Investigating the Concentration of Citric Acid


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Investigating the Concentration of Citric Acid


Problem
-------

I have been set the task of determining the concentration of a sample
of citric acid, using only a 1.0 M solution of sodium hydroxide and
any laboratory glassware that is required. I have also been given the
RMM of citric acid as 210g and I have been told that one mole of
citric acid will react with 3 moles of sodium hydroxide.


My Experiment
-------------

Using this information I have determined that the best way to conduct
this experiment is to do a titration. This will allow me to determine
the amount of impure citric acid required to neutralise a known volume
of 1.0 M sodium hydroxide. Consequently, by calculating the correct
volume of pure citric acid that would be required to neutralise this
volume of sodium hydroxide, I can calculate the percentage purity of
the citric acid.


Calculating the Amount of Citric Acid
-------------------------------------

In order to make this experiment a good titration, I must use a
reasonable amount of citric acid. This is so that the amount of citric
acid needed to neutralise the sodium hydroxide is not so small that it
is difficult to read off the burette, and so that it is not so large
that it takes more than 50cm3, and so 2 burettes must be used. I think
that I can safely assume that in this experiment the percentage purity
will be greater than 50%. Therefore, if I calculate the amount of pure
citric acid required to neutralise 25cm3 of sodium hydroxide when the
citric acid is 100% pure, I should easily be within the confines of my
50 cm3 burette. I will therefore now calculate the required mass of
sodium hydroxide:

Ratio citric acid : sodium hydroxide

1 : 3

Concentration of sodium hydroxide = 1.0 M

Therefore, require concentration of citric acid = 1.0 M /3 = 0.33 M

I will be using a volumetric flask which can hold 250cm3 of liquid.

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Therefore, in order to make a 1.0 M solution of citric acid I will
require 210 / 4 as I am not using a whole litre of water.

210 / 4 = 52.5g of citric acid.

However, I need to make a 0.33 M solution and therefore I will require
52.5 / 3 grams of citric acid.

52.5 / 3 = 17.5g of citric acid required to make a 0.33 M solution of
citric acid, assuming that the acid is 100% pure. It is clear that the
concentration of citric acid in my solution will not be 0.33 M, but
this calculation allows me to make an approximation and therefore
perform my titration experiment.


Method
------

1) I will carefully weigh out 17.5g of citric acid and dissolve it in
distilled water. I will then take the dissolved citric acid and place
it carefully in a volumetric flask of volume 250cm3. I will then wash
out the beaker, in which I have dissolved the citric acid carefully,
with distilled water and add the washings to the volumetric flask. I
will repeat this process three times to ensure that all of the citric
acid has been transferred from the beaker to the volumetric flask. I
will then fill up the volumetric flask with distilled water adding the
last few drops with pipette filler until I have exactly 250cm3 of
water in the volumetric flask. I will then mix the concentrated citric
acid with the water by turning the flask upside down and then upright
three times.

2) I will then use the citric acid solution that I have made up, to
fill the burette up to the 0cm3 mark. I will fill it by first pouring
the citric acid solution into a small beaker and then pouring this
carefully into a small filter funnel, which is inserted in the
burette.

3) I will then take a pipette and measure out exactly 25cm3 of the 1.0
M sodium hydroxide solution and place this carefully into a conical
flask along with several drops of phenolphthalein indicator.

4) I will then do a trial titration to gauge the approximate amounts
necessary for neutralisation to occur. I will be able to see this when
the phenolphthalein indictor, which is pink in alkali, turns
colourless.

5) I will then do 4 accurate titration experiments in order to ensure
that my results are correct. I will then record the average value for
my results.

6) I should now be able to calculate the percentage purity of the
citric acid using my average value for the amount of citric acid
required to neutralise the 25cm3 of sodium hydroxide.


Fair Test
---------

In order to keep my experiment as accurate as possible, I will keep
the amount of sodium hydroxide in the conical flask constant at 25cm3,
I will also keep the concentration of the sodium hydroxide constant at
1.0 M. I will measure the amount of citric acid needed to turn the
phenolphthalein colourless, and record this accurately.


Safety
------

Throughout this experiment, I will ensure that I wear safety goggles
and a protective lab coat so that I my eyes and clothes are protected
from the acids and alkalis and no damage is likely to be caused to
either.


Results
-------

Titration Results

Trail Titration (cm3)

Titration 1(cm3)

Titration 2 (cm3)

Titration 3 (cm3)

Titration 4(cm3)

Initial Volume in Burette

0.7

0

0

0

0

Final Volume in Burette

46.8

45.4

45.2

45.3

45.3

Volume citric acid used

46.1

45.4

45.2

45.3

45.3

Average volume used

45.3


Calculation to work out the percentage purity of the citric acid

Volume sodium hydroxide used: Volume citric acid

25cm3 : 45.3 cm3

Number of grams in 25 cm3 of sodium hydroxide = 1.0M contains 1 mole
of sodium hydroxide per 1 dm3 . Therefore in 25 cm3 (1 dm3 / 40 = 25
cm3) amount of moles of sodium hydroxide in 25 cm3 = 1.0/40 = 0.025
moles.

Number of moles = Mass/RMM

O.25 = Mass/40 (RMM NaOH = 23+16+1 = 40)

0.25*40 = Mass

1g = Mass

Therefore, the mass of sodium hydroxide in 25 cm3 = 1g

In 250 cm3 of impure citric acid solution there is 17.5g of impure
citric acid.

Therefore in 1 cm3 there is 17.5/250 grams of impure citric acid.

17.5/250=0.07

I used 45.3 cm3 of impure citric acid solution.

0.07*25 = 3.171g

Ratios of volume should be 1:1 as I calculated the exact amount of
pure citric acid to exactly neutralise the 25 cm3 of sodium hydroxide
solution in a 1:1 ratio. Therefore if the citric acid were pure, it
would take 25 cm3 to neutralise the sodium hydroxide.

Therefore the correct volume of citric acid is:

0.07*25 = 1.75g

Percentage purity = (correct value/impure value) * 100

Percentage purity of the citric acid = (1.75/3.171) * 100


Conclusion
----------

According to my results, I can conclude that the percentage purity of
the sample of citric acid I have been given is 55%


Evaluation
----------

I think that my results are accurate within the boundaries of
experimental error. However, I think that at this early stage of the
IB course my experimental technique in areas such as doing titrations
and making up standard solutions leaves a good deal to be desired and
I could certainly make my results more accurate if I took into account
the following factors in future.

1) I find that when I transfer the concentrated solution into the
volumetric solution whilst making the standard solution, I have often
dissolved the citric acid in too much water and therefore, I have
difficulty in washing out the three times as this would overfill the
flask. I think to improve my technique in this area I need to be more
patient whilst dissolving the solid in the water, rather than assuming
that my solution is saturated too early.

2) I also find that in transferring the concentrated solution and the
washings into the volumetric flask, a drop of liquid will often
trickle down the side of the beaker and is consequently lost. This
makes the experiment considerably inaccurate. I think that this
situation is due to the size of the funnel that I am using, as this
means that I have to pour the liquid very slowly and consequently
drips escape. To rectify this predicament, I could either use a larger
funnel, or if this would not fit in the neck of the volumetric flask,
I could control the flow of the liquid by using a large funnel above a
small one. I think that the solution is particularly good as it would
allow me to keep the flow of liquid constant, whilst any escaping
drips would fall into the larger funnel.

3) It is also possible that my lack of experience at reading values
from burettes, volumetric flasks and pipettes may give rise to minor
inaccuracies. However, I would consider these inaccuracies to be
extremely minor as I am fairly confident in my own ability to read off
accurately. I would say, however, that my own lack of experience in
using a burette and finding the exact point when the indicator changes
colour may have given rise to a degree of inaccuracy. It is quite
possible that I have misread the colour change on one or more
occasions, or even added too many drops to allow the indicator to
change colour.

However, despite these minor points which could possibly give rise to
some degree of inaccuracy, I am confident enough in my experimental
technique to say that my results are fairly accurate within the limits
of experimental error.


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