L4. pH

INTRODUCTION:

The pH is a quantitatively measure to degree of acidity and basicity in a solution. A solution with pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline. Pure water (distilled water) has a pH close to 7, this solution is neutral. 

The equation of pH is this:

                            

pH= -log (H + )

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

1. Universal indicator paper: It's an absorbent paper that has been impregnated with universal indicator. This method consists as from about pH 2 to pH 10. 
2. pH-meter: Is an electronic device used for measuring he exact pH af a solution. It consists of a glass electrode connected to an electronic meter that measures and displays de pH reading. 

We made this experiment because this week we studied the dissolutions and dispersions in biology class. The objectives were: measure different pH values of organic and inorganic solutions and prove different methods of measuring pH.

MATERIALS:

- Distilled water

- Milk

- Wine

- Lemon

- Tomato

- Coffee

- Carbonated beverage

- 10% NaOH solution

- 10% HCl solution

- NH3 solution

- Soap solution

- Universal indicator paper (strips)

- pH-meter

- Acetic Acid

- Tongs

- 8 x 100 mL Beakers

- 2 Clock glass

- 1 x 250 mL Beaker

- 5 tests tubes

- test tube rack

- 10 mL pipet

- Funnel

- Graduated Cylinder

PROCEDURE:

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

1. Squeeze the lemon and tomato in two clock glasses.
2. Take a pice of indicator paper and place one end of it into the soution. Leave for at least 20 seconds.
3. Remove the indicator paper and compare its colour with the appropriate colour chart. 
4. Repeat points 1 to 2 with as many others solutions as you are provided with.
5. Record your results.

How does concentration affect pH?

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

1. Prepare a test tub track with 5 tests tubes cleaned with distilled water. Mark the tubes with the labels: A, A1, A2, A3 and B.
2. Add 10 mL of lemon juice to tubes A and B.
3. Take the A tube and put 5mL of its lemon juice to test 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 1,2 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

                - Concentration (%)= ( Volume of Juice/ Total Volume) x 100

• Find the pH of the test tube:

TEST	VOLUME OF	TOTAL	CONCENTRATION	pH
B	10 mL	10 mL	100%	3,38
A1	2,5 mL	10 mL	25%	3,37
A2	1,25 mL	10 mL	12’5 %	3,33
A3
A	5 mL	10 mL	50%	3,35

RESULTS AND CONCLUSIONS:

We measure the pH of different substances, and we observed that if the pH was acid the color of the indicator, was red, pink, orange, yellow. LIke the HCl (pH=1) or the wine (ph=3) the tomato (pH=4) and vinegar (pH=2) and the milk that present a neutral- acid pH (pH=6-7) are acid too.

And if the substance present a basic pH, the color of the indicator was more blue and purple like the soap (pH=10), the bleach, that present an a high pH (pH=12-14) and NaOH (pH=14). 
And in a neutral pH is when this solution have pH of seven, the color is more green and this substances present the same number of H+ and oH-, like the distilled water. 

Then we do an other experiment with the lemon juice, with  tubs (A,A1, A2, A3, B) we do a different concentrations of lemon juice and distilled water and with the pH- meter we can calculate, measure the exact pH of the solution. We can see the results in the above table. With this results we can confirm that the lemon juice are acid. It shows a high concentration of hydrogen ions.

QUESTIONS:

1.- Which of the solutions gave an acid pH?

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

2.- Which of the solutions was alkaline?

Soap, bleach and NaOH.

3.- Which of the solutions were neutral? Did you expect this results? Explain.

Distilled water, because it has the same concentration of H+ and OH, and thats why we expect that water is with a pH neutral.  

                                                                                                  

4.- How does a pH of 3 differ from pH of 4 in terms of H+ concentrations?
10 times large value.

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:

• Which is the dependent variable? The pH.
• Which is the independent variable?The concentration of lemon juice.
• Which is the problem that we want to solve? The problem that we want to solve is equalize the concentrations.
• Which is the control of the experiment? The control is the test B.
• Write the results and conclusions of your experiment: The pH is very similar on the all concentrations, but the normal is that the pH depends of the protons concentrations. 

6.- Which pH do you think that gastric juices might have? Why? Do you think that intestinal pH has the same pH? Why?
They have an acid pH because its used to improve food digestion. I think that all intestinal pH are acid.

7.- Which pH do you think that blood might have? Why?
The blood have an neutral pH, because if is hypotonic the cell bloat, if is an hypertonic the cell crease. The pH causes it to keep the concentration, because we could die.

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

The acid rain is a phenomenon, that have a pH levels lower than normal. Is mainly due to the mixture of water with molecular products of sulfur and nitrogen in the combustion incurred, such as nitrous oxide NO2, NO3 nitric oxide or sulfur dioxide SO2 which when combined with the water generated nitrous acid HNO2, HNO3 nitric acid and sulfuric acid H2SO4. 
Acid rain can have harmful effects on plants, animals and buildings. Causes corrosion of metal structures such as bridges or erosions in stone buildings. It also burns the leaves of vegetation and ecosystems it affects mainly vegetables, such as forests, but can also affect lakes and reservoirs acidifying them.
In Barcelona this rain, is alkaline.