Making And Investigating Buffer Solutions Essay Research

Making And Investigating Buffer Solutions Essay, Research Paper

`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 acidFor 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 acidIn 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.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 pH of 9.0+-0.1 as

once more the pH meter tended to flicker between 8.9 and 9.1.. I think that the incorrect value of the

second buffer solution can be attributed partially to the inaccuracy of the pH

meter, but also to the inaccuracy of my measurement.. My burette for the first part of the experiment had a faulty tap,

I did not realise this until part way through, but as I made the ammonia

solution first, it is likely that this caused the pH of my buffer solution to

be inaccurate.Investigation of my buffer solutions: ........... I chose to

investigate only the buffer solution of pH 9.0 after being instructed to only

investigate only one buffer solution by my teacher.. In order to investigate the buffering capacity, I will add some

1.0M HCl and NaOH to the buffer solution to see how effective my buffer is in

combating the introduction of acid or alkali..

According to buffer theory, the introduction of small amounts of acid or

alkali to the buffer solution should have little effect on the pH of the

solution due to the nature of the buffer solution. I shall set up two burettes

one filled with 0.1M HCl the other with 0.1M NaOH.. I shall then use a pipette to measure out exactly 25cm3

of buffer solution and place it in a small beaker. I shall then repeat this so

that I will have two small beakers each containing 25cm3 of buffer

solution.. To the one I shall add HCl,

to the other NaOH from two separate burettes..

However, I shall add one drop, then 1cm3 and then 5cm3

measuring the pH after each step and recording it in a table. ........... I will also

investigate the effect of dilution on buffer solutions.. I will make two solutions, the first will be

a 1/10 dilution, and for this I shall measure 10cm3 of buffer

solution and

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