Experiment 13 Acid-Base Indicators and ph

Transcription

1 Experiment 13 Acid-Base Indicators and ph Background Most students start chemistry with some idea of what ph is and that it is important in many practical situations. If you have ever maintained a hot tub or swimming pool, you probably considered basic. But what is the time, the concentration is less than 1 mol/l, usually a lot less. See the table below for some examples: [H 3 + ] ph [H ] 0.1 mol/l 1 1 x mol/l mol/l 3 1 x mol/l 1 x 10 5 mol/l 5 1 x 10 9 mol/l 1 x 10 7 mol/l 7 1 x 10 7 mol/l 1 x 10 9 mol/l 9 1 x 10 5 mol/l 1 x mol/l mol/l 1 x mol/l mol/l ph = log [H 3 + ] The log represents the power of the exponent in the concentration, and it is made negative so that most ph values will be positive. This means that neutral solutions are represented with a 7 mol/l H 3 + very small numbers in a more convenient way. What makes a solution acidic? In short, the more acids are dissolved in a solution, the more acidic it will be. Strong acids lower ph the most, but weak acids can lower it to a certain extent as well. A basic solution, of course, will have bases in it rather than acids. The most 2 H 3 + Acid-Base Indicators and ph 81

2 As suggested above, everything that contains water your water bottle, your coffee, your pure water is not strictly pure. There is always some amount, at least 10 7 mol/l, of one of these two ions. Granted, that is not very concentrated, but the concentrations get higher as the ph is farther from neutral. And recall that the exact concentration of the hydronium ions will So if everything that has water also has a ph value, how can we measure it? There are a number of ways. In this experiment we will look at two of the simplest: one is to use an indicator, a substance that changes color with ph. Hot tub test strips, for example, have indicators built into them, and you can read the ph from their color. The other method you will use in this lab is a ph meter, which is a cleverly designed electrode that gives different voltage readings at different ph values. Using a meter is relatively quick and accurate. First, you will make solutions with known ph, then you will study how they affect indicators, Procedure Part I. Preparing solutions of known ph ph values with your indicator. The following indicators may be assigned: thymol, thymolphthalein, methyl red, bromocresol green or bromothymol. Label a set of small test tubes with ph values Add about 10 ml of 0.1 mol/l HCl to the ph 1 test tube. Use a plastic pipet to transfer 1 ml of your ph 1 solution into the p test tube. Use your graduated cylinder to measure of DI water into your p test tube. Mix well by swishing the solution up and down in your pipet. Now transfer 1 ml of your p solution into the ph 3 test tube, add of DI water to the tube and mix well. This process is called serial dilution. For ph values 4-10, add appropriate buffered solutions to your test tubes. For ph 13, add 10 ml of 0.1 mol/l NaH to the test tube. The ph 12 and 11 solutions you will prepare by serial dilution. Add 1 ml of ph 13 solution and of DI water to the ph 12 test tube. Mix well, then transfer 1 ml of ph 12 solution to the ph 11 test tube. Add of DI 82 Santa Rosa Junior College Chemistry Department

3 10 ml 0.1 M HCl Buffers 10 ml 0.1 M NaH 1 ml 1 ml 1 ml 1 ml Figure 1. Preparation of ph 1-13 solutions Now all your test tubes should be prepared. Add a few drops of your assigned indicator to each Set up your test tubes with a white piece of paper behind them so your classmates can look at your work and record data from it. Write down the name of the indicator and how many some common solutions. Part II. Testing ph of common solutions Now that you know the colors of the various indicators used in this experiment, your job is to every the unknown solutions with every single indicator. If you know a solution is acidic, for example, you don t need to test it with an indicator that changes color at ph 10. However, you are free to test whatever you think may help you narrow down the possible ph of each solution. For each indicator tested with each solution, record the observed color. nce you have enough information, give a range Part III. Using a ph meter. Now that you have tested the ph of common solutions with simple indicators and estimated range that you estimated in part II. Acid-Base Indicators and ph 83

4 Assigned indicator: Part I. Testing the colors of ph indicators ph thymol thymolphthalein methyl red bromocresol green bromothymol 1. By what factor does the concentration of H 3 + ions drop when the ph is increased by 1? Hint: consider how you made the ph 1 3 solutions. 2. In the table below, give a brief rule summarizing how each indicator works. For example, if you tested thymolphthalein, you could say that it turns dark at ph 11 and higher, light at ph 10, and is colorless at ph 9 and below. Indicator Rule(s) thymol (TB) thymolphthalein (T) methyl red (MR) bromocresol green (BG) bromothymol (BB) 84 Santa Rosa Junior College Chemistry Department

5 Parts II/III. Testing ph of common solutions thymol (TB) thymolphthalein (T) methyl red (MR) bromocresol green (BG) bromothymol (BB) ph (as a range) ph (by meter) Sprite Vinegar Baking soda solution Ammonia cleaner Aspirin solution 3. For two of your unknown solutions, explain the process you used to narrow down the ph your explanation is complete enough to stand up in court. Solution #1: Solution #2: Acid-Base Indicators and ph 85