Thursday, July 18, 2019
IB Chemistry Lab Design Ã¢â¬ compare the effect of temperature on the concentrations of Vitamin C and Vitamin A in solution Essay
Research Question: To compare the effect of temperature on the concentrations of Vitamin C and Vitamin A in solution. Background Information: Many researchers claim that the vitamin content in the food we eat decreases as we cook the food, since it is being exposed to high temperatures. This really intrigued me since cooking is one of my hobbies, and I always try and check the nutritional value of meals that I cook. With this in mind, it was quintessential for me to see for myself if these claims hold true. To narrow the scope of the investigation, I chose Vitamins A and C to do this study upon. The claims state that the enzyme in whose form Vitamin C is found, gets denatured (or oxidised) at temperatures over 70Ã °C as well as at low temperatures, in the freezer. Also, studies claim that Vitamin A (in the form of retinol) gets oxidised at high temperatures, during cooking. Once these vitamins get oxidised, they are lost to us. This investigation will compare the effects of high temperature on Vitamin A and Vitamin C solutions. http://chemmovies.unl.edu/chemistry/smallscale/SSGifs/SS054Ascorbic.gif The ascorbic acid enzyme gets denatured at high temperatures because the tertiary structure of the protein, which the enzyme is made up of, unravels, causing the active site of the enzyme to change in shape. This leads to the substrate being unable to fit into the active site, and we say that the enzyme is denatured. This denaturing can take place at extremes of pH too.The Vitamin A (retinol) gets oxidised because, at high temperatures, it reacts with oxygen in the air to form a carboxylic acid (retinoic acid). In the laboratory, ascorbic acid found in fruits and vegetables can be simulated by adding ascorbic acid crystals to water, to form a mildly acidic solution. This solution can also be used as the standard solution during titration to find concentration of ascorbic acid. To simulate Vitamin A, we can simply add retinol to water to form a standard solution. Hypothesis: At higher temperatures, both the concentration of Vitamin C and Vitamin A can be expected to decrease. However, I expect the decrease to be greater in the case of the Vitamin C solution since (having protein-like properties) it is more sensitive to extremes of temperature. Also, the alcohol retinol has a high boiling point, so I believe that it will be more resistant to oxidation too. Variables: Independent Variable Why and How it is Changed Temperature to which the Vitamin C/A solution is heated The factor whose effect is being studied on the concentration of Vitamin C/A in a solution is the temperature the solution is heated to. Therefore, the temperature is the independent variable. In order to change the temperature, equal quantities of the same Vitamin C/A solution are heated to different temperatures. The different temperatures taken are Ã¢â¬â 30Ã °C, 50Ã °C, 70Ã °C and 90Ã °C. As a control, one solution is placed at room temperature. A thermometer is used to measure the temperature of the solution. Investigation at each temperature will be repeated 3 times, to ensure reliability. Dependent Variable Why and How it is Recorded Concentration of Vitamin C/A in solution (in mol dm-3), after exposure to temperature The effect of temperature on the concentration of Vitamin C/A in a solution is being studied, thus the concentration is the dependent variable. As the temperature moves further away from room temperature (above or below) the concentration of Vitamin C/A in the solution should decrease due to denaturing or oxidation. The concentration of Vitamin C is calculated by doing an iodine titration. A starch solution is added to a standard Vitamin C solution which is the titrant. Into this, a solution of potassium iodide and potassium iodate is titrated till a blue colour is obtained (end point). This is repeated thrice. The average volume of iodine solution used is calculated, and divided by the concentration of Vitamin C. Then, the solutions of unknown concentrations are titrated and unitary method is used to calculate their concentrations. The concentration of Vitamin A is calculated using a redox titration. This is done by making a solution of acid dichromate, potassium iodide and starch. This is titrated against a solution of sodium thiosulfate of known concentration. The volume of thiosulfate used is noted. Then, the Vitamin A solutions are added to a similar solution of dichromate, KI and starch, and titration is carried out with thiosulfate. For every 1 less mole of thiosulfate used there is 0.25 mole of alcohol in the sample (according to the chemical equations) which was tested. Controlled Variable Why and How it is Maintained pH of Vitamin solution Extremes of pH can also lead to the denaturing of enzymes, so if pH changes it will interfere with the results, potentially giving inaccurate results. Thus, the pH needs to be kept a constant. This can be done by adding a few drops of acidic buffer to the initial solution. Presence of Antioxidants Antioxidants including salts such as sodium chloride tend to Ã¢â¬ËprotectÃ¢â¬â¢ ascorbic acid from being oxidised, and thus their presence may lead to inaccurate results. Thus, they need to be eliminated. This can be done by using distilled water (without any salts) while preparing the solution of ascorbic acid. Head Space Present in System The ascorbic acid gets denatured because of oxidation by air. Thus, if the volume of air present in the system changes, the results will also change invariably. To prevent this, the amount of head space present in the system must be kept constant. This can be done by placing a lid on top of the beaker in which the acid solution is heated. Initial Concentration of Ascorbic Acid solution If the initial concentration of ascorbic acid in the solution is different, then the final concentration will also be affected. This can be avoided by adding the same mass of ascorbic acid to the same volume of water while preparing all the sample solutions. Volume of Ascorbic Acid solution The volume of acid solution used for each temperature and each trial should be the same since otherwise it will affect the volume of iodine solution used. Therefore, the volume has to be measured accurately using a pipette for each temperature and trial (each titration). Concentration of starch, potassium iodide and potassium iodate solutions The concentration of any of these solutions will affect the volume of solution titrated during each trial. Thus, it needs to be kept a constant. This can be done by ensuring that equal masses of these reagents are added to equal volumes of water, for all the trials. Final temperature of solution The final temperature of the solution may affect the concentration of the acid in the solution, as rapid heating and cooling can encourage oxidation. Thus, to avoid errors, the solutions will be allowed to rest till they reach room temperature, and only then will they be titrated to calculate concentration. Chemicals 1. L-ascorbic acid Ã¢â¬â 3.52 g to make 1 dm3 of 0.002 M solution of acid 1. Glucose Ã¢â¬â 50 g to add to acid solution, to simulate fruit juice 1. Potassium Iodide Ã¢â¬â 10.0 g to make 1 dm3 iodine solution 1. Potassium Iodate Ã¢â¬â 0.536 g to make 1 dm3 iodine solution 1. Starch (soluble) Ã¢â¬â 0.25 g to make 50 ml of 0.5% starch solution 1. 3.00 M Sulphuric Acid Ã¢â¬â 60 ml to add to iodine solution 1. Distilled Water Ã¢â¬â To make all the solutions and washing Other Materials 1. Weighing Scale 1. Bunsen Burner 1. Tripod Stand 1. Wire Gauze 1. Mortar and Pestle 1. Pipette Filler 1. Lid (for beaker) Ã¢â¬â 4 1. Thermometer Procedure: Preparing Ascorbic Acid Solution of concentration 0.002 M 1. Measure 3.52 g of L-ascorbic acid using the weighing scale and the weighing boat (which has to be completely dry). 1. Place the weighed crystals in the mortar and use the pestle to crush the crystals into a fine powder, to aid with dissolving it in water. 1. Place the powdered acid into a 500 ml beaker and add a little distilled water to dissolve the acid. Use the glass rod to stir. 1. Once it seems that the acid has fully dissolved, add some more water to the solution, to ensure that all the acid has actually dissolved. Then, transfer the solution into the 1000 ml standard flask using a washed funnel and the glass rod. 1. Wash the beaker with water and pour into standard flask, to remove any remaining solution. Repeat this process 3 times. 1. Wash the funnel and the glass rod, letting the water run into the standard flask. 1. Make up the solution to the 1000 ml mark. Place the stopper and mix the solution thoroughly. Transfer approximately 500 ml of this solution to the 500 ml beaker, for ease of use. Preparing the 0.5 % starch solution 1. Measure 0.25 g of starch using the weighing scale and weighing boat. 1. Bring 50 ml of distilled water nearly to a boil, and then add the measured quantity of starch powder to it. Allow to cool. Preparing the Iodine Solution 1. Measure 10.0 g of potassium iodide and 0.536 g of potassium iodate using the weighing scale and weighing boat. Transfer this to a 500 ml beaker. 1. Dissolve the solids in approximately 400 ml of distilled water. Stir using the glass rod, to aid in dissolving. Add the 60 ml of 3.00 M sulphuric acid to the solution at this point. 1. Once it seems that the solids have fully dissolved, add some more water to the solution, to ensure that all of it has actually dissolved. Then, transfer the solution into a 1000 ml standard flask, using a washed funnel and the glass rod. 1. Wash the beaker with water and pour into standard flask, to remove any remaining solution. Repeat this process 3 times. 1. Wash the funnel and the glass rod, letting the water run into the standard flask. 1. Make up the solution to the 1000 ml mark. Place the stopper and mix the solution thoroughly. Transfer approximately 500 ml of this solution to a 500 ml beaker, for ease of use. Titration Set-up and Final Steps 1. Transfer 65 ml of Vitamin C solution each into 5, 250 ml beakers. 1. Keep one of the containers in a trough containing melting ice (0Ã °C). Keep one at room temperature (as a control + standard solution). Heat the other three to 30Ã °C, 60Ã °C and 90Ã °C respectively. Ensure that all the beakers are covered with a lid during heating or cooling. 1. Wash the pipette, first using tap water and then distilled water. Rinse the pipette thoroughly with the Vitamin C solution at room temperature. 1. Use a pipette to transfer 20 ml of the Vitamin C solution, at room temperature, into a conical flask. This is the standard solution (and the control) since its concentration is known (0.002 M). 1. Add 10 drops of the starch solution to the conical flask. Swirl the contents to mix properly. 1. Wash the burette with tap water followed by distilled water. Then, rinse the burette with the iodine solution. 1. Fill the burette with iodine solution till the 0.0 ml mark. 1. Titrate the iodine solution into the conical flask, swirling the conical flask at all times. The end point is reached when a blue colour is obtained that persists even after 20 seconds of swirling. Note down the volume of iodine solution used. 1. Re-fill the burette to the 0.0 ml mark. Repeat the titration process 2 more times. Note down these two values for volume of iodine solution used as well. Calculate the average volume used. 1. Check that all the solutions that were heated (or cooled) have reached room temperature, with the help of a thermometer. 1. If they have reached room temperature, repeat the entire titration process (steps 18 Ã¢â¬â 24) with the other 4 solution (0Ã °C, 30Ã °C, 60Ã °C and 90Ã °C). Ensure that the burette is re-filled to the 0.0 ml mark after each and every titration, and that the pipette is first washed, and then rinsed with the solution that is going to be placed in the conical flask. 1. Use unitary method, to calculate the concentration of Vitamin C in each solution, after heating or cooling, using the concentration of the solution at room temperature (0.002 M) as the known value.