Buffer solution making and investigation.

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Buffer solution making and investigation

Aim

I will attempt to prepare two buffer solutions. The first buffer solution will have a pH value of 5.2 and will be made from a mixture of 1.0M ethanoic acid and 1.0M sodium ethanoate solution. The second buffer solution will have a pH value of 8.8 and will be made from a mixture of 1.0M ammonia and 1.0M ammonium chloride solution.

Plan

In order to make these buffers, I will need to calculate the exact proportions of salt solution and acid/alkali to add in order to obtain the necessary pH value:

pH = pKa - log [HA]

[A-]

Given that:

The pKa value for 1.0M ammonia and 1.0M ammonium chloride solution is 9.3.

The pKa value for 1.0M ethanoic acid and 1.0M sodium ethanoate solution is 4.8.

For the making of the buffer pH 5.2:

Equations for the dissociation of ethanoic acid and sodium ethanoate respectively:

CH3COOH + H2O ↔ CH3COO- + H3+O

CH3COONa → CH3COO- + Na+

Using the equation previously stated:

5.2 = 4.8 - log [HA]

[A-]

5.2 - 4.8 = - log [HA]

[A-]

0.4 = - log [HA]

[A-]

10-0.4 = [HA]

[A-]

0.398 = [HA] : [A-]

CH3COOH + H2O ↔ CH3COO- + H3+O

(100 - x) x x

= 0.398/1

100 x 0.398 = 28.46 cm3 salt

1.398

Therefore:

100 - 28.46 = 71.54 acid

For the making of the buffer pH 8.8:

Equations for the dissociation of ammonia and ammonium chloride respectively:

NH3 + H2O ↔ NH2- + H3+O

NH2Cl → NH2- + Cl-

Using the equation previously stated:

8.8 = 9.3 - log [HA]

[A-]

5.2 - 4.8 = - log [HA]

[A-]

0.5 = - log [HA]

[A-]

10-0.5 = [HA]

[A-]

0.316 = [HA] : [A-]

CH3COOH + H2O ↔ CH3COO- + H3+O

(100 - x) x x

= 0.316/1

100 x 0.316 = 24.01 cm3 salt

1.316

Therefore:

100 - 24.01 = 75.99 cm3 acid

In order to make these buffer solutions, I must accurately measure the volumes of the acid and the salt. Unfortunately, the most accurate volumetric measuring equipment available to me is a burette that is only accurate to one decimal place. However, this should be sufficient to obtain a pH value correct to one decimal place. I will use two burettes, one filled with acid, the other with salt. I will run the required amount of acid and salt into a beaker in each case. I will then measure the pH using a digital pH meter, calibrated using buffer solutions of pH 4 and 7. Hopefully, the intended values of the buffer solutions will be the same as the accurate values +-0.1.

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Results

I had some difficulty in obtaining an accurate value for the pH of my buffer solution due to the unreliability of the pH meters available to me. However, I did manage to obtain a reliable pH meter eventually and when I measured the pH of my buffer solutions with it I found that the buffer solution made from the ethanoic acid and the sodium ethanoate had a pH value of 5.2 +-0.1 as the pH meter tended to flicker between 5.1 and 5.3, and the buffer solution made from the ammonia and the ammonium chloride had a ...

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