ES5: Chlorine + Iodine Sodium Thiosulfate Titration Flashcards
What are the risks of chlorine?
- Toxic and corrosive so must be kept away from the skin and eyes
- Dangerous to be inhaled at high concentrations
- Must be kept away from flammable materials
Why must chlorine be kept away from flammable materials?
Because it is a powerful oxidising agent
Why is inhaling chlorine dangerous?
- Reacts in the lungs to form HCl
- This corrodes lung tissue
- Drowns the person as liquid fills the lungs
What is chlorine stored in as it is transported?
- Specially designed pressurised tank containers
- In cylinders
How does chlorine travel?
By road or by rail
What state is chlorine transported in?
Liquid
Why is chlorine transported as a liquid?
More chlorine can be stored in a fixed volume under pressure as a liquid than a gas
What do tanks used to transport chlorine have if the temperature/pressure gets too high?
Pressure release devices which vent the tank and release chlorine gas
What are the tanks used to transport chlorine made and lined with
Steel
What to do the inside of the tanks used to transport chlorine need to be?
Dry
Why do the inside of the tanks used to transport chlorine need to be dry?
Because chlorine reacts with water to produce HCl which is corrosive
What to the tanks used to transport chlorine have at the top?
Cylindrical protective housing through which all loading and unloading is done
What do the tanks used to transport chlorine have in them for safety during unloading?
A scrubber unit which ensures all air being displaced has chlorine removed
Name 2 uses of chlorine
- Water treatment
- Bleach
How is chlorine used in water treatment?
- To sterilise water for drinking/swimming
How is chlorine used in bleach?
- To remove stains from clothing
- To kill bacteria on surfaces
What are iodine-sodium thiosulfate titrations used for?
To find the concentration of an oxidising agent
Describe a method for an iodine-sodium thiosulfate titration
- Measure out a certain volume of the oxidising agent and pipette into a conical flask
- Add excess potassium iodide + sulfuric acid
- Put sodium thiosulfate in the burette
- After the solution fades to pale yellow, add 2cm3 of starch solution in the flask as an indicator for iodine - turning blue when the iodine is detected
- Add sodium thiosulfate one drop at a time until the blue colour disappears
- Measure the amount of sodium thiosulfate needed to do this
- Repeat
What happens after potassium iodide is added to the oxidising agent? Give an equation with chlorate as the oxidising agent
- Oxidising agent oxidises iodide ions to iodine
ClO- + 2I- + 2H+ =====> I2 + H2O + Cl-
Give an equation for the reduction of iodine ions by sodium thiosulfate.
I2(aq) + 2S2O3^2- (aq) =====> 2I- (aq) + S4O62- (aq)
What are the steps for calculating the concentration of an oxidising agent?
- Convert all cm3 readings to dm3
- Write the 2 ionic equations for the titration and calculate their stoichiometry
- Calculate the moles of sodium thiosulfate
(moles=conc x volume) - Calculate moles of iodine
(moles of thiosulfate/2) - Calculate the moles of the oxidising agent
(Use ratio of iodine produced:oxidising agent) - Calculate the conc. of the oxidising agent
(conc=moles/volume)
What’s the unit for concentration?
moldm-3
Name 5 problems that could arise in a iodine-sodium thiosulfate titration
- Contaminated apparatus could make inaccurate results
- Burette could be read incorrectly
- Random errors
- Solutions could react with oxygen in the air
- Adding starch too soon could make iodine “stick to the starch” and not react properly with the sodium thiosulfate
What is the solution to the problem of contaminated apparatus making inaccurate results?
- Ensure the burette is very clean
- Rinse with sodium thiosulfate (water could dilute the solution)
What is the solution to the problem of the burette being read incorrectly?
- Always read from the bottom of the meniscus
- Keep eyes level with the liquid
What is the solution to the problem of random errors?
Repeat experiment until you obtain 3 results within 0.1cm3 of each other then take an average
What is the solution to the problem of solutions reacting with oxygen air?
Solutions should be made up freshly
What is the solution to the problem of adding starch too soon making iodine “stick to the starch” and not react with the sodium thiosulfate?
Only add the starch when the solution is pale yellow
