water titrations

1. N.B!!! Q: how to make a solution of iodine using the dissolved oxygen in the water sample

add manganese II sulfate (Mn2+) and alkaline (OH-) potassium iodide to water sample, shake until a brown precipitate forms. after, add concentrated sulfuric acid (H+) and shake until iodine (I2) forms.

1. key part to note from 3 eqs (will be given if needed in exam!)

1 mol O2 liberates 2 mol I2

1. suitable indicator for titration

starch solution (only added when close to the endpoint) (same as standardising sodium thiosulfate titration!)

1. colour change observed during titration and why?

red-brown -> yellow -> pale yellow (NOW ADD STARCH!)

as sodium thiosulfate reacts with the iodine, the iodine is used up (same as standardising sodium thiosulfate titration!)

1. colour change observed at end point and why?

(AFTER STARCH ADDED) blue-black -> colourless

starch is added causing a blue-black colour in the presence of iodine. as soon as the iodine is completely used up, blue-black decolourises (same as standardising sodium thiosulfate titration!)

1 Q: why is starch only added when a pale-yellow colour forms in the conical flask i.e. when near the end point?

1. waiting until a pale-yellow colour tells us that the end point is very near so the sodium thiosulfate can be added very slowly in drops, resulting in an accurate endpoint.

2. iodine adsorbs onto the starch, preventing it from reacting with sodium thiosulfate. if the starch was added any earlier, it would cause an inaccurate endpoint (same as standardising sodium thiosulfate titration!)

1. burette and conical flask

burette: sodium thiosulfate S2O3^2-

conical flask: iodine I2

(same as standardising sodium thiosulfate titration!)

1. looking for

Mb (opposite of standardising sodium thiosulfate titration!)

1 Q: what is meant by BOD?

Biochemical Oxygen Demand

amount of oxygen consumed by biological action when a sample of water is kept at 20 degrees Celsius in the dark for 5 days

1 Q: why should the same of water be analysed for dissolved oxygen immediately?

due to BOD. living organisms use up oxygen for respiration so a lower result for dissolved oxygen will be obtained if not analysed immediately

1Q: what's the purpose of adding manganese (II) sulfate to the reaction bottle?

manganese (II) sulfate is a source of Mn2+ ions necessary for the formation of Mn(OH)2

1Q: what's the purpose of adding alkaline conditions/OH-/KOH to the reaction bottle?

provides OH- ions necessary for the formation of Mn(OH)2

1Q: why did a brown precipitate form in the bottle?

due to Mn(OH)3 being formed from the rxn of Mn(OH)2 with oxygen

1Q: what does it mean if a white precipitate forms/no brown precipitate forms?

there's no dissolved oxygen in the water sample. the white precipitate is due to Mn(OH)2 being formed

1Q: why was potassium iodide/I-/KI added to the reaction bottle?

provides I- ions necessary for the formation of iodine (I2)

1Q: explain why a large excess of potassium iodide was required in this titration?

1. to ensure that oxygen is the limiting reagent so ALL of it is used up.

2. the I2 that's produced will react with the potassium iodide forming the triiodide ion I3- which is a soluble version of iodine (same as sodium thiosulfate titration, except potassium manganate VII isn’t the limiting reagent here, O2 is!!!)

1Q: why is concentrated sulfuric acid/H+/H2SO4 added to the brown precipitate?

only in an acidic environment will Mn(OH)3 (brown precipitate) react with I- to liberate iodine (I2)

1Q: what's observed on addition of the concentrated sulfuric acid?

red-brown colour forms due to I2 formation

1Q: why are all the solutions added to the same of water concentrated?

1. minimises the volume of water displaced, hence, minimising dissolved oxygen lost

2. when all the reagents are in excess, oxygen becomes the limiting reagent so ALL of the oxygen reacts

1Q: how was the manganese (II) sulfate solution, the alkaline potassium iodide and sulfuric acid added to the bottle containing the water sample and why were they added in this manner?

using a dropper that's under the level of the water before adding the solution in order to avoid bubbling any air (oxygen) into the water

1Q: what safety precaution should be taken when stoppering and shaking the bottle after addition of every solution?

take care not to bubble any air (oxygen) into the water

1Q: what name is given to calculating the concentration of dissolved oxygen by titration by liberating iodine from oxygen in this way?

Winkler method

1Q: Winkler's reagent

manganese (II) sulfate, alkaline potassium iodide, and sulfuric acid

1Q: explain why the Winkler method is unsuitable for chlorinated water?

chlorine in water would react with iodide ions to form iodine. the extra iodine would cause the titre value to be too large, leading to an inaccurate end point

Mr Na2S2O3⋅5H2O

Mr = 248

1 calculation note when to use balanced eq?

only use balanced eq when ur asked to find parameters of I2 after the first step!!!!!! when u know mol sodium thiosulfate (not always needed e.g. 2019 didn't require using balanced eq!)

1 calculation note how to get dissolved oxygen?

q will always give u a piece of info e.g. "for every 1 mol of dissolved oxygen, 2 mol I2 are liberated" or "4 mols S2O3^2- were required for every 1 mol of dissolved O2 present originally" so set up an eq w just those two, fill in molar ratios & the mols of the thing u solved, then multiplication/division to get dissolved O2 mols!!!!

1 calculation note Q states "calculate number of mols O2 that released the I2 detected in each titration"

standard procedure using sentence in q to set up own eq with two compounds, fill in molar ratios, fill in mols of what u already solved then multiplication/division to get mol dissolved O2. once u have this bc q states "that released the I2 detected" that just means dissolved O2 mols u solved are in Vb!!!

1 calculation note Q gives info abt Na2S2O3⋅5H2O, what to do?

just use it's mr for the full thing, normal procedure!