Serial dilutions are a mechanisms for creating solutions of different concentrations for use in the laboratory. We will initially create a series of concentrations, each of which is 1/10 of the previous concentration. This proceedure allows for the rapid creation of a set of solutions for determining which concentrations work best within the range of operation of the laboratory equipment.

Proceedure

Place 10 ml of stock starch solution within a test tube. Into each of 6 other test tubes, place 9 ml of distilled water. Label tube 1 FS, tube 2 -1, rube 3 -2 tube 4 -3, tube 5 -4, tube 6 -5 and tube 7 -6. Pipette 1 ml of solution from the first tube to the second, mix. Pipette 1 ml from the second to the third, mix. Pipette 1 ml from the third to the fourth, mix. Pipette 1 ml from the fourth to the fifth, mix. Pipette 1 ml from the fifth to the sixth, mix. Pipette 1 ml from the sixth to the seventh, mix.

You have now created a series of solutions with a range of concentrations from 1 to one-milionth of the original concentration. Add a 100 microliters of iodine to each test tube, which ones turn blue? Over what range of concentrations is this amount of iodine a good indicator for the presence of starch?

Take readings in the spectrophotometer for each of your serial dilution tubes. Record this data in a table in your notebook. Note which solutions are too dark to obtain any reading. Note the page number of this table in your table of contents.

Now, you need to prepare 70 ml of the starch concentration that was just too dark to read in the spectrophotometer, i.e., the concentration just before the concentration at which you were able to obtain a reading. Determine the concentration of your stock solution is grams per ml. Based upon your serial dilution record, calculate the concentration of this ideal starch solution concentration: You will use this concentration for the Enzyme Lab Experiment, this will be your Enzyme Lab Stock Solution Concentration.

Prepare one tube with 9.9 ml of this solution and 6 tubes with 9 ml of this starch concentration. Into the first tube place 100 microliters of iodine solution. Mix this tube up and add 1 ml of this solution to the next tube and so on to create another series of serial dilutions. Place the tubes in the spectrophotometer and record your results in a table in your notebook. Note this table in your table of contents.

Results

What concentrations of starch and iodine indicator provide the best range or reading on the spectrophotometer? Use these numbers for your enzyme experiment.