Unit 2 - Practical Scientific Procedures and Techniques
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Unit 2 Assignment A
Aimee Baish FSC year 1
Titration and colorimetry
Introduction
Titrations are used to find the unknown concentration of a solution, where a solution with a known
concentration is added into the solution with the unknown concentration until the pH is 0 or until
the mixture is neutral, which is normally a change in colour (shown by using indicator) or by using a
pH meter. Using the volume of solution (with a known concentration) used, for the change to
happen, you can determine the concentration of the other solution. Colorimetry is the measure of
how much of a colour wavelength a sample will soak up or send out, this can lead to determining the
concentration of a substance by putting a sample of the substance into a colorimeter, and then
comparing it to different concentrations on a graph. A colorimeter works by passing light through a
sample you’ve put in the cuvette and then put in the machine, and it measures how much of the
light the colorimeter transmitted was absorbed into the sample, and how much the sample
transmitted.
Equipment calibration
To calibrate a pipette, you would measure 10cm 3 of water and then weigh it (it should weigh 10g),
and then you repeat this 10 times. The closer you are to 10g the better, however you should mostly
be a maximum of 0.02g out, this is the absolute error of the pipette itself.
To calibrate a pH meter, you would make sure the pH meter is turned on and in calibration mode (it
will say CAL on the screen) and then when the screen starts flashing 4, you would put the pH meter
in the buffer for 4, then when it starts flashing 7, put the pH meter in the buffer for 7. After this, the
pH meter will have a small 4 and 7 in the top corner, that shows that its calibrated.
To calibrate scales all you would do is weight 5g of a substance on the scales after setting it to 0.
Repeat this a few times, and then use all the other scales available. For example, kitchen scales (go
to 2 decimal places), balances (go to 3 decimal places) and an analytical balance (goes to 4 decimal
places). This would show that each scale/balance has a different absolute error, this would show
that if you were even 0.0001g away for 5 grams on an analytical balance, but it wouldn’t do this on a
kitchen scale.
, To calibrate a colorimeter you put some water in a cuvette and then after the colourimeter has been
turned on, you would put the cuvette of water in the machine and press the T button, which runs
the test, and then when the water a true 0 reading you would then continue testing the rest of your
solutions.
Making the titrant solution (Na2CO3) method
1) Using a balance (which has an absolute error of ±0.1g) weigh around 5.3g of Na2CO3 into
the small beaker.
2) Using the balance with an absolute error of ±0.01g, weight the small beaker and its contents
accurately.
3) Record the mass from step 2.
4) Move the contents of the small beaker into the large beaker.
5) Then weigh the small beaker again, using the same balance from step 2 and record the mass.
6) Then take the mass of step 3 and the mass from step 5 and find the difference between the
(take the step 3 mass away from the step 5 mass). This is the mass of sodium carbonate in
your beaker. My result, 107.51-102.01=5.5g of NaCO3
7) Add de-ionised water carefully down the side of the large beaker.
8) Use about 150cm3 of water and then swirl the beaker carefully to mix the contents around.
9) Then, stir the contents of the large beaker using a glass rod to help dissolve the solid
Na2CO3 completely.
10) Pour the solution into a volumetric flask, using a funnel while pouring down the glass rod
(this ensures that any extra Na2CO3 on the rod would go into the volumetric flask). Then get
the last drops of the solution off the glass rod, and onto the funnel.
11) Wash out the beaker, rod and funnel with the de-ionised water from the wash bottle,
making sure to let the washings go into the volumetric flask.
12) Add more de-ionised water up to the mark on the volumetric flask if needed. Then, add the
stopper into the flask, make sure it’s in properly, then shake the flask to mix the solution
thoroughly.
Making the titrant solution (Na2CO3) apparatus
-White tile (bench mat) used to prevent the surface from getting any solution on it.
-100cm3 beaker (small beaker) used to measure the mass of the sodium carbonate.
-250cm3 beaker (large beaker) used to mix the solution.
-250cm3 volumetric flask with stopper used to make the solution in and keep the solution contained.
-Filter funnel used to pour the solution from the beaker into the volumetric flask, to prevent
spillages.
-Glass rod used to stir the mixture and used to break down any extra solids left in the mixture.
-Spatula used for putting the sodium carbonate into the beaker.
-De-ionised water used to make the solution and used to transfer the washings.
-Sodium carbonate (Na2CO3) used to make the solution with.
-Balance with an absolute error of ±0.1g used to measure the mass of the sodium carbonate.
-Balance with an absolute error of ±0.01g used to measure the mass of the beaker and its contents.
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