# PH Indicators / Dilution (NaOH)

Scenario:

You took 7 test tubes. In #1 you mixed 5ml of 0.1M NaOH and 5ml of H2O together. In the other 6 test tubes, you added 9ml of H2O. You took test tube #1 (with 5ml 0.1M NaOH and 5ml H2O) and added 1ml to test tube #2. Take test tube #2 and add 1ml to test tube #3. take test tube #3 and add 1ml to test tube #4 and so on till test tube #7. Add 2 drops (in each test tube) of Bromothymol Blue to ALL 7 test tubes.

Questions:

1. Calculate the concentrations of OH- and the pH of each solution. Since NaOH donates OH- ion for each NaOH molarity, the OH- concentration is equivalent to the NaOH molarity. The pOH is calculated as -log [OH-] and the pH is calculated as 14- pOH.

2. First, do the equation: (M con) (V con)= (M dil) (V dil)

3. Substitute known values

4. M dil = ____________ M

5. Once you find the concentration, find the pH using the formula:
pH = -log [H2O+]

6. Repeat the entire procedure using Alizain Yellow as the indicator.

7. Repeat the entire procedure using Phenolphtalein as the indicator.

8. Repeat the entire procedure using Red Cabbage Extract as the indicator.

9. Summarize your results for each pH indicator, recording both the pH values and the color of solution at each pH.

10. Estimate the useful range for each indicator, from a minimum to a maximum pH (range usually spans from 1.5 to 2pH units).

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Stephen Allen, MSc

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BSc, Dalhousie University, Halifax, Canada
MSc, Baylor College of Medicine, Houston, Texas
BA, Ambassador College, Big Sandy, Texas
Graduate Certificate, University of British Columbia, Vancouver, Canada

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