Determination of the % of hydrochlorite in bleach Flashcards
Apparatus
Safety glasses, funnel, burette, pipette, conical flask, volumetric flask, retort stand, wash bottle with deionised water white tile.
Materials
Standard solution of sodium thiosulphate, bleach, potassium iodide solution, dilute sulphuric acid.
What is the first step in the method?
All glassware was rinsed with deionised water to clean them,
What happens after the glassware was rinsed?
25 cm³ of original bleach were added to a volumetric flask and deionised water was added to make the solution up to 250cm³, observing the usual precautions.
What happens after the original bleach was added to the volumetric flask and deionised water.
The pipette was rinsed with some of the diluted bleach solution and then 25cm³ of the bleach solution were added to a clean conical flask.
What happens after the pipette was rinsed with some of the diluted bleach solution?
To this solution of diluted bleach about 20cm³ of dilute sulphuric acid were added along with about 10 cm³ of potassium iodide.
What colour was the solution after about 20cm³ of dilute sulphuric acid were added along with 10cm³ of potassium iodide.
Red-brown colour
Why was the solution red-brown colour after the dilute sulphuric acid and potassium iodide was added to the solution?
Due to the liberated iodine.
What happens after the solution after about 20cm³ of dilute sulphuric acid were added along with 10cm³ of potassium iodide?
The burette was filled with the standard sodium thiosulphate solution, observing the usual precautions.
What happens after the burette was filled with the standard sodium thiosulphate solution?
The solutions were titrated until a straw-yellow colour was reached and then some starch indicator was added with turns the solution in the conical flask a blue-black colour.
What indicator was used?
Starch
When did the titration end?
When the colour changed from blue-black to colourless.
What colour was the solution titrated until? (before starch indicator was added)
Straw-yellow colour
What is the ‘last step’?
Three titrations were carried out.
What were the results from the burette?
Rough titration - 28.7cm³
2nd titration - 28.5cm³
3rd titration - 28.6cm³
What is the bleach (hypochlorite ion) in the titration?
An oxidising agent
What does the bleach (hypochlorite ion) do?
It liberates iodine from solutions of excess potassium iodide in a 1:1 ratio.
How can the amount of bleach (hypochlorite ions) be calculated?
By reacting the iodine that was liberated with a previously standardised solution of sodium thiosulphate.
What is the oxidising agent in this reaction?
Bleach
What is the limiting reactant in this reaction?
Bleach
Why was the bleach diluted?
The bleach was too concentrated and would require an excessive of sodium thiosulfate to react with the iodine that the bleach liberated.
Describe how the 25.0cm³ sample of the original bleach solution was diluted to exactly 500cm³. (L.C)
- Pipetted into 500cm³ volumetric flask
- Add deionised water until near mark
- Add dropwise
- Stopper and invert several times and mix thoroughly
What colour developed when the potassium iodide and the sulfuric acid reacted with the diluted bleach in the conical flask? (L.C)
Brown / red
Give two reasons why excess potassium iodide was used. (L.C)
- So that all the bleach has reacted
- To keep the iodine in solution
What was the purpose of standing the conical flask on a white tile during the titration? (L.C)
So that colour-change (end point) is clearer
Name the indicator used in the titrations and state the colour change observed at the end point. (L.C)
Starch
Blue-black to colourless
Why was the bleach diluted before carrying out the titrations? (L.C)
The bleach was too concentrated and would require an excessive of sodium thiosulfate to react with the iodine that the bleach liberated.
At what stage in the titration is the indicator added?(L.C)
When the solutions were titrated until a straw-yellow colour was reached.
What colour change indicates the end point? (L.C)
Blue-black to colourless
Name the indicator used. (L.C)
Starch
Why is excess potassium iodide used? (L.C)
- So that all the bleach has reacted
- To keep the iodine in solution
Identify the piece of apparatus that should be used to transfer the bleach to the volumetric flask. (L.C)
Pipette
