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Unit 19 - Practical Chemical Analysis Assignment 3 (Merit)
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Assignment 3 – ChemistryMerit
Colorimetry is spectroscopic type technique also known as colorimetric analysis. Colorimetry is an
analytical technique which chemists use to determine a coloured solutions concentration. When
carrying out colorimetry there are a few steps which must be carried out and these are as follows;
The first step in colorimetry is to create a standard solution. A standard solution is a solution which
contains a known and accurate concentration (amount) of a substance or element. A standard
solution is typically used to aid in identifying and determining the concentration of a substance
which has an unknown concentration. When attempting to analyse an unknown substance, it’s
important that the chemist is as accurate as possible with all results they obtain – this is where
standard solutions step in. This is because these solutions contain an accurate concentration of a
chemical component, so, they will increase confidence in regard to determining substances with
unknown concentrations. When a standard solution is being prepared, it’s likely that the primary
standard will need to be dissolved in a solution such as distilled water. What is a primary standard?
A primary standard can be described as a type of reagent which has certain characteristics. A
primary standard helps in accurately assessing the substances unknown concentration. A primary
standard is more scientific terms can be described as a typical reagent which is able to be weighed
easily. It is so pure that it weigh is typically representative of the amount of moles present within a
substance. Below are the steps on how a standard solution is created:
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, Assignment 3 – Chemistry
Merit
Firstly, the weight of the substance is needed in order to create a standard solution typically to
250ml using deionised water. Therefore, an empty weighing boat must be placed onto a three figure
balance before tare is pressed to zero it. Then the approximate mass of the substance required must
be added to the weighing boat before removed the boat from the weighing pan before re-zeroing it.
As the boat now contains the substance it should be placed back onto the weighing pan and
reweighed (now it should read heavier than the original weight) – this weight should be recorded.
The sample should ideally be weighed out to plus or minus 0.01 of a gram. As much as possible of
the substance should be placed into a dry, clean beaker and the empty weighing boat reweighed.
The mass of the empty weighing boat should be recorded, this mass should be then subtracted from
the mass of the substance and boat which was recorded previously – this will give the accurate mass
of the substance to three decimal place figures. Then, the substance which was placed into the dry
beaker must be dissolved using a small amount of deionised water. (Ensure that there’s no splashing
through adding the deionised water down the sides of the beaker.) The substance should be stirred
gently using a stirring rod until it is completely dissolved. (If it’s slow to dissolve it’s possible to use
warm water or heat it gently over a Bunsen burner to speed up the dissolving process *It’s
important to note that the hot solution should NEVER be poured into graduated glassware, this is
because the glass will expand and will never return to its original shape, this could have an effect on
the calibration and for further measurements the glassware would be unreliable.*) Then a funnel is
used to transfer this dissolved solid into a volumetric flask. A series of swills using deionised water
are then carried out on the beaker as this ensures that all of the solid is removed from the glass. The
Stirring rod and funnel are also both swilled into the volumetric flask in an attempt to ensure all of
the dissolved solid is present in the standard solution, to make it as accurate as possible. A deionised
water bottle is then used to fill the volumetric to two centimetres below the meniscus of the
volumetric flask (essentially the base of the fluids meniscus should just touch the graduated mark on
the neck of the flask.)The flask should be held at eye level as this will prevent errors of parallax. A
dropping pipette should be used to add the last few drops so that the meniscus is sitting on the line
of the volumetric, the last few drops are vital as if the mark ends up being overshot the solution will
no longer be standard. Finally, a stopped should be placed onto the flask before being inverted
numerous times to ensure that the standard is thoroughly mixed (This is a vital step because if the
mixture hasn’t been efficiently mixed it’ll end up being more concentrated at the based where the
original solution was added, and at the top it would be less concentrated where the deionised water
was added.) Although after inverting the flask the meniscus could now be below the mark NO MORE
WATER SHOULD BE ADDED.
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