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 add to it 90cm3 of distilled water measured out using a

burette.? The second will be a 1/100

dilution, for this I shall measure out 2cm3 of buffer solution and

add to it 198cm3 of distilled water.? I shall then test these dilutions for their effect on the

buffering capacity of my buffer solution.?

I shall use a pipette to measure out exactly 25cm3 of each

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

so that I will have four small beakers each containing 25cm3 of a

dilute buffer solution.? For each dilute

buffer, to one beaker I shall add HCl, to the other NaOH from two separate

burettes.? Initially, I shall add one

drop, then 1cm3 and then 5cm3 measuring the pH after each

step and recording it in a table.Results Vol.

added (cm3) HCl

(undiluted buffer) NaOH

(undiluted buffer) HCl

(1/10 diluted buffer) NaOH

(1/10 diluted buffer) HCl

(1/100 diluted buffer) NaOH

(1/100 diluted buffer) 0 9.0 9.0 9.0 9.0 9.0 9.0 0.1 8.9 9.0 8.8 9.2 7.8 10.3 1.0 8.7 9.2 8.5 9.7 2.0 11.6 5.0 8.2 9.9 2.0 11.7 1.6 12.2 ??????????? ?? Conclusion ??????????? As

expected, the introduction of a small amount of strong acid or alkali to my

undiluted buffer solution made little difference to its pH.? However, the introduction of even small

amounts of strong acid or alkali to the diluted buffer solutions made a

substantial difference to their pH.? In

a previous experiment, I investigated the affect of adding acid or alkali to

water in comparison to a buffer solution, the differences in the change in pH

was marked the water changes pH easily with the introduction of small amounts

of acid or alkali in contrast to the buffer solution.? Similarly, as I added more water to a decreasing amount of? buffer solution, the buffer?s ability to

reduce the effect of additions of acid or alkali is also reduced and the effect

of the water, becomes more prominent, hence the pH either drops or rises far

more rapidly when the buffers are diluted and the more diluted they become, the

greater the effect of introducing acid or alkali to the solution. Therefore, I

can conclude that dilution of a buffer solution has an adverse effect on its

buffering capacity. I was also asked to suggest a reason for the likelihood

that the buffer of pH 8.8 may be less stable over a period of time than many

other buffers.? I would suggest that

this is because the ammonia is normally a gas at room temperature.? As a result, it is likely that the liquid

ammonia will be unstable and prone to evaporate and become a gas once

more.? Mixing the ammonia liquid with a

solution of its salt will have no effect on the unstable nature of the ammonia

and therefore the buffer is more likely than other buffers to break down over

time as the ammonia is likely to evaporate thus reducing its ability as a

buffer and altering its pH.Evaluation ??????????? This

experiment was one of the most difficult that I have attempted.? I found it difficult mainly because of the

time restraints put on me.? I made my

task more difficult by my preparation; I made an initial fundamental mistake in

the calculation of the volumes of acid and salt solution necessary to obtain

the required pH.? This resulted in the

pH of my first two solutions being very wrong, it was only when I realised this

mistake that I was able to correct it and hence continue with the experiment.? This would not have mattered except that

there was a strict time limit on the experiment.? This meant that I had to rush the remainder of the experiment,

i.e. the dilution of the buffer solution and therefore, I did not spend as much

time checking the volumes for the dilution as I would have liked and I used

measuring cylinder instead of a burette for measuring out the distilled water.

Furthermore, when I was adding acid and alkali to the buffer, I did not have

time to let the pH meter settle down totally and I was taking the first stable

reading as correct, as I have discovered this is not always the case.? This having been said, the pH meter that I

eventually found after using several different types was very reliable and

accurate and I feel that my results are reasonable.? I do not feel that my results are inaccurate, but that they could

perhaps be improved if I spent a little more time taking them.? If I was to repeat this experiment, I would

improve it by spending a whole day on it, I would carefully make both buffers

and allow them to settle before diluting them meticulously.? Finally, I would add acid and alkali to both

buffers.? I would contrast their

performance with that of water and work out their buffering capacity in

comparison to each other and water. I would also meticulously note the pH

changes of the diluted buffers and work out whether the decrease in pH is

directly proportional to the amount of water added.