thsnkyou in advance
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Equipment & Materials Available will be a standard solution of 2,6-dicholorophenol indophenol (DCIP) at a high concentration (the concentration will be on the bottle; this is known as your stock solution) and two unknowns (A and B). Your task will be to construct a standard curve from this high concentration standard and to use this standard curve to estimate the concentration of the two unknowns. You will be told the minimum volume to make up for each standard solution of your standard curve. Procedure - Determining the concentration of DCIP 1. Make up a range of concentrations from 0 to the highest available (The concentration of DCIP stock solution provided is 0.1 mM). There are no rules as to what concentrations to use or how many. However, the prelab exercise above should give some idea about what a good range of concentrations might look like. The concentrations that you choose do not have to be linear (e.g. 2, 4, 6, 8, 10 μM etc) but can be not evenly separated (0.5, 1, 3, 5, 7.5, 10 μM). It does not matter from a theoretical point of view how many different concentrations that you use or which concentrations you use because each should fit on the same standard curve. However, from a practical point of view about 7 different concentrations are recommended plus your blank (8 in total) To ensure that there are less errors, duplicates of the concentrations used for the standard curve are usually done. In setting up your experiment, it is good to construct a table that indicates the amount of water that you add, the amount of standard solution, the final concentration of the standard solution and then a column for the absorbance measured a 620 nm for each standard concentration. It is also good to include a couple of rows for your unknowne in the table as well. If you decide to use a table to guide your experiment, make sure that the table has a descriptive label. This by convention is written above the table. 2. Remember to thoroughly mix the solution in each standard because poor mixing will lead to large errors. Do not forget to describe in your methods how you mixed the
solutions. 3. Before measuring your
solutions, compare the colour of your unknown
solutions with your series of standard
solutions and estimate the concentration of your unknowns. Record these estimates. 4. The absorbance of the standard
solutions and the two unknowns should be measured against your blank solution (in this case water). 5. Fill one cuvette with the water that you used to make your dilutions - this is called the blank and serves to eliminate any absorption due to any dissolved substances present in the water. The spectrophotometer is 'zeroed using the blank. 6. Fill other cuvettes with your standard
solutions and blanks. Measure the absorbance of each at 620 nm and record it in your table. (Do not forget to zero the spectrophotometer on your blank before you do this. If you were to forget, then you could take the absorbance reading of blank away from the standard
solutions measures of absorbance to get the true answer. 7. Using a graph paper or Microsoft Excel, plot Absorbance (620 nm) against the known DCIP concentrations for the standard dye
solutions. This is the DCIP standard curve. 8. Use the DCIP standard curve to determine the concentration of two DCIP
solutions of unknown concentration Discussion of methods 1. Describe a process for making up your standards that would give the greatest accuracy for your standard
solutions. Explain why you would do this. 2. Should you plot both sets of data for your standard curves (duplicates) or take the mean of the values? Justify your answer.
3. What do you think would happen to the absorbance if you used 420 nm for the light to be absorbed? Would the absorbance be greater or smaller?