dissolved oxygen

Question 1

why do we use a starch indicator

Answer 1

Starch forms a blue complex with iodine, which disappears at the endpoint of the titration, providing a clear visual signal for precise measurement.

Question 2

why do we hadd in h2so4, then nai/naoh, then mncl2

Answer 2

Acidification dissolves the Mn(OH)₃ precipitate and ensures that the Mn(III) species are available to oxidize iodide (I⁻) to iodine (I₂/I₃⁻). If this step were done later, incomplete dissolution could lead to inaccurate results. The iodide (I⁻) must be present in excess before MnCl₂ is added so that when Mn(III) forms, it can immediately oxidize iodide to iodine, preventing loss of oxygen or side reactions. Manganese(II) must be added last because, in basic conditions, it immediately reacts with dissolved oxygen to form Mn(OH)₃. If added earlier, without the right pH conditions, the reaction might be incomplete or introduce variability.

Question 3

why is a reagent blank used

Answer 3

to correct for impurities, ensure. accuracy, and minimize systemic error

Question 4

what are the steps for iodometric titration of seawater and why do we do each one

Answer 4

1. fixing the oxygen : To “trap” the dissolved oxygen in a measurable form. (forms brown percipitate)
2. releasing oxidized oxygen : To convert the trapped oxygen into a measurable iodine species.
3. quantify oxygen: to determine exact amount of oxygen originally present