Water molecules can dissociate and ionize when a hydrogen atom shared by two water molecules in a hydrogen bond shifts from one molecule to the other. The electron of the hydrogen atom is not transferred, only a single proton. This single proton is referred to as a hydrogen ion (H+), with a charge of +1. the water molecule that lost the proton becomes a hydroxide ion (OH-) with a charge of -1. the water molecule bonding to the hydrogen ion becomes a hydronium ion (H3O+) with a charge of +1. The chemical equation is written:

2 H2O H3O+ + OH –

To simplify the equation, by convention we write:

H2O H + + OH -

2

A solution that has equal concentrations of H+ and OH- is said to be neutral.

An acid is a substance that dissociates when dissolved in water, producing H+. Hydrochloric acid is a strong acid.

HCl H+ + Cl-

A base is a substance that, when dissolved in water, reduces the hydrogen ion concentration. Some bases do this by donating OH- that interact with H+ to form water. Sodium hydroxide is an example of a strong base.

NaOH Na+ + OH-

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A measure of the power of the hydrogen ion concentration in a solution is known as pH.

pH= - log₁₀ [H+]

The pH scale ranges from 0-14 for biological systems. At neutrality, the hydrogen and hydroxide ion concentration is 10⁻⁷ M. Therefore, at neutrality pH is equal to 7. There is an inverse relationship between pH and hydrogen ion concentration; that is, as the hydrogen ion concentration increases, the pH decreases, and vice versa.

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Base: dissociates hydroxide ions into solution or accepts hydrogen ions

pH above 7

Acid: dissociates hydrogen ions into solution

pH below 7

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It is very important for living organisms to maintain a pH around 7.2- 7.4.

Buffers are special solutions that can help maintain pH by resisting any changes to pH caused by the addition of an acid/base.

Buffers are solutions of weak acid/weak base. An example is carbonic acid/bicarbonate in your blood.

H₂CO3 + H₂O ↔ H3O⁺ + HCO3⁻

H2CO3 (carbonic acid) is able to act as an acid by donating H+ when a base is added and HCO3 (bicarbonate) is able to act as a base by accepting H+ when acid is added.

The more the resistance to a change in pH the better the buffering capacity of the solution.

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Anthocyanins are plant pigments that can be used as a pH indicator because they change color in response to changes in pH. These pigments are responsible for red, blue, and purple colors that are found in flowers, fruits, and autumn leaves.

This is an example of the colors obtained for a few pHs.

pH 2 pH3 pH5 pH7 pH8 pH12

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It has a greater accuracy. Before a pH meter can be used, it must be

calibrated. For best results, calibration should be performed

with both pH 7 and pH 4 and it should be done every couple of hours.

The pHs obtained in class were as follows: Solution pH

Distilled water 6.53

Apple juice 4.56

Vinegar 2.90

Soft drink 3.4

Coffee 5.02

Detergent water 8.10

Milk 6.728

pH reading

The pH paper is orange in color before use and changes color depending on the pH being tested.

Solution pH

Distilled water 7

Apple juice 5

Vinegar 3

Soft drink 3

Coffee 5

Detergent water 8

Milk 7

The pHs obtained in class were as follows:

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Determines how good a buffer is in the presence of an acid or a base.

You will record the pH of two solutions (A and B) as their buffering capacities are tested with the addition of acid and then base. By examining your results you will determine which of the two solutions is the better-buffered solution.

Initial pH of solution A= 7Initial pH of solution B = 7

10

ml of acid/base added

HCl

NaOH

ml of acid/base added

HCl

NaOH

1 2 3 4 5 6 7 8 9 10

7 6.9 6.87

6.82

6.8 6.75

6.59

6.47

6.2 6

7 7.01

7.03

7.04

7.10

7.30

7.35

7.60

7.90

8

1 2 3 4 5 6 7 8 9 10

2.39

2.35

2.29

2.20

2.01

1.99

1.96

1.92

1.80

1.73

10.2

10.31

10.4

10.5

10.6

10.7

10.8

10.9

11.01

11.3

pH of Solution A

pH of Solution B

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End

pH

0

12