Summary notes – The air is admitted and the natural fertiliser of biogas PDF

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Summary notes – The air is admitted and the natural fertiliser of biogas Procedure The experiment measures the relative rate of decay of fats in milk at different temperatures but you could adapt the experiment to keep the temperature constant and vary the concentration of lipase.

Principle TOP OF PAGE and sub-index on cycles and decay

I'm not giving precise details of concentrations, and volumes quoted are just typical values.

See also Biofuels & alternative fuels, hydrogen, biogas, biodiesel GCSE chemistry revision notes

What I do describe are the principles of the experiment and how to do it.

Introduction to biogas

For an exam you need to appreciate all aspects of the experiment - design, what is needed (apparatus and chemicals), how to do it and how to process the results and draw conclusions.

Apparatus and chemicals needed Thermostated water bath, test tubes, thermometer, 10 cm3 measuring cylinder Milk, lipase solution, sodium carbonate solution, phenolphthalein indicator solution

Investigation method (with added explanation) For the experiment mixtures: choose a constant volume and constant concentration of the lipase, sodium carbonate and phenolphthalein solutions, so, for a fair test, the total volume in each experiment is the same and the only thing that varies is the temperature of the thermostated bath (see diagram). Procedure: Set the required temperature for the water bath and check it is constant with a thermometer.

(g) Biogas from waste - the process and generator/digester/fermenter designs

Biogas (mainly methane CH4) is produced naturally in marshes, septic tanks and sewers oxygen deficient places anywhere anaerobic bacteria thrive! Compost consists of decomposed organic material including plant waste from the garden or food waste from the kitchen. When enclosed in a compost bin it gradually decomposes into a rich organic material that is a really good natural fertiliser - a good example of partially recycling the biomass from photosynthesis.

Measure a volume of the lipase solution into a test tube, enough for several experiments. Measure out a volume of milk in another 2nd test tube and add a few drops of phenolphthalein indicator to the milk - then add a measure of the sodium carbonate solution to this mixture. NOTE: The solution should turn pink because the solution is alkaline and phenolphthalein turns pink above pH 10, but becomes colourless below pH 8. Both test tubes are placed in the water bath and left to reach the ambient set temperature of the water bath. When ready, using a calibrated dropping pipette, you measure 1 cm3 of the lipase into the milk mixture, shake gently to mix thoroughly (or stir with clean glass rod) and start the stopwatch. The enzyme will immediately start to decompose the milk producing an acidic product (a fatty acid). Stop the watch and measure the time taken for the pink colour of the indicator to become colourless as the alkaline sodium carbonate is neutralised by the fatty acid formed. Repeat the experiment several times for each temperature and repeat the whole experiment at different temperatures e.g. 10, 15, 20, 25, 30, 35, 40, 45, 50oC recording everything in a neat clear table - all the values should be recorded and the average time for each temperature too. You can use ice cubes to cool the water bath to temperatures below room temperature, but its tricky to keep the temperature constant.

(c) doc bResults The reciprocal of the time gives you a measure of the rate of the decay reaction.

e.g. if the reaction time was 40 seconds, the rate is 1/40 = 0.025 s-1. Using the average times and rates for each temperature, plot a graph of the rate versus temperature. The rate of reaction is basically a measure of a fixed quantity of decay (unit of fatty acid formed) per unit time.

Conclusion You should find the rate: (i) increases at first (normal rate of reaction rule from chemistry), (ii) goes through a maximum at the optimum temperature (typical of an enzyme) (iii) and the rate falls away at higher temperatures as enzyme lipase protein becomes denatured. For more details of enzyme theory see Enzymes - structure, functions, optimum conditions, investigation experiments, digestion gcse biology revision notes

Extension to investigation You can adapt the experiment to keep the temperature constant and vary the concentration of lipase. You can choose a constant temperature close to the optimum e.g. 30oC for the thermostated bath. For the experiment mixtures: choose a constant volume of lipase solution, BUT using different concentrations, choose a constant concentration AND volume of sodium carbonate and phenolphthalein

solution, so, for a fair test, the total volume in each experiment is the same and the only thing that varies is the lipase concentration. All the apparatus, chemicals and method are the same for the temperature varying experiment previously described. Your results should look something like the graph above-right. Initially the rate of milk decay should be proportional to the enzyme concentration - as long as everything else is kept constant.

The background chemistry to this investigation lipases Lipids, like many organic molecules, only contain the elements carbon, hydrogen and oxygen. Lipase enzymes break down lipids like natural fats and oils (triglyceride esters) into glycerol and long chain fatty acids. Lipids are NOT polymers because they are not very long chain molecules. Enzyme reaction word equation: lipid == lipase enzymes ==> glycerol + long chain fatty acids

The sort of molecular change that takes place details you do not need to know for GCSE level biology. However in GCSE chemistry you would be expected to recognise the acidic carboxylic acid group -COOH, an important 'molecular feature' in understanding this decay experiment. See also

Enzymes - structure, functions, optimum conditions, investigation experiments, digestion gcse biology revision See also Enzymes and Biotechnology (gcse chemistry revision notes)...