Titration analysis method-oxidation-reduction titration method (4)

(2) Iodometry-determination of dissolved oxygen in water

The basic reaction formula of iodometric method is as follows:

I ₂ is a weak oxidizing agent and can only be used to titrate stronger reducing agents directly; I ^- is a medium-strength reducing agent that can indirectly determine a variety of oxidizing agents.

Under acidic conditions, the oxidizing substance in the water sample reacts with KI to release I ₂ quantitatively .

The reaction formula is as follows:

1) Preparation of Na ₂ S ₂ O ₃ standard solution

The method of preparing 0.

Weigh 0.

When preparing the Na ₂ S ₂ O ₃ solution, freshly boiled and cooled distilled water is required to remove CO _{2 from the} water and kill bacteria, and a small amount of Na ₂ CO _{3 is} added to make the solution weakly alkaline, thereby inhibiting the growth of bacteria

2) Calibration of Na ₂ S ₂ O ₃ solution

Using starch as an indicator, titrate with Na ₂ S ₂ O ₃ solution until the blue color disappears.

The reaction conditions of K ₂ Cr ₂ O ₇ and I ₂ are as follows

a.

The reaction rate of bK ₂ Cr ₂ O ₇ with KI is slow.

c.

Titration to the end point, if a few minutes later, the solution appears blue again, this is caused by air oxidation I-, does not affect the analysis results, if the titration to the end point, blue soon, it means K ₂ Cr ₂ O _{7 The} reaction with KI is not complete and should be re-calibrated

3) Dissolved oxygen and its measurement
.

a.
Dissolved oxygen
.
The oxygen dissolved in water is called dissolved oxygen, often expressed as DO, and the unit is mgO ₂ /L
.
The saturation content of dissolved oxygen in water is closely related to factors such as atmospheric pressure and water temperature
.
As atmospheric pressure decreases, so does dissolved oxygen
.
As the temperature rises, the dissolved oxygen also drops significantly
.

Under normal conditions, clean surface water contains dissolved oxygen that is close to saturation
.
When the water contains algae, oxygen is released due to photosynthesis, which can oversaturate the dissolved oxygen in the water
.
On the contrary, if the water body is polluted by organic substances, the dissolved oxygen in the water will continue to decrease
.
When the oxidation progresses too fast, and the water body cannot absorb sufficient oxygen from the air in time to supplement the oxygen consumption, the dissolved oxygen in the water body will gradually decrease, or even approach zero
.
At this time, anaerobic bacteria multiply and become active, and the organic matter has a putrefaction effect, which makes the water smelly
.
The content of dissolved oxygen in wastewater depends on the process before the wastewater is discharged.
Generally, the content is low and the difference is large
.
The determination of dissolved oxygen is of great significance to the study of water self-purification
.
Dissolved oxygen is also an important comprehensive indicator of water quality in the control of water pollution and wastewater biological treatment technology
.

b.
Determination of dissolved oxygen
.
The determination of dissolved oxygen generally adopts the iodometric method
.
When measuring, add MnSO ₄ and NaOH solutions to the water sample.
O _{2 in the} water will oxidize Mn ²⁺ into hydrated manganese oxide MnO(OH) ₂ brown precipitate, which fixes all the dissolved oxygen in the water, and the more dissolved oxygen , The darker the precipitation color
.
The reaction formula is as follows:

MnO(OH) ₂ is dissolved with acid in the presence of I ^- , and quantitatively releases I ₂ equivalent to dissolved oxygen .
Using starch as an indicator, titrate the released I ₂ with a Na ₂ S ₂ O ₃ standard solution .
The reaction formula is as follows:

Dissolved oxygen calculation formula:

In the formula, the concentration of C—Na ₂ S ₂ O ₃ standard solution, mol/L;

V——The amount of Na ₂ S ₂ O ₃ solution consumed by the water sample , ml;

V _water -the volume of the water sample, ml;

8-Conversion factor of oxygen
.