Methodology:
- Rinse all apparatus to be used with distilled water. Pat dry using paper towels.
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Collect sea water in a 100cm3 beaker and mark it as such.
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Pipette 25cm3 of sea water into the volumetric flask. Tap the pipette lightly against the flask to ensure all of the solution has been transferred.
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Carefully fill the flask with distilled water upto the 250cm3 mark, adding drop by drop as the bottom of the meniscus reaches the mark.
- Close volumetric flask and mix diluted sea water thoroughly by inverting and shaking vigourously.
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Collect silver nitrate solution (F1) in other 100cm3 beaker and mark it as such.
- Attach clamp to retort stand and ensure that it is secure.
- Attach burette to retort stand. Ensure that the burette is securely held.
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Rinse the burette with F1 solution, collecting all the solution in the third 100cm3 beaker.
- Fill the burette with the remaining F1 solution using a filter funnel. Ensure that the funnel has been removed after filling the burette and that there are no air bubbles in the tip of the burette. In the presence of one, open the tap to let it run out.
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Pipette 10cm3 of diluted sea water into a 250cm3 conical flask. Tapping the pipette lightly against the flask to ensure all of the solution has been transferred.
- Add 5 drops of indicator, potassium chromate, into the conical flask and swirl the flask. It should result in a faint yellow colour solution.
- Note the initial reading of the burette. Ensure all readings are read at the bottom of the meniscus.
- For the rough trial, add solution from the burette slowly until solution in conical flask has a red tinge that does not disappear even after vigourous swirling.
- Record the final burette reading.
- Wash the conical flask with plenty of distilled water.
- Repeat 13, 14 and 15. Top up the burette if necessary (Step 10).
- Repeat titration to collect another set of readings. However, this time stop before the end point recorded in 15 and add drops of F1 until the solution in conical flask changes colour.
- Record the final burette reading.
- Collect third reading by repeating 16, 17, 18 and 19.
Safety:
- Safety goggles have to be worn at all times to prevent chemicals used in the lab from coming into contact with the eyes when touch the face with our hands.
- Wash hands after handling the chemicals to prevent irritation of the skin. Silver nitrate solution can leave a yellow stain on hands, clothes, papers.
- Hair has to be tied up and away from the face to avoid hair falling into the chemicals.
- Handle all apparatus carefully so as to avoid breakage. In the case, of a broken apparatus, report to the lab teacher so that the area can be cleared.
- Ensure that you workspace is clear and uncluttered so that in the case of spillage, papers, etc will not be stained yellow and the spill can be cleared quickly.
- Take care when fitting the pipette filler onto the pipette. Fit gently to avoid breaking the mouth of the pipette.
Precautions:
- Wash all the apparatus with distilled water to remove impurities that will result in inaccurate readings.
- Remove the filter funnel after filling the burette to prevent drops of solution from falling into the burette from the funnel that will also lead to inaccurate readings.
- Ensure that there are no air bubbles in the tip of the burette.
- Ensure that readings are taken at the bottom of the meniscus.
- When filling the volumetric flask, add distilled water drop by drop so that the mark in not overshot.
- Shake volumetric flask vigourously to ensure even mixing.
- Swirl the conical flask vigourously when adding F1 to ensure that all of it has reacted with the chloride ions.
Data Recording & Processing:
Observations: The solution was initially faint yellow in colour. Addition of Ag+ ions resulted in the solution turning cloudy and a white precipitate, silver chloride, being formed. More Ag+ ions resulted in giving the solution a reddish-brown colouration as silver chromate starts to form. The second and third titration resulted in a change from faint yellow to a orange solution.
Total % uncertainty = 0.2 + 0.24 + 0.2 + 0.5
= ±1.14%
Total absolute uncertainty =× 0.4625
= ±0.00527 moldm-3 (3 s.f)
= ±0.005 moldm-3 (1 s.f)
∴ The molarity of cholride ions in sea water is (0.463±0.005) moldm-3.
Conclusion and Evaluation:
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The concentration of chloride ions in the sea water is (0.463±0.005) moldm-3. To test the validity of results obtained, the experimental value must be compared with the literature value of concentration of chloride ions in sea water. The literature value is 0.469 moldm-3.
% total error =× 100%
=× 100%
= 1.28% (3 s.f)
% random error = 1.14%
% systematic error = % total error – % random error
= 1.28% - 1.14%
= 0.14%
The % difference in experimental and literature value is the total error of the experiment. It is relatively small difference of 1.28%. Thus, it can be said that the results obtained were accurate. Also, the titration readings were within 0.1cm3 of each other. Thus, they were precise and concordant each other.
It should noted that the literature value of 0.469 moldm-3 is the average molarity of sea water. Thus, when comparing, an assumption is made that the chlorinity of sea water is homogeneous across all seas.
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Further assumption was made that the titration was carried out under conditions of pH 6.5 – 9. At higher pH silver ions may be removed by precipitation with hydroxide ions, and at low pH chromate ions may be removed by an acid-base reaction to form hydrogen chromate ions or dichromate ions, affecting the accuracy of the end point.
- The first reading taken was rough and as observed by the colour change, it overshot the endpoint. Thus, only 2 readings were used to calculate the average. If time allowed, more readings could be taken and the average would reduce random error.
After the initial rough titration, nearing the endpoint, silver nitrate was added drop by drop. A single drop resulted in the final colouration of the solution in the conical flask to be orange. However, the ideal endpoint is when there is a light orange solution. The persistant over-shooting of the endpoint can be a source of systematic error.
- Since we know the concentration of chloride ions in sea water, the salinity of sea water per cubic decimetre can be calculated.
Salinity of salt (NaCl)
= 0.4625 × (22.99 + 35.45)
= 27. 03gdm-3
- This method of titration can be used to find the concentration of bromide ions in a solution as well.
Ag+(aq) + Br-(aq) → AgBr(s)
When all of the bromide ions has reacted to form silver bromide, a pale yellow salt that is insoluble, the addition of more Ag+ ions will react with the CrO42- ions. The indicator is still potassium chromate.
However, if the solution contains both chloride and bromide ions in significant amounts, this titration method will not be accurate to find the concentration of either ions.
- Residue containing silver ions can be save for later recovery of silver metal. The solution remaining in the burette can be reacted with any metal that is of a higher reactivity then silver such as copper or zinc. Silver will be displaced. The metal will form an aqueous solution. Thus, it is possible to obtain silver.
