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Titration Practical

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Titration Practical

Planning assessment

The apparatus to be used:

· Beaker

· 250cm 3 volumetric flask

· Funnel

· Burette

· Pipette

· White tile

· Conical Flask

· Balance

· Spatula

· Weighing bottle

· Glass rod

Method

Making a standard solution:

A weighing bottle was accurately weighed and approximately 5g of
anhydrous sodium carbonate was added and the weight of the bottle plus
the solid recorded. The anhydrous sodium carbonate was then
transferred into a 100cm3 clean beaker. The bottle was carefully
rinsed out two or three times with water and the washings were
transferred to the beaker each time. About 25cm3 of water was poured
into the beaker and stirred with a glass rod until the solid had
completely dissolved. This solution was then added to a 250cm3
volumetric flask using a funnel. The beaker and funnel were swilled
thoroughly using a small amount of water these washing were then added
to the volumetric flask. Water was then added to the volumetric flask
until it was about 1cm below the graduation mark. The water was then
added slowly from a clean pipette so that at eye level the bottom of
the meniscus was just touching the graduation mark. The volumetric
flask was then stoppered and then inverted.

Carrying out a titration:

A conical flask was swilled out with water and a pipette and pipette
filler were used to withdraw 25.0cm3 of the sodium carbonate solution
from the volumetric flask and transfer it to the conical flask. A
burette was first swilled with sulphuric (VI) acid using a clean, dry
beaker and a funnel and then filled to below the zero mark. A little
of the solution was then run out of the burette into the beaker and
the funnel removed. A white tile was then placed underneath the
conical flask and a few drops of the indicator methyl orange was added
to the sodium carbonate solution. The amount of sulphuric (VI) acid in
the burette recorded by seeing at eye level what mark the bottom of
the meniscus touched to the nearest 0.05cm3. The sulphuric (VI) acid
was then added to the sodium carbonate solution in small volumes until
the sodium carbonate solution had turned orange. The amount of
sulphuric (VI) acid left in the burette was measured and the amount
added to the sodium carbonate was then recorded by subtracting the
final reading of the burette from the initial one. This was a rough
titration; more titrations were done until there were three volumes
which agreed to within 0.1cm3.

Justifications

The funnel was removed from the burette before reading it as drops of
sulphuric (VI) acid could have fallen into the burette after the
reading was taken which would have made the readings inaccurate and
larger then it should have been. A white tile was placed underneath
the conical flask so that any colour changes could be observed more
clearly as the white would contrast with orange. The burette was read
at eye level to avoid any parallax errors, as this is where the best
view of the meniscus is, and from different angles the water can look
different. A rough titration was carried out to see how much of the
sulphuric (VI) acid was roughly needed to react with the hydrous
sodium carbonate. The conical flask was swilled as the solution from
the burette was added so that all of the solutions reacted with each
other. The burette and pipette were washed out with the solutions that
were to go in them before they were used to ensure that any other
substances are rinsed out and they are the only substances in the
glassware. If this was forgotten and the burette was not washed out
the reading would be inaccurate and larger then they should have been
as it would seem more sulphuric acid would be needed to neutralise the
sodium carbonate. The conical flask was swilled with water to get rid
of any impurities in the conical flask, the solution going into the
flask already contained water as a solvent so no new substances were
added. The solution was released from the burette drop by drop when
the end point was near to find the exact drop at which the sulphuric
(VI) acid stops reacting with the sodium carbonate solution. If it
were let out very quickly there would be a larger percentage error, as
the readings would have a large difference in values. The experiment
was repeated until concordant results were obtained so that reliable
and accurate readings were acquired. Approximately 5cm of the
anhydrous sodium carbonate was used as this was a logical amount; not
too much so that it would not completely dissolve, but not too little
so that only a small amount of the sulphuric (VI) acid would be needed
to neutralise it as a smaller reading gives a larger percentage error.
250cm3 standard solution of sodium carbonate was made so that the same
solution could be used when doing repeats because this ensures that
the concentration of the sodium carbonate solution used for each
titration is the same. 25.0cm3 of sodium carbonate was used for the
titration as this was a reasonable amount; enough to be able to stir
it and see a colour change, not too much so that large amounts of
sulphuric (VI) acid would be needed to neutralise it. Only a few drops
of the methyl orange indicator was added to the sodium carbonate
solution as if too little was added then the colour would be too faint
to see; but if too much was added the colour would be too deep for the
colour change to be seen clearly, which would lead to less accurate
results.

Methyl Orange is a weak acid so just a small proportion of their
molecules ionise in aqueous solution. We can show this as an equation:

YELLOW

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HIn represents the methyl orange which is a weak acid, and In– the ion
it forms when it ionises. HIn and In– have different colours: HIn is
red and In– is yellow. Because it is a weak acid it will be mostly in
the HIn form and therefore coloured red. But when added to the sodium
carbonate solution, this will react with the H+ ions and then more HIn
will ionise so as to replace them. So more In– will be formed and
eventually the indicator will appear yellow. This will show up when
the In– concentration is about 10 times greater than the HIn
concentration.

The reaction is reversible, as we add the sulphuric (VI) acid from the
burette the neutralsation will occur but because the sulphuric acid is
a strong acid and sodium carbonate is a weak alkali the point of
neutralisation for such a titration will occur when the pH of the
solution is in the region of pH3.2 and pH4.4 which corresponds to the
pH range over which point the methyl orange changes colour from yellow
to orange, so it is appropiate to use this indicator

Risk assessment

As sodium carbonate is an irritant goggles and a lab coat must be worn
to prevent any irritation to the eyes or skin. Try not to inhale
vapours as sodium carbonate can irritate the respiratory system. If
swallowed drink plenty of water and seek medical attention. If
substance gets into eyes flood the eyes with gently running tap water
for 10 minutes and seek medical attention. If sodium carbonate
solution is spilt onto skin or clothes immediately remove contaminated
clothing and wash skin with plenty of water. Soak contaminated
clothing ands rinse repeatedly. If any of the substance is spilt in
the lab scoop up the solid and if any solution is spilt add mineral
absorbent and scoop into a bucket. Rinse the area of the spill and the
cloth or mop thoroughly. If methyl orange is swallowed immediately
wash out mouth, have a glass or two of water and seek medical
attention. If the indicator gets into the eyes flood eyes with gently
running tap water for 10 minutes and seek medical attention. If methyl
orange is spilt on skin or onto clothes remove contaminated items and
flood area with water and then wash thoroughly with soap and cold
water. If spilt in the lab, wash the area thoroughly. Sulphuric (VI)
acid is a corrosive and can cause burns to the skin so a lab coat must
be worn at all times. If swallowed, wash out mouth and have a glass or
two of water. Do not induce vomiting and seek medical attention as
soon as possible. If splashed into the eye flood eye with gently
running tap water for 10 minutes and seek medical attention. If the
acid is spilt onto skin or clothes remove the contaminated clothing
and quickly wipe as much liquid off the skin as possible with a dry
cloth before drenching the area with a large excess of water. If a
large area is affected or blistering occurs seek medical attention. If
spilt in the laboratory wear eye protection and gloves. Cover with
mineral absorbent and scoop it into a bucket. Add anhydrous sodium
carbonate over the mixture and leave to react, and then add lots of
cold water. Rinse the area of the spill several times with water.

Sources of information

AS Chemistry At Esher College Work Book 1 “The Elements of Life” and
“Developing Fuels”, pg. 4-5

Hazcards, Cards 98, 95 and 32

http://www.chemistry-react.org/go/Faq/Faq_8219.html

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