7. pH changes in acid–alkali
neutralisations and choice of indicator for a specified titration
Index of all GCSE level (~US grades 8-10) notes on acids, bases
and salts
What next?
Associated Pages
Summary
pH titration curves for a neutralisation reaction: How does
the pH change during a neutralisation reaction? What indicator do you use for a
particular acid–alkali titration? Litmus, phenolphthalein, methyl orange, methyl
red colour changes are given for determining the end–point of a titration. These
revision notes and pH curve graphs for acid alkali titration neutralisations and
their pH changes, should prove useful for the new AQA chemistry, Edexcel
chemistry & OCR chemistry GCSE (9–1, 9-5 & 5-1) science courses.
Doc Brown's
chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE chemistry, O level
& ~US grades 8, 9, 10 school science courses for ~14-16 year old science
students for national examinations in chemistry topics including acids
bases alkalis salts preparations reactions
7.
What
pH changes go on in a neutralisation reaction?
Typical neutralisation reactions involving mixing a
soluble acid with a soluble base (alkali) include
sodium hydroxide + hydrochloric acid
==>
sodium chloride + water
NaOH(aq) + HCl(aq)
==>
NaCl(aq) + H2O(l)
sodium hydroxide + sulfuric acid ==> sodium
sulfate +
water
2NaOH(aq) + H2SO4(aq)
==> Na2SO4(aq) + 2H2O(l)
potassium hydroxide + nitric acid ==> potassium nitrate +
water
KOH(aq) + HNO3(aq)
==> KNO3(aq) + 2H2O(l)
Apart from the water,
all the actual species are ions eg
H+, Cl–
and SO42– ions from the acids, Na+,
K+ and OH– from the alkali.
The only ions that
change are
the hydrogen ion (H+
causes acidity), if pH <7 acid, H+ concentration is
more than OH– concentration
the hydroxide ion (OH–
causes alkalinity), if pH >7 alkaline, OH–
concentration is more than H+ concentration
these two ions
combine to form neutral water in these particular
neutralisation reactions –
this is the
real neutralisation reaction
H+(aq)
+ OH–(aq)
==>
H2O(l)
Note:
At pH 7 there are very tiny, but
equal, concentrations of H– and OH– ions.
This means the Cl–,
SO42–, NO3–, Na+
and K+ ions do NOT change and are called spectator
ions and their concentrations only fall on mixing due to the
obvious dilution effect of having a bigger volume they are dissolved
in.
These are important
chemical points when appreciating what is going on when the pH of a
solution changes ie understanding what a pH curve represents.
These two graphs, on the same set of axis, show how the pH changes when an alkali
(soluble base) and an acid neutralise each other and what you see visually using
universal indicator (univ. ind.). These simple curves represent what happens
when eg hydrochloric acid and sodium hydroxide are mixed or nitric acid and
potassium hydroxide (1 : 1 molar equations) BUT the curves are complicated for
acids like sulfuric acid where the molar ratio is NOT a 1 : 1 molar ratio with
the alkali. You need to consult more advanced notes via links at the end of the
page.
The pH
neutralisation curves
Strictly speaking, they only apply to a strong acid
and strong soluble base (alkali), but this pattern of pH change illustrated by
the graph is what is happening in the
salt preparation method (a) or in
acid and alkali titrations.
Note: You can prepare a
salt, or analyse an acid or alkaline solution by doing an acid–alkali addition either way round but in either case the
volume of acid or alkali needed for neutralisation = the volume reading X at pH
7 (univ. ind.
green).
This strictly speaking only applies if it is a strong
acid reacting with a strong alkali (see
section 10).
At first on adding one to the other,
the pH only changes gradually, but then you get a much more dramatic
change as you approach the end-point i.e. the complete neutralisation
point.
It is at this point, the end-point, you get the sharpest
change in pH, and, any indicator you choose to use, MUST change colour
the most sharply at this point on the pH curve, to get the end-point
accurately, hence the titration volume of acid or alkali accurately.
You
can do the experiments by measuring out a given volume of an
acid/alkali of known concentration, and adding the acid/alkali in
small portions from a burette and measuring the pH with a pH meter
after every small addition e.g. 1 cm3 of the acid/alkali solution.
See another page
for the apparatus and how to use it.
Red graph line:
If you add a strong acid
to a strong alkali (univ. ind. = blue), the pH starts at about 13-14 and only falls little at first
as the colour changes from purple ==> blue. Then the pH falls much more steeply
as the indicator colour changes from 'bluey' green ==> dark green ==> pale
green. The solution is then neutralised at pH 7. This is the point where
the salt is 100% formed. With further addition of excess acid, the pH falls and then
levels out to about pH 1 as the colour changes further from green ==>
yellow ==> orange ==> red
In terms of H+ and
OH– ions: Initially a high concentration of OH–,
so solution very alkaline, but as the H+ is steadily added,
the OH– ions are neutralised to water. Therefore the OH–
concentration steadily falls as does the pH because the solution becomes
less alkaline. At pH 7, neutral there are very tiny equal concentrations
of H+ and OH–. If excess acid is added, the pH
steadily falls to around 1 as the concentration of H+ from
the acid rises.
Blue graph line:
If you add
a strong alkali to a strong acid (univ. ind. = red), the pH starts at about
0-1 and only rises
a little
at first with the colour still quite red. Then on further addition of
alkali the pH rises more sharply
as the colour changes from red ==> orange ==> yellow and eventually at the
neutralisation point at pH 7 the univ. ind. is
green. This is the point where the salt is 100% formed. With excess alkali the pH
continues to rise
and then levels out to about 13 as the indicator colour changes through
dark green ==> blue ==> purple.
In terms of H+ and OH–
ions: Initially a high concentration of H+, very
acid, but as the OH– of the alkali is steadily added, the H+
ions are neutralised to water. Therefore the H+ concentration
steadily falls and the pH rises as the solution becomes less acid. At pH
7, neutral there are very tiny equal concentrations of H+ and
OH–. If excess alkali is added the pH steadily rises from 7
to around 13 as the concentration of OH– from the alkali
rises ie becoming a much more alkaline solution.
The pH titration
curves involving weak acids and weak alkalis are explained in section
10.
Universal indicator, and most
other acid–base indicators, work for strong acid and alkali titrations, but
universal indicator is a somewhat crude indicator for other acid–alkali
titrations because it gives such a range of colours for different pH's. So,
to get accurate titration results you need to use a special indicator
for a particular acid–alkali titration. The complications arise because
not all acids and soluble bases (alkalis) are as strong as each other.
There is more on weak/strong acids in section 10.
More on Acid–Base Theory and Weak and Strong Acids
Examples of more accurate and 'specialised' indicators
-
Apparatus used in titrations – pipette, conical flask and a
burette
- Note that the first mentioned is in the
flask and the second is in the burette.
- titrating a strong alkali with a
strong acid (or vice versa):
- e.g. for sodium/potassium hydroxide (NaOH/KOH)
– hydrochloric/sulfuric acid (HCl/H2SO4)
titrations,
- the pH at the endpoint is around 7, so
you can use ...
- phenolphthalein indicator (pink
in alkali, colourless in acid–neutral solutions), the end–point is the
pink <==> colourless change,
- Litmus works too, the end point is the
red <==> purple/blue colour change AND with universal indicator you see green
at the endpoint, but these two indicators are NOT as accurate as
phenolphthalein and other indicators.
- They are ok as simple demonstration to
illustrate the principles of a titration, BUT, they should not be used
for quantitative work.
titrating a weak soluble base (weak alkali) with a
strong acid:
- e.g. for titrating ammonia (NH3)
with hydrochloric/sulfuric acid (HCl/H2SO4),
- the end-point is typically around pH 5,
so you have to use ...
- methyl orange indicator (red in
acid, yellowish–orange in neutral–acid), the end–point is an 'orange'
colour, not easy to see accurately,
- or, screened methyl orange indicator
is a slightly different dye–indicator mixture that is reckoned to be
easier to see than methyl orange, the end–point is a sort of 'greyish
orange', but still not easy to do accurately,
- or, methyl red indicator (red in
acid, yellow in neutral–alkaline), the end–point is 'orange'.
titrating a weak acid with a strong alkali:
- e.g. for titrating ethanoic acid
(CH3COOH) with sodium hydroxide (NaOH),
- the end-point is typically around pH 9,
so you have to use ...
- phenolphthalein indicator (pink
in alkali, colourless in acid–neutral solutions, pink in alkali), the
end–point is the first permanent pink.
titrating a weak acid with a weak
alkali (or vice versa):
- These are NOT practical
titrations because the pH changes at the end–point are not great
enough to give a sharp colour change with any indicator.
See section 10. for more details on
acid-alkali titrations and calculations and procedures.
What next?
Associated Pages
Index of all my GCSE level (~US grades
810) notes on acids, bases
and salts
Email doc
brown comment? query?
HOME PAGE of Doc Brown's Science
UK KS3 Science Quizzes for
students aged ~11-14, ~US grades 6 to 8
Biology * Chemistry
* Physics notes for GCSE
level
students aged ~14-16, ~US grades 8 to 10
Advanced Level pre-university Chemistry
notes for students aged ~16-18 ~US grades 11 to 12
GCSE/IGCSE Acids & Alkalis revision notes sub–index:
Index of all pH, Acids, Alkalis, Salts Notes 1.
Examples of everyday acids, alkalis, salts, pH of
solution, hazard warning signs : 2.
pH scale, indicators, ionic theory of acids–alkali neutralisation : 4.
Reactions of acids with
metals/oxides/hydroxides/carbonates, neutralisation reactions : 5.
Reactions of bases–alkalis
like ammonia & sodium hydroxide : 6. Four methods
of making salts : 7. Changes in pH in a
neutralisation, choice and use of indicators : 8. Important formulae
of compounds, salt solubility and water of crystallisation :
10.
More on Acid–Base Theory and Weak and Strong Acids
See also
Advanced Level Chemistry Students Acid–Base Revision
Notes – use index
Section
(2) lists common indicators
-
Multiple choice revision quizzes and other worksheets
-
GCSE/IGCSE foundation–easier multiple choice quiz on pH, Indicators, Acids,
Bases, Neutralisation and Salts
-
GCSE/IGCSE higher–harder multiple choice quiz on pH, Indicators, Acids,
Bases, Neutralisation and Salts
-
GCSE/IGCSE Structured question worksheet on Acid
Reaction word equations and
symbol
equation questions
-
Word
equation answers and
symbol
equation answers)
-
GCSE/IGCSE word–fill worksheet on Acids,
Bases, Neutralisation and Salts
-
GCSE/IGCSE
matching pair quiz on Acids, Bases, Salts and pH
-
See also
Advanced Level Chemistry Students: Acid–Base Revision
Notes – use index
-
Advanced Level Chemistry Students: Acid–base indicator theory, pH curves and
titrations
-
Advanced Level Chemistry Students: Salt
hydrolysis,
Acid–base titrations–indicators, pH curves and buffers
|