required to chemically oxidise organic compounds in water.
What is the function of potassium dichromate in COD determination?
Fundamentally, it is the amount of oxidant consumed during oxidation of organic substances present in water samples. As per the standard methods of detecting COD, potassium dichromate (K2Cr2O7) or potassium permanganate (KMnO4) are generally used as oxidizing agents.
Why do we use reflux method for determining COD?
It is used to indicate organic matter contamination and it helps in knowing overall organic load to the receiving body. There are two methods for COD determination. The first method: open reflux method is suitable for a wide range of wastes where large volume of sample is required (for samples with COD= 50 mg O2/L).
Why HgSO4 is used in COD test?
The most general COD determination uses potassium dichromate (K2Cr2O7) in excess in an acidic medium with the aid of silver sulfate (Ag2SO4) as a catalyst, and mercuric sulfate (HgSO4) added to remove interference of chlorides.
Why can’t we use nitric acid instead of dilute sulfuric acid in COD?
HCl and HNO3 are not used in the titration of Mohr’s salt in place of sulfuric acid because: HCl is a mild reducing agent and it reacts with some of the indicator that is KMnO4 solution so the final value of the indicator used come to be more than the actual value disturbing the experiment.
What is the difference between BOD and COD?
Biochemical oxygen demand (BOD) is the amount of oxygen required by the microorganisms to break down the organic materials, whereas chemical oxygen demand (COD) is the amount of oxygen required to break down the organic material via oxidation.
Why COD is measured?
COD is an indicative measure of the amount of oxygen that can be consumed by reactions in a measured solution. It is commonly expressed in mass of oxygen consumed over volume of solution, which in SI units is milligrams per liter (mg/L). A COD test can be used to easily quantify the amount of organics in water.
Which chemicals are used in COD?
The most commonly used oxidant is potassium dichromate, which is used in combination with boiling sulphuric acid. It is important to note that the chemical oxidant is not specific to organic or inorganic compounds, hence both these sources of oxygen demand are measured in a COD assay.
What are the two methods of determining the COD value of a water sample?
There are two methods available for COD determination namely open reflux and closed reflux.
What is the color change for ferroin indicator?
For the indicator titrations, 20.0 mL aliquots of this unknown solution are titrated with the K2Cr2O7 solution using ferroin as the indicator. The color change is from red to yellow green. (The best is just as it changes color which is when it is bluish in color).
What is composition of ferroin?
1,10-Phenanthroline iron(II) sulfate complex, o-Phenanthroline ferrous sulfate complex. Linear Formula: [Fe(C12H8N2)3]SO4. CAS Number: 14634-91-4.
What is the Colour of the reduced form of ferroin indicator *?
pH independent
| Indicator | E, V | Color of Reduced form |
|---|---|---|
| Nitrophenanthroline (Fe complex) | +1.25 | red |
| N-Phenylanthranilic acid | +1.08 | colorless |
| 1,10-Phenanthroline iron(II) sulfate complex (Ferroin) | +1.06 | red |
| N-Ethoxychrysoidine | +1.00 | yellow |
Why starch is used as indicator in COD?
Principle: A known volume of potassium dichromate oxidises the organic matter (C and H and not N2) and then excess O2 is allowed to react with potassium iodide to liberate iodine in amounts equal to the excess oxygen which is estimated titrimetrically with sodium thiosulphate solution using starch as indicator.
What is the limit of COD?
The chemical oxygen demand (COD) concentration in a treated effluent, if observed to be persistently greater than 250 mg/l before disposal to any receiving body (public sewer, land for irrigation, inland surface water and marine coastal areas), such industrial units are required to identify chemicals causing the same.
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