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How accuracy may be ensured in techniques used Essay

M1: Explain how accuracy may be ensured in the techniques used Cynthia Nzeh

Task 1

1) Discuss how your choice of equipment and how it affected the accuracy of your method. Discuss good volumetric technique.

2) Calculate the apparatus error for the method used.

3) Given the value calculated by the senior technician calculate your error and comment on this error in relation to the apparatus error of the method. In the titration, I used these available instruments to ensure my results would be as accurate as possible. Volumetric burette: This instrument was filled with the sodium hydroxide solution that would be gradually added to the vinegar solution.

The burette was used instead of a measuring cylinder as it gives a finer volume reading. It has a tap which enables the user to quickly and easily stop the amount of sodium hydroxide solution that pours out. However using a piston burette instead of a volumetric burette may have increased the chances of gaining more accurate results. This is because the piston burette delivers precision bore and plunger.

White tile: The tile was used to be able to observe the colour change of the solution in the conical flask better. By placing the white tile it allowed us to see the colour change more easily and quickly therefore increasing our chances of closing the burette tap on time and giving us closer and possibly more accurate results. If non clear paper was used or no paper at all, the colour change of the solution would not have been spotted as quickly therefore increasing the time taken to close the burette tap to stop sodium hydroxide being added to the vinegar solution. The increase of sodium hydroxide solution would mean that the results obtained would not be the accurate or close to the accurate value.

Funnel: This piece of equipment was added to the top of the burette and it was used to channel the sodium hydroxide solution into the burette. The mouth of the burette was too small to pour the sodium hydroxide solution in safely so the plastic funnel was used to prevent any spillage of the solution and enabled the sodium hydroxide solution to be poured into the burette with precision and no spillage. A disposable paper funnel would not have been an appropriate piece of equipment to use, this is because the paper funnel would have absorbed some of the correctly measured sodium hydroxide solution (liquid) and changed the total volume of the sodium hydroxide going into the burette. The funnel is then removed before the titration takes place; this is to prevent any extra droplets, of the solution remaining in the funnel, to be added to the solution as it could affect the overall volume in the burette.

Conical flask: The conical flask was used to store the vinegar with drops of phenolphthalein before sodium hydroxide was added to the solution through the burette. A conical flask was the most ideal piece of equipment to use. The narrow head minimises the risk of splashes from the flask, this means that the solution remains in the flask and does not come into contact with experimenter. Using a conical flask over a beaker is more ideal. This is due to the fact that a beaker has a wide open mouth which means that solutions can splash out when poured in. Also, the shape of the instrument make it is easier to swirl the contents of the flask.

Bulb pipette (and filler): This instrument was used to measure out 25.0cm3 of sodium hydroxide to be added to the conical flask. There is a line of meniscus which indicates where the volume should be, this makes the pipette produce an accurate 25.0 volume. The pipette has a narrow portion which slows down the speed at which the solution gets drawn up. This enables the volume to be more accurate as it is easier and quicker to stop the pipette drawing up more of the solution after drawing up the desired amount.

A measuring cylinder may have also been used and may be more ideal than the bulb pipette. This is due to the fact that it has a measuring scale and the amount poured into the instrument is more controlled compared to the bulb pipette as this only has one line of measurement and it is tricky to get a solution to fall on the line of the meniscus. Good Volumetric Techniques

To achieve a good volumetric technique, the experimenter needs to be able to correctly complete certain procedures.

Before beginning an experiment, it is good to calibrate any equipment that needs calibration. This ensures that results obtain will be as accurate as possible or as near to the true value. All equipment should be inspected to ensure that there are no chips, cracks or general damage to the equipment as these could cause problems during the experiment which could affect the overall results gained.

Equipment such as burettes, pipettes, flasks and beakers must be rinsed with distilled water to get rid of any impurities in the equipment. If solutions are to be poured in any piece of equipment then the equipment should also be rinsed out with the solution being used, this will maintain the pH level in the instrument. Once a solution is transferred from the pipette to the flask, touching the tip of the pipette on the side of the flask will drain any extra drops that may still remain on the tip of the pipette.

When filling up the burette it is important that a funnel is used, however as the solution reaches the 0 mark it is ideal that the funnel be removed and a pipette used instead to reach the 0 mark, this is to achieve greater precision. During the experiment, it is important to swirl the flask continuously with one hand whilst the other hand opens and closes the tap of the burette, this allows the solution to mix well enough for the colour change to be observed quicker.

The titrant should be added in drop by drop as it reaches the endpoint as it prevents too much of the solution in the burette to be added to the solution in the conical flask. Also, the tap should be shut properly to prevent any extra unwanted solution from pouring out. The burette and pipette must be read at eye level to gain the actual result. To ensure reliability, the experiment should be repeated until a concordance between two results is acquired.

Apparatus error
To calculate the apparatus error the equation is:

Maximum error
Measure Value

25cm3 pipette = (± 0.1cm3 ÷ 25) x 100 = 0.4
Burette = (± 0.15cm ÷ 22.75) x 100 = 0.66 Mass Balance = (0.01 ÷ 4.05) x 100 = 0.25
Volumetric flask = (0.1 ÷ 200) x 100 = 0.05
= 1.36% apparatus error

Experimental error = 100 x (real answer – experiment answer) Real answer

100 x (0.056 – 0.0546)
0.056
100 x 0.0014 = 2.5%
0.056

The experimental error is greater than the apparatus error. This could have been down to a number of different factors. Too much sodium hydroxide may have been added to the solution which could have changed the volume of sodium hydroxide used. The burette could have been read wrong and not at the same eye level or from the bottom of the meniscus line. These faults may have been the cause of the experimental error or they may have added to the experimental error and this is why the experimental error is much greater than the apparatus error.

Task 2 & 3

There were certain procedures done in order to ensure the results obtained would be as accurate as possible.

Flame test

Before beginning the experiment, the wire loop is dipped into dilute hydrochloric acid then held in a Bunsen burner flame. This is to get rid of any residue remaining of the loop. Each test tube was correctly labelled to avoid a mix up.

The solutions where look at behind a white background, this was to enable the correct colour to be identified easier. Different instruments for different solution were used.

Food test

For this test it was important to keep the amount of food samples used equal for each food group. The food samples were placed in separate pellets to avoid cross contamination between the foods. Each test tube was correct labelled to avoid a mix up.

Equal drops of indicators were added to each food sample.

Sources
http://nobel.scas.bcit.ca/courses/wpmu/chem0011/files/2010/06/Lab8-Introduction-to-Volumetric-Techniques-I.pdf http://chemwiki.ucdavis.edu/Analytical_Chemistry/Quantitative_Analysis/Titration http://www.thestudentroom.co.uk/showthread.php?t=363038

http://wiki.answers.com/Q/How_do_you_calculate_percentage_error_for_equipment http://www.chemguide.co.uk/inorganic/group1/flametests.html


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