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Aim: To determine the concentration of chloride ions in sea water by titration with silver nitrate solution of known concentration.

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Introduction

CHEMISTRY INVESTIGATION: VOLUMETRIC ANALYSIS Aim: To determine the concentration of chloride ions in sea water by titration with silver nitrate solution of known concentration. Hypothesis: To determine the concentration of chloride ions, sea water will be titrated with silver nitrate solution of known concentration. Silver ions form insoluble white silver chloride precipitation when added to a solution containing chloride ions: Ag+(aq) + Cl-(aq) � AgCl(s) Potassium chromate (VI) can be used to indicate the end-point of the titration, the point at which all the chloride ions have been precipitated. Silver ions combine with chromate (VI) ions to form a red precipitate of silver chromate (VI): 2Ag+(aq) + CrO42-(aq) � Ag2CrO4 (s) However, no silver chromate (VI) is precipitated until all the chloride ions have been removed. Therefore, the instant a permanent red tinge appears in the solution that does not vanish with vigourous swirling, the addition of silver nitrate solution should be stopped as the end-point of the titration has been achieved. It should be noted that silver nitrate is expensive, and is normally used in fairly low concentration. Thus, in this titration, only 0.05 mol dm-3 AgNO3(aq) is used. To obtain sensible results, it is necessary to dilute the sea water to give a concentration of chloride ions comparable to that of the silver nitrate. Apparatus: 1) 3 100cm3 beakers 2) 250 cm3 conical flask 3) (50.00�0.05)cm3 burette 4) Retort stand with clamp 5) Filter funnel 6) (10.00�0.05)cm3 pipette 7) (25.00�0.06)cm3 pipette 8) Pipette filler 9) (250.0�0.1)cm3 volumetric flask 10) Safety glasses 11) ...read more.

Middle

16) Wash the conical flask with plenty of distilled water. 17) Repeat 13, 14 and 15. Top up the burette if necessary (Step 10). 18) 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. 19) Record the final burette reading. 20) Collect third reading by repeating 16, 17, 18 and 19. Safety: 1) 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. 2) Wash hands after handling the chemicals to prevent irritation of the skin. Silver nitrate solution can leave a yellow stain on hands, clothes, papers. 3) Hair has to be tied up and away from the face to avoid hair falling into the chemicals. 4) 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. 5) 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. 6) Take care when fitting the pipette filler onto the pipette. Fit gently to avoid breaking the mouth of the pipette. Precautions: 1) Wash all the apparatus with distilled water to remove impurities that will result in inaccurate readings. ...read more.

Conclusion

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. 4) 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 5) 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. 6) 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. ...read more.

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