ANALYTICAL CHEMISTRY
DR. MOHAMMAD KHANFAR
EIGHTH LECTURE
ACID BASE TITRATION
SHAMS W. ALSHIKHLY
SUNDAY 6TH OF JULY ,2014
In the last lecture we talked about back
titration
and figured out that the method is used for:
Parts of Titration
Volatile substances
Analyte: strong acid/base, weak acid/base.
Titrant: always strong acid or base.
Insoluble substances
Indicator: The same type of analyte (acid or base) but
Rapid reaction when excess is added
Substances which require heating
What is the principle of back titration?
Addition of excess of standard solution (titrant) to a
weighed amount (or volume) of sample, and
determination of excess (un-reacted) titrant using
secondary titrant.
E.g. Silver with Phosphate.
weaker, the most important properties of the
indicator are:
1) pKa value should be lower than the analyte when
it is base, and higher when it is acid.
2) pKa value should fall within inflection region1.
Let’s assume that we have an inflection point
between (4-8 pH), the best indicator would be the
one that has pKa = 6 or ± 1
Titration Curves
Analyte = Acid
Titrant = Base
Now we will start with:
Acid/Base Titration
(neutralization titration)
In acid/base titration,
the analyte is either
strong acid, strong base
or weak acid, weak base.
Titrant is always strong
acid or base. Indicator is
always weak acid or
weak base depending on
the analyte and it’s
always weaker than the
analyte.
Strong acids
HCl
H2SO4
HClO4
Strong Bases
NaOH
KOH
CH3ONa
R-O-Na+
Infliction region: the point when the analyte is completed with
one drop excess of the titrant giving me sharp difference in pH.
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ANALYTICAL CHEMISTRY
DR. MOHAMMAD KHANFAR
EIGHTH LECTURE
Analyte = Base
Titrant = Acid
ACID BASE TITRATION
SHAMS W. ALSHIKHLY
SUNDAY 6TH OF JULY ,2014
The stronger the base or acid the sharper the
inflection AFTER the equivalence point
So, it would be easier to detect the endpoint.
⬆Sharpness ⬆Accuracy in end point.
Analyte = HCl or Ch3COOH
Titrant = NaOH
What is the effect of strength in the
acidity or basicity on the titration curve?
Analyte = HCl
Titrant = NaOH or CH3ONa
Post equivalence point of CH3COOH is little bit higher
than HCl because the product of CH3COOH is
conjugate base but the product of HCl will be neutral
compound neither acidic nor basic.
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ANALYTICAL CHEMISTRY
DR. MOHAMMAD KHANFAR
EIGHTH LECTURE
Indicators
Phenolphthalein:
ACID BASE TITRATION
SHAMS W. ALSHIKHLY
SUNDAY 6TH OF JULY ,2014
base, so I will have a basic indicator , when I start
titration , the basic indicator should have a pKa
HIGHER than the analyte .
One of the most used indicators is Phenolphthalein,
Look at its curve below:
Methyl Orange:
pKa = 9.4
Phenolphthalein is colorless by itself, but when it
loses a proton, it turns into pink; which means that it
is acidic, a very weak acid; because of its high pKa.
When using phenolphthalein we have to be sure that
the inflection point of the titration curve will have
pKa=9.4 in the middle of its pH values or ± 1
Its red in the acidic form (pKa = 3.7), and yellow in
the basic form (pKa 10.3).
Phenolphthalein is used as indicator, it is used as acid
in acidic analytes, can I use it as base?
It can be used in acidic analytes and basic analytes, at
the first place it is an acid but when added into a
basic solution its will turn into basic indicator.
Its conjugate base has pKa = 4.6, so, yes! I can use
the conjugate base when the analyte is basic.
We have to be sure that the pKa will fall within the
inflection point before choosing it as indicator.
Did you understand?
Ok, let me clarify
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At the first place we use phenolphthalein as acidic
indicator, but it is very weak acid, when we put it in a
basic analyte, it will turn rapidly into a conjugate
ANALYTICAL CHEMISTRY
DR. MOHAMMAD KHANFAR
EIGHTH LECTURE
ACID BASE TITRATION
SHAMS W. ALSHIKHLY
SUNDAY 6TH OF JULY ,2014
For soft and colored copy go to www.ispatula.net
Before any base is added, how much is the pH (at
zero time)?
Other indicators:
pH = − log[𝐻𝐶𝑙]
pH = − log 0.05 = 1.3
After addition of 10 ml (before equivalence
point) of NaOH, how much is the pH?
Look at the above table, pKa should always fall within
the pH range.
Calculations
How can we calculate pH on any point
within the titration curve?
I added 10 ml NaOH which is consumed
by HCl completely, so my solution here
is the excess of HCl. Now I have to
determine how many moles of HCl are
remained after the addition of NaOH,
we said that to reach the equivalence
point we need 25 ml of NaOH, which means all
of the HCl is consumed and I have excess of
NaOH, but in 10 ml I have the opposite
condition.
Total moles of HCl = 0.05 M X 0.05 L = 0.0025 mole
Total moles of NaOH = 0.1 M X 0.01 L = 0.001 mole
Number of moles after the addition of 10 ml NaOH =
0.0025 – 0.001 = 0.0015 mole HCl
Example 1:
pH = − log[𝐻𝐶𝑙]
Titration of NaOH 0.1 M with HCl 0.05 M, 50 ml. Analyte
is HCl, NaOH is the titrant
pH = − log [
0.0015
0.05
] = 4.1
How much is pH on the equivalence point?
7, always 7 when the titrant and the analyte are
STRONG
How much NaOH I need to reach the equivalence point?
M1V1=M2V2
see the footnote 2
Our greatest weakness lies in giving up. The most
certain way to succeed is always to try just one
more time.
Thomas A. Edison
50 X 0.05 = 0.1 X VNaOH
= 25 ml NaOH
Only when the reaction ratio is 1:1,e.g. H2SO4 is not 1:1 so it
can’t be used directly
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