Water: BOD

Identify the compound usually added to the water to provide the Mn²⁺ ions

MnSO₄ (Manganese(II) sulfate)

Explain why a large excess of KI was required in this analysis

Ensures all O₂ detected /

ensures O₂ limiting /

ensures iodine kept in solution

What change was observed when the concentrated sulfuric acid was added and mixed thoroughly with the contents of the sample bottle?

Changes to brown

Describe how the conical flask was prepared for a titration and then used during the titration to help ensure that an accurate end point was reached

Rinsed with deionised water / use clean (dry) flask //

swirl (to mix contents) /

wash down sides with deionised water /

stand on white tile

Give one way of ensuring that the level of liquid in the burette was at eye level before taking a reading

Use clamp (so liquid is at eye-level) / place burette on lower surface / suspend burette over edge of bench / bend (crouch, kneel) until eye at level of liquid

What sequence of colours was observed in the conical flask from the start of the titration until the end point was reached?

Red-brown → to pale yellow,

starch added,

blue-black → colourless

What colour changes were observed at the titration stage of the experiment

i) up to the point when the indicator was added

ii) at the end point?

i) brown → pale straw yellow

ii) blue black → colourless

Calculation 2019 Q1 part d)

Why was it important to avoid trapping air bubbles each time a stopper was inserted into the water sample bottle and when using the dropper?

More oxygen (O₂) would dissolve / increase dissolved oxygen (O₂)

Describe how burette was rinsed and filled for use in the titrations

Rinse with deionised water //

rinse with a little of the thiosulfate solution //

add solution to burette an clamp vertically //

use a funnel to fill / adjust to mark at eye-level //

open tap to fill part below tap

Name the indicator in the titrations

Starch

Calculation 2013 Q1 part f)

What conclusion should have been reached had a white precipitate been observed instead of the brown precipitate after the first two additions of reagents to the bottle filled with river water

There was almost no dissolved oxygen in the water

Kits, designed for use in the field, allow the dissolved oxygen concentration to be measured immediately on collection of the sample. Why is the immediate determination of dissolved oxygen considered best practice?

Biochemical (biological) reactions (photosynthesis, respiration, metabolism) occur / action of micro-organisms (bacteria, algae)

Why was it necessary to analyse the sample of river water immediately?

So that oxygen content doesn’t change due to photosynthesis /

So that oxygen content doesn’t change due to respiration /

So that oxygen content doesn’t change due to activity of organisms in the water

In making the additions to the sample, why should the solutions be concentrated?

To minimise the amount of the water sample that is displaced / minimise the change in the oxygen dissolved in the sample

Describe how the additions of the concentrated solution of manganese(II) sulfate (MnSO₄) and alkaline potassium iodide (KI) to the bottle of river water should be carried out.

What essential precaution should be taken when replacing the stopper of the bottle after these conditions are made?

Remove a few cm³ of river water from the bottle //

make additions under the level of the water //

using a dropper //

do not bubble air into the water in the process.

Do not trap air bubbles

Describe the procedure for using a pipette to measure exactly 50 cm³ portions of the iodine (I₂) solution into the titration flask

Rinse with water followed by iodine //

fill pipette using a pipette filler //

adjust to have bottom of meniscus on mark / read at eye level //

remove droplets adhering to outside //

drain into titration flask //

touch tip of pipette against side of flask //

do not blow out drop inside pipette

Calculations 2005 Q1 parts f) and g)