Preparation of 0.05 n sodium thiosulphate solution
Answers
(a) Preparation of 0.05M sodium thiosulphate solution (Na2S2O3)
Approximately 0.791g of Na2S2O3 was weighed on a weighing paper and transferred
into a clean 100 ml glass bottle. Then, 100 ml deionized water was added to dissolve the
powder. The solution was shaked thoroughly to obtain uniform concentration.
(b) Preparation of starch indicator
About 10.0g soluble starch was suspended in 100 ml water. 15.0g potassium hydroxide
(KOH) pellets was added into the solution and was stirred until dissolve. The solution
was diluted with 900 ml water and let stand for one hour. Concentrated HCl, followed
by 2 ml acetic acid glacier (as preservative) were then added into the solution.
(c) Standardization of 0.05M sodium thiosulphate solution
Potassium dichromate powder (K2Cr2O7) was oven-dried at 180-200°C. Approximately
2.0g of dichromate was dissolved in water and diluted to exactly 500 ml. Approximately
3.0g of KI was dissolved in 45 ml of water in a 500-ml iodine flask. 10 ml of 3M Hcl
solution was added.
50 ml of dichromate was pipette into the mixture of KI and HCl. The solution
was swirl gently and allowed to stand in the dark for five minutes. It was diluted with
100-200 ml of water and then titrated with 0.05M Na2S2O3 until the solution is light
yellow in colour. 4 ml of starch indicator was added and titration was continued slowly
to a green end point. Volume of Na2S2O3 used, vs was recorded. The following
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expression was used to calculate the molarity of standardized 0.05M sodium
thiosulphate solution,
Molarity of standardized 0.05M sodium thiosulphate
2.039 x weight of dichromate used (g)
= ___________________________________
volume of sodium thiosulphate used, vs (ml)