normality factor and selection of indicator in acid base titration
Sometimes, it becomes difficult to calculate the exact weight of the solute. Under this condition, the concentration of prepared solution becomes different from proposed solution and the term normality factor must be introduced.
Mathematically,
Normality factor= (Weight taken) / (weight to be taken)
Normality factor indicates by how many times the concentration of prepared solution is different from proposed solution.
For eg: If we have to prepare 500ml decinormal oxalic acid solution, the weight we have to take is 3.15gm. In lab due to certain difficulties, if weight is taken as 3.10 gm, then normality factor will be equivalent to
=3.1/3.15 = 0.98 and concentration is expressed as N/10 (f=0.98)
It indicates that the concentration of prepared solution is 0.98 times than that of proposed concentration.
Selection of indicator in acid base titration:
In acid base titration the completion of reaction is usually indicated by using the third substance (beside titrant and titrand) which is known as indicator. The indicator are weak organic acid or base possessing different colour in acidic or basic medium.
The colour change of the indicator isn't sudden and abrupt but takes place over certain range which is called as PH range of the indicator. The PH range of various indicator are as follows.
Indicator PH- range Acid color Basic color
(i) Methyl orange 3.1-4.4 Red Yellow
(ii) Methyl red 4.2-6.3 Red Yellow
(iii) Phenol 6.8-8.4 Yellow Red
(iv) Phenolpthalein 8.3-10 colourless pink
If graph is plotted between PH of solution vs volume of base added from burette following type of curve is obtained.
fig: Titration curve
The steep portion in titration curve is known as vertical portion.
The nature of the curve will be same whatever may be nature of acid or base. But only difference will be vertical.
The vertical region range of various nature of acid or base are as follows.
(i) S.A + S.B -----> 3-11
(ii) S.A + W.B -----> 3-8
(iii) W.A + S.B -----> 6-11
For indicator to be suitable in any titration, PH range of indicator must fall within vertical region range.
(a) For Strong acid and Strong Base
The nature of the titration curve is as shown below.
fig: titration curve
In this type of titration, vertical region extends from 3-11 , PH range of methyl orange is 3.1 to 4.4, methyl red is 4.2 to 6.3, phenolpthalein is 8.3 to 10 fall on the vertical region. So, all type of these 3 indicator can be useful for such type of titration.
(b) For strong acid and weak base
The nature of the titration curve is as shown below.
fig: titration curve.
The vertical region extend from 3 to 8. The PH range of methyl orange is 3.1 to 4.4, methyl red is 4.2 to 6.3, fall within vertical region. Therefore, these two indicator can be used for these titration but PH range of phenolpthalein is 8.3 to 10 doesn't fall within vertical region. So, can't be used for these sort of titration.
(c) For weak acid and strong base
The nature of the titration curve is as shown below.
fig: titration curve
Here, PH range of phenolthalein indicator is 8.3 to 10 fall within vertical region. So, can be used for this sort of titration. But PH range of methyl orange indicator is 3.1 to 4.4 , doesn't fall within region. So, can't be used for these titration. Here certain points of PH ranges of methyl red indicator fall within vertical region. So, it can also be used for these sort of titration. But the best indicator is phenolthalein.
Note:1 Methyl orange can't be used as indicator for the titration of weak acid and strong base.
Note2: Phenolpthalein can't be used as indicator for titration of strong acid and weak base.
Mathematically,
Normality factor= (Weight taken) / (weight to be taken)
Normality factor indicates by how many times the concentration of prepared solution is different from proposed solution.
For eg: If we have to prepare 500ml decinormal oxalic acid solution, the weight we have to take is 3.15gm. In lab due to certain difficulties, if weight is taken as 3.10 gm, then normality factor will be equivalent to
=3.1/3.15 = 0.98 and concentration is expressed as N/10 (f=0.98)
It indicates that the concentration of prepared solution is 0.98 times than that of proposed concentration.
Selection of indicator in acid base titration:
In acid base titration the completion of reaction is usually indicated by using the third substance (beside titrant and titrand) which is known as indicator. The indicator are weak organic acid or base possessing different colour in acidic or basic medium.
The colour change of the indicator isn't sudden and abrupt but takes place over certain range which is called as PH range of the indicator. The PH range of various indicator are as follows.
Indicator PH- range Acid color Basic color
(i) Methyl orange 3.1-4.4 Red Yellow
(ii) Methyl red 4.2-6.3 Red Yellow
(iii) Phenol 6.8-8.4 Yellow Red
(iv) Phenolpthalein 8.3-10 colourless pink
If graph is plotted between PH of solution vs volume of base added from burette following type of curve is obtained.
fig: Titration curve
The steep portion in titration curve is known as vertical portion.
The nature of the curve will be same whatever may be nature of acid or base. But only difference will be vertical.
The vertical region range of various nature of acid or base are as follows.
(i) S.A + S.B -----> 3-11
(ii) S.A + W.B -----> 3-8
(iii) W.A + S.B -----> 6-11
For indicator to be suitable in any titration, PH range of indicator must fall within vertical region range.
(a) For Strong acid and Strong Base
The nature of the titration curve is as shown below.
fig: titration curve
In this type of titration, vertical region extends from 3-11 , PH range of methyl orange is 3.1 to 4.4, methyl red is 4.2 to 6.3, phenolpthalein is 8.3 to 10 fall on the vertical region. So, all type of these 3 indicator can be useful for such type of titration.
(b) For strong acid and weak base
The nature of the titration curve is as shown below.
fig: titration curve.
The vertical region extend from 3 to 8. The PH range of methyl orange is 3.1 to 4.4, methyl red is 4.2 to 6.3, fall within vertical region. Therefore, these two indicator can be used for these titration but PH range of phenolpthalein is 8.3 to 10 doesn't fall within vertical region. So, can't be used for these sort of titration.
(c) For weak acid and strong base
The nature of the titration curve is as shown below.
fig: titration curve
Here, PH range of phenolthalein indicator is 8.3 to 10 fall within vertical region. So, can be used for this sort of titration. But PH range of methyl orange indicator is 3.1 to 4.4 , doesn't fall within region. So, can't be used for these titration. Here certain points of PH ranges of methyl red indicator fall within vertical region. So, it can also be used for these sort of titration. But the best indicator is phenolthalein.
Note:1 Methyl orange can't be used as indicator for the titration of weak acid and strong base.
Note2: Phenolpthalein can't be used as indicator for titration of strong acid and weak base.
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