Unit 2- Practical Scientific Procedures and Techniques
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Unit 2 Aim A: Titrations and Colorimetry (DISTINCTION)
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Unit 2- Practical Scientific Procedures and Techniques
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Unit 2- Practical Scientific Procedures and Techniques
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Practical Scientific Procedures and Techniques
1. Acid Base Titration Neutralisation (Standardisation) of HCl by ~0.1mol dm-³ Standard of
Sodium Carbonate, Na₂CO₃.
Titrations are precise analytical procedures, performed under carefully controlled
environments. They can be used in industry for purposes such as testing accuracy in
labelling of consumer products. In this case, the aim was to find the concentration of the
Hydrochloric Acid used.
The equipment needed to perform this titration includes: volumetric flask, burette, pipette,
Erlenmeyer flasks, retort stand, clamp, funnel, lab wash bottle. Also needed is: around
2.4500g anhydrous Sodium Carbonate, Hydrochloric acid (unknown concentration),
phenolphthalein indicator, distilled water.
Health and Safety:
Hydrochloric acid is corrosive to the skin, eyes and mucous membranes so contact and
inhalation of fumes must be avoided and the skin/eyes washed immediately if contact
occurs. Contact may cause burns, ulceration and scarring; inhalation may cause pulmonary
irritation and upper respiratory lesions. Hydrochloric acid must not be ingested - this may
cause corrosion of the mouth, oesophagus and stomach, vomiting and diarrhoea. (EPA,
2000).
Sodium Carbonate can cause respiratory irritation, coughing, shortness of breath and
pulmonary edema if inhaled - always work at a distance and do not inhale. Contact with the
eye can cause burns and permanent injury to the cornea - avoid contact with the eyes and
wash out with water for at least 15minutes if contact occurs. Wash skin immediately if
contact occurs, as sodium carbonate can cause irritation. (EHS, 2015)
Be careful with all glass equipment to avoid breakages.
PPE including goggles and laboratory coat must be worn at all times.
Figure 1. Table of Results for the Titration.
As seen in figure 1, the start level, end level and titre (or volume used, calculated using: titre
= end level - start level) are recorded before a mean of the 8 repeats is calculated; for this
set of results, the mean titre was 22.3cm³ or 0.0223dm³. It is repeated 8 times to ensure
accuracy in the results, providing a smaller margin of error. The first, rough titration is not
,included in the mean as it was not carried out accurately enough and may alter the results.
Any anomalous results, such as the result of 26.4 above, should not be included in the mean
either, as there must have been an error in execution.
These results will be used to find the concentration of Hydrochloric acid. First, find the
relative formula mass of Na₂CO₃ by adding the relative atomic masses of its components, (2
x 23) + 12 + (3 x 16) = 106. Then divide the mass of Na₂CO₃ used, 2.45, by the Mᵣ, 106, to
find the moles of sodium carbonate used to be 0.023 moles. This is then divided by the
volume of the solvent used (this must be in dm³), in this case 0.0223dm³, to find that
0.092mol dm-³ was the concentration of sodium carbonate solution. Times this concentration
by 0.025 to find that 0.0023 moles of Na₂CO₃ were in the 25.0cm³ used for each individual
titration run. The molar ratio of Na₂CO₃ to HCl is 2:2, as you can see from the balanced
equation, Na₂CO₃ + 2HCl = 2Na₂CO₃ + H₂O + CO₂. This means 0.0023 moles of HCl were
used also. This titration ran to the exact halfway point, neutralisation , rather than completing
it, so the mean titre must be doubled to account for this (0.0223 x 2 = 0.0446dm-³). To find
the concentration of hydrochloric acid, simply divide 0.0023 moles by the 0.0446dm-³ mean
titre, giving an answer of 0.1031390135mol dm-³; it is important not to round this number.
The margin of error for these results can be calculated using the equation, percentage error
= (concentration of acid in titration / actual concentration of acid) - (actual concentration of
acid x 100). The actual concentration of the acid used here was 0.100mol dm-³, so
(0..100) - (0.100 x 100) = 3.14. These results are therefore 96.86% accurate.
3.14% margin of error shows the results to be highly accurate, as well as precise, shown by
the mostly concordant results for titre. This was possible by ensuring no solvent was spilled
or left in the container and all equipment was rinsed of any solution using distilled, not tap,
water, this is important because of the highly accurate nature of titrations themselves. In just
25.0cm³ of the sodium carbonate solution, there are 1.3850921 x10²² atoms, so even losing
one drop of the solution could mean losing thousands of atoms, decreasing accuracy
considerably. The exact mass of anhydrous sodium carbonate had to be recorded so that all
calculations were accurate, as well as adding precisely the correct volume of the solution
into the flask using a pipette and looking from eye level to avoid meniscus interference. The
burette had to be rinsed with HCl before being used to avoid residue from previous uses
from interfering. It is important that the colour change of the indicator was spotted
immediately so that the titre was correct, this meant swirling the flask as the acid poured into
it from the burette only drop by drop once close to the rough titration’s end point and having
a white tile placed beneath it to make the colour change more easily visible, the faint pink
colour at which neutralisation was said to be complete had to be consistent across each
repeat to prevent discordant results.
One problem encountered was misuse of the pipette, which could have caused any
discordant or anomalous results recorded, so practising using one beforehand, ensuring it
was working correctly or finding an easier alternative to this could be beneficial. Another
problem was overfilling the flask with distilled water when making the sodium carbonate
solution, this could be prevented by using small, plastic pipettes to fill it. Sometimes the
funnel became blocked with Na₂CO₃ powder, so using a wider-mouthed funnel or using a
wash bottle to rinse distilled water through as it was emptied into the flask could help. A
chart to show the colour at which neutralisation is truly achieved could reduce
inconsistencies in titre volume and improve reproducibility as there may have been
confusion about the exact colour that shows neutralisation has been achieved. Since so
many atoms could be lost, reducing transfer of solutions is essential in improving the
method. As well as this, the equipment used should be to a high degree of accuracy in its
measurements.
, This titration was only done to exactly half way, neutralisation, rather than completing it by
adding a methyl orange indicator and producing an alkaline solution. This was because the
second half would use extra time and resources without adding any value to the results. To
account for this, the mean titre was doubled during calculations.
This titration used 25.0cm³ of 0.092mol dm-³ sodium chloride solution to neutralise an
unknown concentration of hydrochloric acid, using a phenolphthalein indicator to observe a
colour change which showed neutralisation. Stoichiometry and molarity laws were then used
to calculate the concentration of hydrochloric acid, 0.1031390135mol dm-³. The margin of
error was also calculated, to be 3.14%, showing high accuracy. (Tan, 2021)
2. Standardisation of NaOH Using HCl Standard From Last Titration With use of pH Meter
and pH / Volume Graph.
Titrations are precise analytical procedures, performed under carefully controlled
environments. They can be used in industry for purposes such as testing accuracy in
labelling of consumer products. In this case, the aim was to find the concentration of the
Hydrochloric Acid used.
The equipment needed to perform this titration includes: volumetric flask, burette, pipette,
Erlenmeyer flasks, retort stand, clamp, funnel, lab wash bottle, pH probe. Also needed is:
around Hydrochloric acid (unknown concentration), NaOH solution, distilled water.
Health and Safety:
Hydrochloric acid is corrosive to the skin, eyes and mucous membranes so contact and
inhalation of fumes must be avoided and the skin/eyes washed immediately if contact
occurs. Contact may cause burns, ulceration and scarring; inhalation may cause pulmonary
irritation and upper respiratory lesions. Hydrochloric acid must not be ingested - this may
cause corrosion of the mouth, oesophagus and stomach, vomiting and diarrhoea. (EPA,
2000).
Sodium Chloride can cause irritation if in contact with skin or eyes - avoid contact and wash
with water immediately if contact occurs. Do not inhale - may cause respiratory irritation -
work at a distance and move to fresh air if inhaled. (Fisher Scientific, 2008)
Be careful with all glass equipment to avoid breakages.
PPE including goggles and laboratory coat must be worn at all times.
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