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
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.