COD_2016

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1 | P a g e FACULTY : CIVIL & ENVIRONMENTAL ENG. EDITION:

DEPART : WATER & ENVIRONMENTAL

ENG. REVISION NO: 02

LAB : ENVIRONMENTAL ENGINEERING EFFECTIVE

DATE: 28/12/2015 EXPERIMENT : CHEMICAL OXYGEN

DEMAND (COD)

AMENDMENT DATE: 1.0 OBJECTIVES

To determine the organic oxidizeable matters content of water samples. 2.0 LEARNING OUTCOMES

At the end of the study, the students will be able:

i. To understand the oxidation of oxidizeable matter with a known amount of potassium chromate, the titration of excess chromate, and the calculation of oxygen used. ii. To identify the organic oxidizeable matter pollutant content in water samples.

iii. To evaluate the water quality status at the location of water or wastewater sampling point. iv. To choose the appropriate analytical methodology for measuring COD parameter.

3.0 THEORY

Chemical Oxygen Demand (COD) test is commonly used to indirectly measure the amount of organic compounds in water. Most applications of COD determine the amount of organic pollutants found in surface water (e.g. lakes and rivers), making COD a useful measure of water quality. It is expressed in milligrams per liter (mg/L), which indicates the mass of oxygen consumed per liter of solution. Older references may express the units as parts per million (ppm). The basis for the COD test is that nearly all organic compounds can be fully oxidized to carbon dioxide with a strong oxidizing agent under acidic conditions.

The dichromate reflux method is preferred over procedures using other oxidants because of superior oxidizing ability, applicability to a wide variety of samples, and ease of manipulation. Oxidation of most organic compounds is 95-100% the theoretical value. Most types of organic matter are oxidized by a boiling mixture of chromic and sulfuric acids. A sample is refluxed in strongly acid solutions with a known excess of potassium dichromate (K2Cr2O7). After dilution the remaining unreduced K2Cr2O7 is

titrated with ferrous ammonium sulphate to determine the amount of K2Cr2O7 consumed and,

oxidizable organic matter is calculated in terms of oxygen equivalent. Keep ratios of reagent weights, volumes and strengths constant when sample volumes other than 50 ml are used. The standard 2 hours reflux time may be reduced if it has been shown that a shorter period yields the same results. Dichromate does not oxidize ammonia into nitrate, so this nitrification can be safely ignored in the standard chemical oxygen demand test. The International Organization for Standardization describes a standard method for measuring chemical oxygen demand in ISO 6060 [1].

Potassium dichromate is a strong oxidizing agent under acidic conditions. (Acidity is usually achieved by the addition of sulfuric acid.). In the process of oxidizing the organic substances found in the water sample, potassium dichromate is reduced (since in all redox reactions, one reagent is oxidized and the other is reduced), forming Cr3+. The amount of Cr3+ is determined after oxidization is complete, and is used as an indirect measure of the organic contents of the water sample.

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DEMAND (COD)

AMENDMENT DATE: 4.0 EQUIPMENT

i. COD Reflux System – consisting Erlenmeyer flask (250 mL or 500 mL) with ground-glass 24/40 neck and 300-mm jacket Liebig West, or equivalent condenser with 24/40 ground-glass joint, and a hot plate having sufficient power to produce at least 1.4 W/cm2_{of heating surface.}

ii. Burette iii. Pipette iv. COD Vial

4.1 REAGENT

i. Standard potassium dichromate 0.01667M  1000ml distilled water

 4.903g potassium dichromate (K2 CR2 O7) dry 2 hour 150’C  167ml acid sulfuric (H2 SO4)

 33.3g mercury sulfate (Hg SO4) ii. Acid sulfuric reagen

 5.5g silver sulfate (Ag SO4)  1kg acid sulfuric (H2 SO4)

iii. Standard ferrous ammonium sulfate

 39.2g ammonium iron II sulfate 6-Hydrate {Fe(NH4)2(SO4)2. 6H2O}  20ml acid sulfuric (H2 SO4)

 1L distilled water

iv. Ferroin indicator 5.0 PROCEDURE

5.1 STANDARD METHOD 5220 C

5.2 SAMPLING PROCEDURE

i. Collect the samples in glass bottles, if possible. Use of plastic containers is permissible if it is known that no organic contaminants are present in the containers.

ii. Biologically active samples should be tested as soon as possible. Samples containing settleable material should be well mixed, preferably homogenized, to permit removal of representative aliquots.

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3 | P a g e FACULTY : CIVIL & ENVIRONMENTAL

ENG. EDITION:

DEMAND (COD)

AMENDMENT DATE: 5.3 LABORATORY PROCEDURES:

1) The substances must be added in COD Vial following the sequences below: a. Place in COD Vial 1 and 2 with:

i. 1.5ml Potassium Dichromate Reagent ii. 3.5ml Acid Sulfuric Reagent

NOTE : Mixture reflux thoroughly before applying heat to prevent local heating of flask bottom and possible blowout of flask contents

b. COD Vial No. 1 : 2.5ml sample

COD Vial No. 2 : 2.5ml distilled water

2) Entering the refluxing mixture at COD Reactor with temperature 150 for 2 hours. At a meantime, please proceed to Procedure no. 6.

3) After 2 hours, cool wash down the condenser with distilled water. Continue cooling the condenser to room temperature.

4) Disconnect reflux condenser. Transfer the solution to the conical flask and dilute the mixture up 150ml with distilled water. Add with 3 drops ferroin indicator. Place the magnetic bars and stir it with magnetic stirrer.

5) Titrate K2Cr2O7 with Ferrous Ammonium Sulfate (FAS). First and last reading should be jot

down in Table 2. Take as the end point of the titration the sharp colour to blue-green. Stop titration, add 3 drops of Ferroin Indicator. Continue titration until the colour change to reddish brown that persist for 1 minute or longer. Stop titration and jot down the last reading in Table 2. Take a precaution that the end point colour changes of titration is fast. Calculate the

volume of FAS used in titration Table 2.

6) Preparation of Standard Ferrous Ammonium Sulfate (FAS) titrant as below:

a. Dilute 5ml Potassium Dichromate K2Cr2O7 standard (0.01667M) with 10ml distilled

water in the conical flask 250ml. swirl and mix slowly.

b. Slowly and carefully add 30ml conc H2SO4 and cool. Refer to Figure 1, titrate with the FAS titrant using 3 drops of ferroin indicator. First and last Reading should be jot down in Table 1.

c. Take as the end point of the titration the first sharp colour change to blue-green. Stop titration, add 3 drops of ferroin indicator. Continue titration until color change to reddish brown that persists for 1 minut or longer.

d. Take precaution that the end point colour changes of titration is fast. Jotted down the colume of FAS used in titration Table 1.

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4 | P a g e For all organic matter to be completely oxidized, an excess amount of potassium dichromate (or any oxidizing agent) must be present. Once oxidation is complete, the amount of excess potassium dichromate must be measured to ensure that the amount of Cr3+_{can be determined with accuracy.}

To do so, the excess potassium dichromate is titrated with ferrous ammonium sulfate (FAS) until all of the excess oxidizing agent has been reduced to Cr3+_{. Typically, the oxidation-reduction indicator}

Ferroin is added during this titration step as well. Once all the excess dichromate has been reduced, the Ferroin indicator changes from blue-green to reddish-brown. The amount of ferrous ammonium sulfate added is equivalent to the amount of excess potassium dichromate added to the original sample

6.0 RESULTS

Table 1 : FAS Standard Titration Data:

Reading of buret/pipet Cone Flask (Standard)

First Reading Last Reading

Volume of FAS standard (ml)

6.1 Compute the molarity for the FAS standards using formula given:

𝑀𝑜𝑙𝑎𝑟𝑖𝑡𝑦 𝑜𝑓 𝐹𝐴𝑆 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 =𝑉𝑜𝑙𝑢𝑚𝑒 0.01667𝑀 𝐾2𝐶𝑟2𝑂7 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑡𝑖𝑡𝑟𝑎𝑡𝑒𝑑, 𝑚𝑙

𝑉𝑜𝑙𝑢𝑚𝑒 𝐹𝐴𝑆 𝑢𝑠𝑒𝑑 𝑖𝑛 𝑡𝑖𝑡𝑟𝑎𝑡𝑖𝑜𝑛, 𝑚𝑙 𝑋 0.100

Note : The 0.1 is the Standard Potassium Dichromate solution, 0.01667M K2Cr2O7 undergoes a

six-electron reduction; the equivalent concentration is 6 X 0.01667M or 0.100N Table 2 : COD test Data Collections:

Normality of FAS (N)

Sample volume (mL)

Volume of FAS used in the original sample, b

(mL)

in the blank sample, s (mL)

6.2 Compute the COD concentration in mg/L for the samples using formula given:

𝐶𝑂𝐷 𝑎𝑠 𝑚𝑔 𝑂2

𝐿

⁄ =(𝐴 − 𝐵)𝑋 𝑀 𝑋 8000 (𝑚𝑙) 𝑠𝑎𝑚𝑝𝑙𝑒

Where:

A = mL FAS used for blank B = mL FAS used for sample M = Molarity of FAS

8000 = miliequivalent weight of oxygen X 1000ml/L

Note: The 8000 multiplier is to express the results in units of miligrams per liter of oxygen since 1 liter contains 1000ml and the equivalent weight of oxygen is 6.

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DEMAND (COD)

AMENDMENT DATE: 7.0 ANALYSIS

(1) Which volume is larger, in the blank sample or in the original sample. (2) How to obtain the normality of FAS equal 0.25N.

(3) If the original sample has to be diluted, how can you calculate the value of COD. 8.0 DISCUSSION

(1) What is the purpose of using blank sample in this experiment

(2) What is the objective of COD test and what is the different between COD and BOD test (3) Why the COD’s value needs to be monitor for the polluted surface water such as in lakes and

rivers as well as for wastewater.

(4) Give your opinion, if you need to compare the results of COD test to BOD and permanganate value (test COD by using potassium permanganate, KMnO4) tests.

(5) Why the value of COD increase when the amount of pollutant increased in surface water. (6) Explain briefly the steps of COD measurement that you should follow during this experiment. (7) Briefly explain whether the value of COD obtain in this experiment are suitable to be

discharged to the stream.

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