Embedded Files
titrations - pH and curves
titrations - pH and curves
Titration Curves - what are they?
Titration Curves - what are they?
There are four titration curves that need to recognised and sketched:
There are four titration curves that need to recognised and sketched:
1) Weak acid - strong base (shown on the right)
1) Weak acid - strong base (shown on the right)
2) Weak base - strong acid (shown below)
2) Weak base - strong acid (shown below)
3) Strong acid - strong base (shown below)
3) Strong acid - strong base (shown below)
4) Weak acid- weak base (shown below)
4) Weak acid- weak base (shown below)
Key points are indicated on all curves.
Key points are indicated on all curves.
weak acid - strong base
weak acid - strong base
weak base - strong acid strong acid - strong base
weak base - strong acid strong acid - strong base
weak acid - weak base
weak acid - weak base
titration curves - key points recap
titration curves - key points recap
Buffer Zone - What is it?
Buffer Zone - What is it?
For more in depth information on buffers:
- use the side navigation
- Buffers page , or
- use the QR code
To sketch a titration curve the pH at the following places needs to known / calculated:
To sketch a titration curve the pH at the following places needs to known / calculated:
pH at the start of the titration. This is due to the intial acid or base being titrated against
pH at the start of the titration. This is due to the intial acid or base being titrated against
pH at the equivalence point. This is dependent on the type of titration
pH at the equivalence point. This is dependent on the type of titration
pH at the end of the titration. This is due to the acid or base that is being used to titre with
pH at the end of the titration. This is due to the acid or base that is being used to titre with
pH halfway to the equivalence. This is the middle of the buffer zone and is not relevant for a strong acid-strong base titration
pH halfway to the equivalence. This is the middle of the buffer zone and is not relevant for a strong acid-strong base titration
Strong Acid - Strong Base titration
Strong Acid - Strong Base titration
such as HCl - NAOH
0.100 Mol L-1 HCL v 0.100 Mol L-1 NaOH
pH AT START
pH AT START
The pH at the start is due to the 0.100 molL-¹ HCl.
The pH at the start is due to the 0.100 molL-¹ HCl.
pH = - log10 [H+]
pH = - log10 [H+]
pH = - log10 [0.100]
pH = - log10 [0.100]
pH = 1.00
pH = 1.00
ph AT EQUIVALENCE POINT
ph AT EQUIVALENCE POINT
The pH at equivalence point is due to the auto ionisation of water.
The pH at equivalence point is due to the auto ionisation of water.
or
or
The concentration of both ions is 1.00 x 10-7 molL-¹.
The concentration of both ions is 1.00 x 10-7 molL-¹.
pH = - log10 [H+] or pOH = - log10 [OH-]
pH = - log10 [H+] or pOH = - log10 [OH-]
pH / pOH = - log10 [1.00 x 10-7]
pH / pOH = - log10 [1.00 x 10-7]
pH of 7
pH of 7
pH AT END OF TITRATION
pH AT END OF TITRATION
It is important to use the new [NaOH] to calculate the final pH as the concentration has be diluted. For example if we want to calculate the pH after 75 mL of NaOH has been added then the new concentration of NaOH is worked out using the excess volume of NaOH after equivalence point, which is 25 mL (Remember 50.0 mL was needed to neutralise the 50.0 mL of HCl) divided by the total volume, which is 75.0 mL multiplied by the initial concentration, which is: 0.1 x (25/75) = 0.0333 molL-¹
It is important to use the new [NaOH] to calculate the final pH as the concentration has be diluted. For example if we want to calculate the pH after 75 mL of NaOH has been added then the new concentration of NaOH is worked out using the excess volume of NaOH after equivalence point, which is 25 mL (Remember 50.0 mL was needed to neutralise the 50.0 mL of HCl) divided by the total volume, which is 75.0 mL multiplied by the initial concentration, which is: 0.1 x (25/75) = 0.0333 molL-¹
If the titration starts with the strong base then the curve will just be reversed, starting with a pH of 13 due to the 0.100 molL-¹ , the pH at equivalence point is 7.00, and the end pH is due to the excess, diluted HCl, which will be 1.48
pOH = - log10 [OH-]
pOH = - log10 [OH-]
pOH = - log10 [0.0333]
pOH = - log10 [0.0333]
pOH = 1.48
pOH = 1.48
Therefore pH = 14 - pOH = 14 - 1.48 = 12.5
Therefore pH = 14 - pOH = 14 - 1.48 = 12.5
Weak Acid - Strong Base titration
Weak Acid - Strong Base titration
Such as CH₃COOH and NaOH
*acetic acid is a common name for ethanoic acid
Start numbers
Start numbers
[CH₃COOH] =0.0869 molL-¹
[CH₃COOH] =0.0869 molL-¹
Vol (CH₃COOH) = 20.0 mL
Vol (CH₃COOH) = 20.0 mL
[NaOH] = 0.100 molL-¹
[NaOH] = 0.100 molL-¹
The videos below show how to calculate pH at the relevant points.
The videos below show how to calculate pH at the relevant points.
pH AT START
pH AT START
pH AT EQUIVALENCE POINT
pH AT EQUIVALENCE POINT
and
and
HALFWAY TO EQUIVALENCE POINT
HALFWAY TO EQUIVALENCE POINT
Halfway to equivalence point through the buffer region (weak acid-strong base)
Halfway to equivalence point through the buffer region (weak acid-strong base)
pH = pKa = 4.76
pH = pKa = 4.76
The volume of base added halfway to the equivalence point is half the volume needed to reach equivalence
The volume of base added halfway to the equivalence point is half the volume needed to reach equivalence
This means half the moles of acid have been neutralised and the concentration of the acid and base are the same
This means half the moles of acid have been neutralised and the concentration of the acid and base are the same
pH AT THE END OF THE TITRATION
pH AT THE END OF THE TITRATION
EXTRA: pH AT OTHER POINTS
EXTRA: pH AT OTHER POINTS
Weak Base - Strong Acid Titration
Weak Base - Strong Acid Titration
Such as NH₃ and HCl
The embedded slide show shows how to calculate pH through this titration
The embedded slide show shows how to calculate pH through this titration
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