Q1. * Determination of the % of hypochlorite in bleach Flashcards
What form does chlorine take in bleach?
The form of sodium hypochlorite NaClO
What is the formula for sodium hypochlorite?
NaClO
What is the name of NaClO?
Sodium hypochlorite
What is contained in sodium hypochlorite?
The hypochlorite ion, ClO-
What is the formula for the hypochlorite ion?
CIO-
What is CIO-?
A hypochlorite ion
What is the more modern name for the hypochlorite ion?
Chlorate (I) ion
What is the hypochlorite ion?
2
It is an oxidising agent ]
It converts iodide ions to iodine
Is the hypochlorite ion an reducing or oxidising agent?
An oxidising agent
What does the hypochlorite ion convert into iodine?
Iodine ions
What does the hypochlorite ion convert iodine ions into?
Iodine
Give the equation for the conversion of iodide ions to iodine using hypochlorite as an oxidising agent.
Give the equation for the reaction that occurs during this experiment.
ClO- + 2l- +2H+ –> Cl- + I2 + H2O
In general, how is the concentration of sodium hypochlorite in household bleach determined?
(2)
A sample of the bleach is reacted with a solution of iodide ions
The liberated iodine is then titrated against a solution of sodium thiosulfate of known concentration
Why must the bleach be diluted?
2
As household bleach is usually fairly diluted
If not diluted a very large volume of sodium thiosulfate would be required to react with the large amount of I2 liberated
What is added to the conical flask of diluted bleach?
Dilute sulfuric acid
Why is dilute sulfuric acid added to the dilute bleach?
2
To ensure the acid is in excess
To supply sufficient H+ ions so that all of the hypochlorite ions, ClO- are completely reduced to chloride ions Cl-
What colour is the solution of sulfuric acid and dilute bleach?
Colourless
What affect does adding the dilute sulfuric acid have on the colour of the bleach solution?
It has no affect on the colour - remains colourless unless coloured by the bleach’s brand
What does the sulfuric acid supply?
H+ ions
Why are sufficient H+ ions needed?
To ensure all of the hypochlorite ions, ClO- are completely reduced to chloride ions Cl-
What ensures all of the hypochlorite ions, ClO- are completely reduced to chloride ions Cl-?
The H+ ions supplied by the sulfuric acid
What is added to the solution of dilute bleach and sulfuric acid?
Potassium iodide solution in excess
What colour change can be seen when the potassium iodide solution is added to the solution of bleach and sulfuric acid?
Changes from colourless to reddish-brown
What causes the colour change from colourless to reddish-brown?
The formation of iodine
Why is the potassium iodide solution added in excess?
2
To ensure all of the bleach has reacted to produce the maximum amount of iodine
To keep the iodine produced in solution
What colour change takes place when the solution of bleach, sulfuric acid and potassium iodide is titrated against a solution of sodium thiosulfate of known concentration?
(2)
Changes from reddish-brown to pale yellow
Brown colour becomes less intense and gradually changes to a yellow colour then a pale yellow colour
Why is there a colour change from reddish-brown to pale yellow?
As the iodine becomes used up which results in the reddish-brown colour becoming less intense and fading to pale yellow
When is the indicator added?
When the solution in the conical flask becomes pale yellow or straw coloured
What indicator is used?
Starch
What colour change takes place when the titration is carried out in the presence of starch?
Blue-black to colourless
What colour does the solution become when starch is added?
Pale yellow to blue-black
What does the pale yellow colour indicate?
It indicates that there is little iodine left in solution
What causes the blue-black colour?
Starch reacting with the small amount of iodine left in solution
What does the fact that the blue-black colour disappears indicate?
(2)
there is no more iodine left in solution
i.e. all of the iodine has reacted with the thiosulfate
What formula is used for the calculations?
Vo x Mo/No = Vr x Mred/Nr
What is the oxidising agent in this experiment?
Hypochlorite / ClO-
What is the reducing agent in this experiment?
Thiosulfate S4O6^2-
What is the formula for thiosulfate?
S4O6^2-
