L3. pH

Introduction:

The pH is a measure of acidity or basicity of a solution. Solutions with a pH less than 7 are said to be acidic and solutions with pH grater than 7 ara basic or alkaline.

For measuring the pH of a solution at the lab we have different pH indicators:

Universal indicator paper: it's an absorbent paper that has been impregnated with universal indicator. This method consists as a continuous colour change from about pH 2 to pH 10.

Ph-meter: is an electronic device used for measuring the exact pH of a solution. It consists of a glass electrode connected to an electronic meter that measures and display de pH reading.

Material:

- distilled water
- milk
- black wine
- lemon
- tomato
- coffee
- soap solution
- bleach
- 10% NaOH solution
- vinegar
- 10% HCl solution
- indicator paper
- ph- meter
- tongs
- 2 clock glass
- beakers
- 5 test tubes
- test tube rack
- 10 mL pipet
- funnel
- graduated cylinder

Procedure

Solutions:

To mesure the pH of different solutions we are going to put the different solutions in small beakers of 250 mL.

1. Put into the beakers the milk and the black wine, and squeeze the tomato in a clock glass.
2. Take a pice of indicator paper and place one end of it into the solution. Leave for at least 10 seconds.
3. Remove the indicator paper and compare its colour with the appropriate colour chart.
4. Repeat points 1 and 2 with as many others solutions as you are provided with.
5. Record your results in a result table.

Lemon juice:

Squeeze the rest of the lemon inside a beaker and filter the solution with a funnel and cellulose paper.

1. Prepare a test tub rack with 5 test tubes cleaned withe distilled water. Mark the tubes with the labels: A, B, A1, A2, A3.
2. Add 10 mL of lemon juice to tubes A and B.
3. Take the A tube and put 5 mL of its lemon juice to tube A1.
4. Take the A1 tube and put 2.5 mL of its lemon juice to tube A2.
5. Take the A2 tube and put 1.25 mL of its lemon juice to tube A3.
6. Add distilled water to each test tube until it has the same volume as test tube B (10 mL)
7. Calculate the concentration of each test tube with the formula you have below:

%= 100 x (volume of juice/total volume)

Questions

1. Which of the solutions gave and acid pH?

HCl, vinegar, black wine, tomato, coffee, milk.

2. Which of the solutions was alkaline?

Soap, bleach, NaOH

3. Wich of the solutions were neutral? Did you expect the results? Explain

Distilled water, yes I expect this results becouse distilled water doesn't have salts.

4. How does a pH of 3 differ from pH of 4 in terms of H+ concentrations?

10 vegades més gran

5. In the second part of the experiment, you have compared the pH of the same product (lemon juice) in different concentrations. In this case explain:

a. Which is the dependent variable?
pH o acidesa

b. Which is the independent variable?
concentracio de llimona

c. Which is the problem that we want to solve?
How does concentration affect pH?

d. Wich is the control of the experiment?
The tube that only have lemon (B)

e. Write the results and conclusions of your experiments:

Concentrations about lemon juice

Results

6. Which pH do you think that gastric juices might have? Why? Do you think that intestinal pH has the same pH? Why?
pH 2 becouse the gastric juice has HCl. I think intestinal ph is

7. Which pH do you think that blood might have? Why?

Neutral becouse the intern fluids need to have neutral pH.

8. What is acid rain? Which are the consequences in the ecosystems and how is its formation pattern? Is rain in Barcelona acid or alkaline?

The water contain more acid. The consequences in the ecosystems are the pollution.

material