• Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month

Determination of Chemical Oxygen Demand (COD) of a Given Sample of Waste Water

Extracts from this document...

Introduction

BCH 2005 Principles of Environmental Chemistry Experiment 4 Determination of Chemical Oxygen Demand (COD) of a Given Sample of Waste Water * Introduction The chemical oxygen demand(COD) 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 , making COD a useful measure of water quality. It is expressed in the unit of mgL-1. In this practical, COD of a given sample is measured by back titration of the FAS solution with the remaining dichromate ions after reflux. For details, the sample is refluxed in concentration sulphuric acid with excess potassium dichromate solution. After reflux, the remaining K2Cr2O7 is titrated with FAS to determine the amount of potassium dichromate solution remained. As we have known the original amount of potassium dichromate in advanced, we can find out the amount of dichromate reacted with the organic solvent during reflux. Hence, that is the COD. This method is suitable for a wide range of wastes where a large sample size is preferred. COD values of > 50 mg O2 l-1 can be determined using this method. * Objective To determine the COD of a given sample of wastewater * Procedure Please refer to the lab manual * Results 1) ...read more.

Middle

Hence, no. of mole of FAS titrant used to titrate with blank solution = 0.2502 x 25.20/ 1000 = 0.0063 mole no. of mole of K2Cr2O7 = 0.0063 / 6 = 0.0011mole Volume of water sample used = 25 ml = 0.025dm3 Weight of oxygen generated = 48,000 mg COD of my sample =?(0.0011 -0.00072 ) x 48000?/ 0..025 = 729.60 mg O2 L-1 The chemical oxygen demand of the sample is 729.60 mg O2 L-1. 3. Ag2SO4 can act as a catalyst to speed up the oxidizing reaction. However, problem arises as samples containing halide ions will form precipitates with Ag+, as the following equation: Ag+ (aq) + X- (aq) --> AgX (s) where X is halide ions. These precipitates are insoluble. As from the principle of catalysis, catalysts should not take part in the reaction, this principle would be violated if the silver precipitates are formed, which is not desirable. And, the silver sulphate may be used up in the reaction that cannot act as a catalyst. As the amount of silver sulphate decreases, it affects the rate of the reaction. Mercuric sulphate was added, as it can remove halide ions in the sample solution before refluxing instead of adding Ag2SO4. It is because HgSO4 can form complexes with the halide ions. So HgSO4 will not from precipitate with halide ion that can also suppress the oxidation of chloride ions in the COD test mixture. ...read more.

Conclusion

Can we use other reagent to oxidise the sample apart from potassium dichromate? The main point to choose the reagent is to consider if it is a strong oxidising agent or not. Thus, potassium permanganate solution can also be considered as a choice of oxidising agent. Precaution 1. Whenever adding concentrated sulphuric acid, add it little at a time slowly along the side of the flask with swirling, so that to release the heat gently and lower the temperature inside the flask. Continuous addition and swirling throughout. 2. Handle with care of the concentrated sulphuric acid. Wear gloves and safety goggles all the time. 3. Point the mouth of the flask away from any person including ourselves. 4. Ensure swirling is in a proper manner, not a very vigorous nor very gently ones. 5. Cool down the flask by water bath until cool if required. 6. Do not put the flask directly into the ice otherwise it may cause breakage of the glassware. 7. Watch the heating mantle tightly. Time is counted from the first drop condensed. Turn the power a bit lower with continuous dropping of the condensed liquid. 8. A small beaker was placed on the reflux tube to prevent foreign substances falling into it. * Conclusion The volume of FAS used to titrate with standard K2Cr2O7 and excess K2Cr2O7 in the sample solution are 10.00cm3 and 17.20cm3 respectively. The molarity of FAS titrant is 0.2502 M while the chemical oxygen demand (COD) of the sample is 729.60 mg O2 L-1. ...read more.

The above preview is unformatted text

This student written piece of work is one of many that can be found in our AS and A Level Inorganic Chemistry section.

Found what you're looking for?

  • Start learning 29% faster today
  • 150,000+ documents available
  • Just £6.99 a month

Not the one? Search for your essay title...
  • Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month

See related essaysSee related essays

Related AS and A Level Inorganic Chemistry essays

  1. Peer reviewed

    Determining the concentration of acid in a given solution

    5 star(s)

    Measuring the sodium carbonate solution 1 2 I need to make 250cm3 of a sodium hydroxide solution with the concentration of 0.1 mol dm-3. I will use solid hydrated sodium carbonate with the formula Na2CO3�10H2O.1 Mr (Na2CO3�10H2O) = (23+23+12+48+20+160) = 286 250cm3 of 0.1 mol dm-3 contains (0.1 x 250/1000)

  2. effects Concentration and Temperature on the Rate of Reaction

    Secondly I need to find the best way of stirring the mixture whilst the reaction occurs. Preferably, I need a way of stirring which will reduce the time taken for the solution to go colourless. This would mean that I can fit in more repeats and more concentrations of potassium bromide can be tested.

  1. Lab report Determination of Enthalpy Change of Neutralization

    -44.9 KJ mol-1 Reaction 4: Mass of the solution = (25 + 25) /1000 = 0.05 Kg Heat given out = (0.05 * 4200 + (3/29 * 0.02073 + 3/26 * 0.02234) * 840) * 13.0 = 2781.679552 J H2SO4 (aq) + 2NaOH (aq) � Na2SO4 (aq) + 2H2O (l)

  2. Aim; 1)To prepare a standard potassium dichromate solution 2)To standardize the ammonium iron(II) sulphate ...

    0.00 24.40 0.00 21.30 Volume of titrant used (cm3) 21.40 21.20 21.30 21.20 Average volume: 21.23 cm3 Calculation: Number of mole of KMnO4 = (0.02123)(21.23/1000) = 0.000450712 ? KMnO4 : H2O2 = 2 : 5 ? Number of mole of H2O2(dil) = (0.000450712)(5 / 2) = 0.0011268 Molarity of H2O2(dil)

  1. Aim To study the effect of concentration of iodide ion ...

    The concentration of iodide solution would be manipulated, acting as independent variable. Using a standard KI 1.0 mol dm-3 solution as stock solution, a dilution will be performed to produce solutions of varying concentrations (0.8, 0.4, 0.2, and 0.1 mol dm-3).

  2. Scientific Practical Techniques

    not be seen through nicked eyes, haemocytometer a device originally used to count blood cells it is now used to count cells and many types of microscopic particles, micropipette a pipette designed for the measurement of very small volumes mostly used in hospital and also a growing yeast culture placed in a bakers at 30 degree temperature.

  1. The Effects of Strong and Weak Acids on the Order of a Reaction.

    Before even undertaking this experiment I knew that I wanted to use Magnesium as it was far higher in the Reactivity table than Zinc, even so, I needed to be sure. After a test run using 0.1g of Magnesium ribbon and 0.1g of Zinc ribbon, I found that my initial assumptions were correct.

  2. The purpose of this experiment was to prepare a sample of Aspirin and measure ...

    smell, however when it does come in to contact with any type of moisture it releases a smell of acetic acid (RSoC, 2014). In cold water Aspirin is insoluble but as the water heats up the solute starts to dissolve.

  • Over 160,000 pieces
    of student written work
  • Annotated by
    experienced teachers
  • Ideas and feedback to
    improve your own work