5.7在线视频

Hello everyone.

In this micro-lecture, let’s learn about

the pretreatment and application of redox titration.

Why should we do the redox pretreatment?

In practical applications,

In practical applications,

the valences of the analytes are generally different.

We have to unify the valence before redox titration

We have to unify the valence before redox titration

We have to unify the valence before redox titration

It’s classified into pre-oxidation treatment

and pre-reduction treatment.

and pre-reduction treatment.

For instance,

For instance,

the valence of iron in iron ore

the valence of iron in iron ore

are both trivalent and divalent.

are both trivalent and divalent.

When determining Fe% in iron ore,

When determining Fe% in iron ore,

the Fe3+ should be firstly reduced to Fe2+,

then the whole iron is titrated

then the whole iron is titrated

by potassium dichromate (K2Cr2O7).

The requitrments of pretreatment agent:

First, the reaction is

quantitative, complete and rapid;

quantitative, complete and rapid;

Second, excess pretreatment agent

is easy to eliminate;

is easy to eliminate;

Third, the redox reaction has good selectivity

that only reacts with the analyte.

The common pretreatment methods

firstly contain the one based on chemical reactions,

such as excessive selenium chloride(SnCl2),

such as excessive selenium chloride(SnCl2),

which is often oxidized by mercury chloride (HgCl2).

Secondly, it can be removed by filtration,

such as NaBiO3 is insoluble in water

such as NaBiO3 is insoluble in water

which can be removed by filtration.

which can be removed by filtration.

Third,

it can be treated by heating decomposition,

it can be treated by heating decomposition,

such as hydrogen peroxide(H2O2)

such as hydrogen peroxide(H2O2)

is easily removed by heating decomposition,

is easily removed by heating decomposition,

and ammonium persulfate ((NH4)2SO4)

and ammonium persulfate ((NH4)2SO4)

can also be removed by heating.

can also be removed by heating.

Althrought the redox reaction is widely used

based on many reagents available

with various different redox ability,

with various different redox ability,

the requirement of titrant

is relatively consistent,

which should be stable enough in the air

which should be stable enough in the air

to perform the quantitative calculation.

Sodium hyposulfite (Na2S2O3)

and ferrous sulfate (FeSO4)

are commonly used as reduced titrant.

Oxidization titrants include

potassium permanganate (KMnO4),

potassium dichromate (K2Cr2O7), Iodine (I2),

bromic acid potassium salt (KBrO3),

cerous sulfate (Ce2(SO4)3·8H2O) and so on.

First, let’s learn about the KMnO4 method,

which is established based on

the strong oxidation of MnO4-.

In weakly-acidic, neutral,

and weakly-alkaline condition,

manganese dioxide (MnO2)

manganese dioxide (MnO2)

is formed from permanganate ion.

Because manganese dioxide is precipitate,

which may affect the judgement of the end point.

which may affect the judgement of the end point.

In strong alkaline condition,

the permanganate ion will

be converted to manganese ion,

be converted to manganese ion,

which is seldom used

which is seldom used

because of the fast reaction speed.

While in strong acidic condition,

permanganate ion can

be converted into divalent manganese ions,

be converted into divalent manganese ions,

which is often used in redox titration.

which is often used in redox titration.

In this reaction we must note that

the acidity is adjusted by sulphuric acid

instead of nitric acid or hydrochloric acid.

instead of nitric acid or hydrochloric acid.

Because chlorid ion can induce the reaction,

while nitric acid has a certain oxidation.

This method is widely used

and no need to add indicators

because the permanganate ion has its own color

and can indicate

and can indicate

the end point of the redox reaction.

the end point of the redox reaction.

But it also has shortcomings:

the KMnO4 solution is not stable

and there is serious interference during the titration.

and there is serious interference during the titration.

So, how to prepare the KMnO4 solution easily?

Indirect preparation is commonly used.

Indirect preparation is commonly used.

We usually select As2O3, H2C2O4`2H2O,

Na2C2O4, pure Fe wire and so on

Na2C2O4, pure Fe wire and so on

to play the standard substance.

The stoichiometric ratio of

permanganate ion and oxalic acid is 2:5,

which is commonly used to

calibrate potassium permanganate solution.

The applications of KMnO4 method.

Direct titration can determine

divalent iron ions, oxalate ions,

divalent iron ions, oxalate ions,

trivalent arsenic ion,

trivalent arsenic ion,

and hydrogen peroxide solution.

and hydrogen peroxide solution.

While direct titration can determine

manganese dioxide and lead dioxide.

Next, what’s the reaction process?

First under acidic conditions,

manganese dioxide reacted with excess oxalate

to form bivalent manganese ions,

and excess oxalate

and excess oxalate

can be determined quantitatively

by potassium permanganate titration.

After that,it is not difficult to calculate

the amount of manganese dioxide by subtraction method.

it is not difficult to calculate

the analyte itself does not have

oxidation or reduction properties,

oxidation or reduction properties,

such as calcium ion.

The process is as follows:

First the calcium ions react with

quantitative and excess oxalate,

quantitative and excess oxalate,

then the generated calcium oxalate

can be easily removed by filtration.

can be easily removed by filtration.

The remaining oxalate

The remaining oxalate

is still titrated by potassium permanganate.

is still titrated by potassium permanganate.

After the necessary calculations,

it is not difficult to find

it is not difficult to find

the content of calcium ions.

K2Cr2O7 method.

It utilizes the oxidation property of dichromate.

Its medium is hydrochloric acid,

which is not limited by the reducibility of chloridion.

Its features are as follows:

K2Cr2O7 is stable, easy to long-term storage

and its standard solution

can be used to titration directly.

These advantages are exactly

complementary to potassium permanganate

complementary to potassium permanganate

and it can be used

and it can be used

to determine divalent iron ions.

to determine divalent iron ions.

The commonly-used indicators,

The commonly-used indicators,

for example, sodium diphenylamine sulfonate

and N-phenylanthranilic acid.

Next,

let’s learn about the determination of iron.

First, the oxides of iron

contains iron ions with different valences,

contains iron ions with different valences,

which should be treated by pre-reduction,

which should be treated by pre-reduction,

that is to say,

selenium chloride

selenium chloride

reduce trivalent iron to divalent iron.

And the excess pretreatment agent,

can be removed by mercuric chloride.

The generated mercurous chloride

is precipitate

is precipitate

and will not affect the titration reaction.

and will not affect the titration reaction.

After adding mixed acid of

sulphoric and phosphoric acid to the system

to adjust its pH,

to adjust its pH,

the whole amount of iron will be easily obtained

by titrating divalent iron ions

with dichromate.

Above all, it’s the process of

determining iron by potassium dichromate.

determining iron by potassium dichromate.

Then, let’s think about a question.

Is it possible to determine iron without mercury?

Because mercury is known to be

an environmentally toxic substance.

an environmentally toxic substance.

Next,

let's sum up the similarities and differences

let's sum up the similarities and differences

between KMnO4 method and K2Cr2O7 method.

KMnO4 has low purity and impurities,

which should be prepared by indirect method and

the standard solution should be calibrated.

The latter is easy to purify,

stable for long-term storage and

can be directly prepared to standard solution.

can be directly prepared to standard solution.

Under acidic conditions,

the former standard electrode potential

is 1.51V,

is 1.51V,

the latter is 1.33V.

the latter is 1.33V.

It’s obvious that

It’s obvious that

the oxidation capability of the former

the oxidation capability of the former

is higher than that of the latter.

is higher than that of the latter.

In 1 mole of hydrochloric acid,

the electrode potential

of the chloride ion couple is 1.33V,

it can react with potassium permanganate,

so hydrochloric acid can not be used as the medium

so hydrochloric acid can not be used as the medium

in the potassium permanganate method.

in the potassium permanganate method.

As the chloride ion

As the chloride ion

will not react with potassium dichromate,

will not react with potassium dichromate,

hydrochloric acid can be used as the medium

in potassium dichromate method.

The principle of iodimetry is

to use the oxidizability of iodine

or the reducibility of iodide ion

to perform the quantitative analysis.

to perform the quantitative analysis.

Its electric couple reaction is iodine (I2)

acquires electrons to generate iodide.

As the solubility of iodine is small,

iodide ions are usually added

iodide ions are usually added

to improve the solubility.

to improve the solubility.

缺少字幕

When the pH is less than 9,

it is not affected by the acidity,

so it has a wide range of applications.

so it has a wide range of applications.

Direct iodimetry is a kind of method

which using the oxidizability of iodine.

The object are some reductive substances

like divalent sulfur ion, dimethyl selenium,

thiosulfate, sulfite and so on.

thiosulfate, sulfite and so on.

Its requirements for acidity are weak acidity,

neutral or weak alkalinity.

neutral or weak alkalinity.

Why not strong acid medium?

Why not strong acid medium?

Because iodide ion is easy to be oxidized

in strong acid environment.

Strong alkaline medium is also not allowed,

because iodine is easy to

take part in the dismutation reaction.

The dismutation reaction is that

the iodide ion reacts with oxygen first,

and then iodine reacts with hydrogen peroxide.

Indirect iodimetry

is a kind of oxidation-reduction titration method

is a kind of oxidation-reduction titration method

which uses the medium strength

reducibility of iodine.

reducibility of iodine.

Its object is oxidable substance

which is a kind of substance

with high electrode potential.

缺少字幕

Its requirements for acidity is neutral

or weak acid.

In strong acid environment,

thiosulfate is easy to decompose,

and iodide ion is easy to oxidize

and iodide ion is easy to oxidize

leading to the end point delay.

In alkaline medium,

iodine and thiosulfate may occur side-effects.

The indicator of iodimetry

is a special indicator,

namely starch indicator,

which requires room temperature,

weak acidity and fresh configuration.

The addition time is different between

direct iodimetry and indirect iodimetry.

direct iodimetry and indirect iodimetry.

This needs our attentions particularly.

This needs our attentions particularly.

The former should be added before titration,

and the latter should be added near the end point.

What is the reason,

What is the reason,

please think about it.

The principle of discoloration

is that the tri-iodide ions formed

is that the tri-iodide ions formed

by iodine and iodide ions

by iodine and iodide ions

complexed with starch and show dark blue.

complexed with starch and show dark blue.

The process is reversible.

Well,let's answer the question above mentioned.

In the indirect iodimetry,

if starch indicator is added too early,

it will strongly adsorb the iodine,

so it can cause the delay of the end point.

The main error of iodimetry

comes from the volatilization of iodine.

We can take the following mentods

to prevent the volatilization of iodine:

to prevent the volatilization of iodine:

adding excessive potassium iodide.

adding excessive potassium iodide.

It plays a role in solubilization,

which can inhibit the volatilization.

which can inhibit the volatilization.

Second,

the temperature of the solution

should not be too high,

should not be too high,

which can effectively

control the volatilization of iodine.

control the volatilization of iodine.

The third is to react in an iodine flask.

The third is to react in an iodine flask.

The fourth is

to avoid shaking violently during titration,

to avoid shaking violently during titration,

which can effectively

control the volatilization of iodine.

Secondly,

the source of error

the source of error

is the oxidation of iodide ions.

is the oxidation of iodide ions.

How to preventive it?

Controlling the acidity of the solution

not too high and avoiding to expose to the light.

not too high and avoiding to expose to the light.

Thirdly,

the iodine should be titrated immediately

after the precipitation.

Fourth,

the catalytic impurities

the catalytic impurities

should be removed before titration.

Preparation and titration of standard solution

Let's introduce sodium thiosulfate solution.

Its preparation needs to pay more attention.

The solution is unstable.

The dissolved carbon dioxide in water

can make it decompose,

the oxygen in air can make it oxidize,

and the microorganism in water

and the microorganism in water

can also make it decompose.

We should use distilled water

after boiling and cooling

after boiling and cooling

to prepare sodium thiosulfate solution,

and add a small amount of sodium carbonate

and add a small amount of sodium carbonate

to make the solution be alkaline and antibacterial.

Then keep it in a brown bottle.

After that,

calibrate it before use.

We often choose potassium dichromate

as the standard substance for calibration

based on indirect iodimetry.

Dichromate reacts with iodide ion to form iodine,

Dichromate reacts with iodide ion to form iodine,

and iodine reacts with thiosulfate.

and iodine reacts with thiosulfate.

In this case,

the analytical concentration of sodium thiosulfate

multiplied by the volume of sodium thiosulfate

multiplied by the volume of sodium thiosulfate

is six times of a fraction,

whose numerator is the mass

of potassium dichromate multiplied by 1000,

and the denominator

and the denominator

is the molar mass of potassium dichromate.

is the molar mass of potassium dichromate.

This formula can be used

to calculate the amount of sodium thiosulfate.

to calculate the amount of sodium thiosulfate.

Another way is

to use iodine standard solution for calibration.

Similarly,

thiosulfate is directly oxidized by iodine.

In this way,

the concentration of sodium thiosulfate

the concentration of sodium thiosulfate

can be calculated,

can be calculated,

which is equal to the twice of the fraction,

whose numerator

whose numerator

is the concentration of iodine

multiplied by the consumed volume,

multiplied by the consumed volume,

and the denominator

and the denominator

is the volume of sodium thiosulfate.

Let' s introduce the preparation

of iodine standard solution.

of iodine standard solution.

Because iodine is volatile,

we can't use analytical balance to weigh it.

In addition,

potassium iodide is added

for dissolving and avoiding light.

Generally,

arsenic oxide standard material

is used for calibration.

In the process,

the pH should be controlled

the pH should be controlled

to be equal to eight,

to be equal to eight,

so that the electric potential

so that the electric potential

of arsenite/arsenate couple

of arsenite/arsenate couple

is smaller than that of iodine/ iodide ion couple.

is smaller than that of iodine/ iodide ion couple.

Sodium thiosulfate can also be

used for calibration.

used for calibration.

The reaction principle is introduced above,

which used Iodine

and sodium thiosulfate for reaction.

and sodium thiosulfate for reaction.

The concentration of iodine

is equal to 1/2 times the fraction,

whose denominator is the volume

of consumed iodine,

and the numerator

is the concentration of sodium thiosulfate

multiplied by the volume

multiplied by the volume

of consumed sodium thiosulfate.

This section mainly

introduces oxidation-reduction titration

pre-treatment and application examples,

including pre-oxidation treatment,

pre-reduction treatment,

potassium permanganate method and so on.

Thank you, goodbye!