Primary and Secondary Standard
Content
• Types of substances used in volumetric analysis
• Primary standard
• Preparation and storage of primary standard
• Secondary Standard
• Preparation and storage of secondary standard
Learning
Objectives
At the end of this
lecture, the student will be able to
• Explain about different types of substances used in
volumetric analysis
• Explain about the primary and secondary standard
substances
• Explain how to prepare and store primary standard
substances: Sodium oxalate, ceric ammonium sulphate
• Explain about the secondary standard substances
• Explain how to prepare and store secondary standard
substance: Sodium hydroxide, Potassium Hydrogen Phthalate, Hydrochloric acid, Sodium
thio sulphate and Potassium permanganate
Different
Types of Substances Used in Volumetric Analysis
1. Primary
standard substances-
The reagent which is ready to be weighed and used to prepare
a solution with known concentration (standard)
Properties of primary
standard substances:
(a) It is extremely pure
(b) Highly stable
(c) It is anhydrous
(d) It is less hygroscopic
(e) Has very high molecular weight
(f) Can be weighed easily
(g) Should be ready to use and available
(h) Should be preferably non toxic
(i) Should not be expensive
2.
Secondary standard:
A secondary standard is a standard that is prepared in the
laboratory for a specific analysis. It is usually standardized against a
primary standard
Standardization:
The process by which the concentration of a secondary standard substance is
determined by titration with a known concentration of primary standard
substance
Examples
for Primary Standard Substances
• Benzoic acid for standardization of waterless basic
solutions: Ethanolic sodium and potassium hydroxide, TBAH, and alkali
methanolates in methanol, isopropanol, or DMF
• Potassium bromate (KBrO3), Potassium dichromate (K2Cr2O7)
for standardization of sodium thiosulfate solutions
• Potassium hydrogen phthalate (usually called KHP) for standardization
of perchloric acid in acetic acid solutions and aqueous base like sodium
hydroxide
• Arsenic trioxide for making sodium arsenite solution for standardization
of sodium periodate solution
• Sodium carbonate (anhydrous) for standardization of aqueous
acids: hydrochloric, sulfuric acid and nitric acid solutions (but not acetic
acid)
• Sodium chloride (anhydrous) for standardization of silver
nitrate solutions
• Sulfanilic acid for standardization of sodium nitrite
solutions
• Zinc powder, after being dissolved in sulfuric or
hydrochloric acid, for standardization of EDTA solution
Examples
for Secondary Standard Substances
Acid base titrations:
Acid: Hydrochloric acid, sulphuric acid, oxalic acid
Base: sodium
hydroxide, potassium hydroxide
Redox titration:
Oxidising agents: Potassium permanganate, Iodine
Reducing agents: Sodium thio sulphate, potassium iodide
Precipitation
titration: Silver nitrate, ammonium thio cyanate
Complexometric
titration: EDTA
Preparation
and Storage of Primary Standard Substances: Sodium Oxalate
Preparation and storage of: Sodium Oxalate: C2Na2O4
Mol weight = 133.99
Eq.wt:67
Chemicals required to prepare sodium oxalate: Sodium Oxalate
and Distilled Water
Preparation:
Preparation of 0.1 N Solution
•Dry sodium oxalate (C2Na2O4) at 100oC to constant mass for
at least two hours and cool
•Dissolve 6.700g of dried material in water and dilute to
1000 ml
Storage:
•Store in a tightly closed container
•Store in a cool, well-ventilated area away from
incompatible substances
•Oxalates slowly corrode steel
Example sum:
1. How do you prepare 0.1N concentration of sodium oxalate
solution of 100 ml volume?
Calculation:
Weight of the
substancex1000
Normality=
-------------------------------------------------
Equivalent weight x volume
required
2. Sodium oxalate of
8.5 g has been weighed for the experiment and dissolved in 500 ml of distill
water.
Calculate the Molarity of the given solution.
Preparation
and Storage of Primary Standard Substances: Ceric Ammonium Sulphate
Preparation and
storage of: Ceric Ammonium Sulphate: 0.1 M (Ce(SO4)2. 2(NH4)2SO4.4H2O
Mol wt: 668.58
Chemicals required to
prepare ceric ammonium sulphate : Ceric Ammonium Sulphate, H2SO4 and
Distilled Water
Preparation:
• Dissolve 67g of ceric ammonium sulphate, with the aid of
gentle heat, in a mixture of 30 ml of sulphuric acid
•Add 500 ml of water cool, filter the solution, if turbid
•And dilute to 1000 ml with water
Ceric ammonium sulphate has 4 moles of water of
crystallization so it is secondary in nature and it is standarised by arsenic
trioxide as per IP
Ceric Ammonium
Sulphate, 0.1 M
Standardise the
solution in the following manner: Procedure
• Weigh accurately about 0.2 g of arsenic trioxide,
previously dried at 105 º for 1 hour
• Transfered to a 500-ml conical flask
• Wash down the inner walls of the flask with 25 ml of a 8.0
per cent w/v solution of sodium hydroxide
• swirl to dissolve, add 100 ml of water and mix
• Add 30 ml of dilute sulphuric acid
• 0.15 ml of osmic acid solution
• 0.1 ml of ferrroin sulphate solution and
• Slowly titrate with the ceric ammonium sulphate solution
• Until the pink colour is changed to a very pale blue
adding the titrant slowly towards the end-point
1 ml of
0.1 M ceric ammonium sulphate
is equivalent to 0.004946 g of As2O3
Ceric Ammonium Sulphate:
Principle involved in standardisation:
• Ceric being an oxidising agent is titrated against arsenic
trioxide, reducing agent in presence of osmic acid as catalyst and sulphuric
acid.
• Simultaneous oxidation and reduction take place, where
ceric gets reduce to cerrous ions and arsenic trioxide gets oxidised to
arsenious acid
• In presence of Ferroin as indicator, endpoint is red to blue
Preparation
and Standardisation of Secondary Standard Substances: Sodium Hydroxide
Introduction:
• Sodium hydroxide (NaOH), also known as lye and caustic
soda is an inorganic compound
• White solid and highly caustic metallic base and alkali
salt
• Available in pellets, flakes, granules
•Prepared solutions: number of different concentrations
•Sodium hydroxide forms an approximately 50% (by weight)
saturated solution with water
•Sodium hydroxide is soluble in water, ethanol and methanol
•Deliquescent and readily absorb moisture and carbon dioxide
in air
Why sodium hydroxide is considered as secondary standard
substances?
Alkaline solutions absorb carbon dioxide from the atmosphere
according to the reaction:
CO2 + 2OH – ßà CO3 2- + H2O
Since hydroxide ion is consumed by this reaction, the
concentration of a standard sodium hydroxide solution will be changed
2 NaOH (s) + CO2 (g) →Na2CO3 (aq) + H2O (l)
Preparation of Sodium
Hydroxide
Sodium hydroxide:
NaOH
Mol Weight= 40
Equivalent weight:
40
To prepare 0.1N
concentration:
Equation to calculate
normality or molarity of a solution
Weight of the substancex1000
Normality/
Molarity= -------------------------------------------------------------------
Equivalent weight/ Mol weight x volume required
Chemicals required to
prepare 0.1N sodium hydroxide solution
Sodium hydroxide and Distilled Water
Preparation and
Storage of: Sodium Hydroxide: 0.1N
1. Weigh about 0.4 gm of A.R. Sodium hydroxide into a 100 ml
volumetric flask
2. Dissolve in small volume of distilled water
3. Make up the mark with distilled water
Storage: Store in a air tight container In a cool dry place
Standardisation of
Sodium Hydroxide
•To determine the concentration of a sodium hydroxide
solution through a titration
•The primary standard must be an acid
•In the present experiment, potassium hydrogen phthalate
(KHP =KC8H5O4) will be used
NaOH(aq) + KHC8H4O4(aq)
→ KNaC8H4O4(aq) + H2O(l)
Preparation
and Standardisation of Secondary Standard Substances: Potassium Hydrogen Phthalate
Introduction:
• Potassium hydrogen phthalate, often called simply KHP, is
an acidic salt compound
• It forms white powder, colorless crystals, a colorless
solution, and an ionic solid that is the monopotassium salt of phthalic acid
• KHP is slightly acidic, and
•Used as a primary standard for acid-base titrations because
it is solid and air-stable, making it easy to weigh accurately
•It is not hygroscopic
•Used as a primary standard for calibrating pH meters
because, its pH in solution is very stable
Preparation and
storage of:
Potassium hydrogen
phthalate: KHC8H4O4
Mol Weight=
204.22 g/mol
Equivalent weight:
204.22
To prepare 0.1N
concentration:
Note: Dry primary
standard potassium hydrogen phthalate at 110°C for two hours before weighing
Chemicals required to
prepare 0.1N potassium hydrogen phthalate solution
Potassium hydrogen phthalate and Distilled Water
Preparation of
Potassium Hydrogen Phthalate
Step: 1. Weigh the required quantity of PHT
Step: 2.Transfer PHT into a 100 ml volumetric flask
Step: 3.Make up the mark with distilled water
Standarisation of
0.1N Sodium Hydroxide Using Potassium Hydrogen Phthalate
Step: 1. Fill the burette with 0.1N potassium hydrogen
phthalate
Step: 2. Pipette out 10 ml of prepared 0.1N solution of sodium
hydroxide into conical flask
Step: 3. Add 2 drops of Phenolphthalein as indicator
Step: 4. Titrate the sodium hydroxide solution against PHT
until pink changes to colorless
Step: 5.Repeat the titration until consecutive concordant
readings are obtained
Step: 6. Calculation:
Formula: N1V1=N2V2
Where,
N1 =
Normality of Primary standard substance
V1 =
Volume of primary standard substance taken for titration
N2 =
Normality of secondary standard substance (Changed one)
V2 =
Volume of secondary standard substance consumed
N1V1 N2V2
-------- = --------
n1 n2
Step: 7. Report:
The exact normality of the secondary standard substance was found to
be:--------- N
Preparation
and Standardisation of Secondary Standard Substances: Hydrochloric Acid
Introduction:
• Hydrochloric acid is a clear, colorless, highly pungent
solution of hydrogen chloride gas in water
• It was historically called acidum salis, muriatic acid,
and spirits of salt
• It is a highly corrosive, strong mineral acid with many industrial
uses
• Hydrochloric acid is found naturally in gastric acid
• Chemically it is a strong acid
Why hydrochloric acid
is considered as secondary standard substances?
• Hydrochloric acid, HCl, and sulfuric acid, H2SO4, are NOT
suitable for use as a primary standard because although they are both
commercially available as concentrated solutions that are easily diluted, the
concentration of the "concentrated" solution is NOT accurately known
•Laboratory grade hydrochloric acid is not sufficiently pure
Preparation of
Hydrochloric Acid
Hydrochloric acid:
HCL Mol Weight= 36.5
Equivalent weight:
36.5
Note: Since
hydrochloric acid is liquid the mass after calculation should be concerted to
volume using the formula:
Preparation of Hydrochloric Acid
Chemicals required to
prepare 0.1N hydrochloric acid solution
Hydrochloric acid and Distilled Water
Preparation and
Storage of Hydrochloric Acid: 0.1N
1. Pipette out 9 ml of hydrochloric acid into a 100 ml
volumetric flask
2. Dissolve in small volume of distilled water
3. Make up the mark with distilled water
Storage: Store in
a air tight container In a cool dry place
Standardisation of
Hydrochloric Acid
• Sodium carbonate (also known as washing soda, soda ash and
soda crystals), Na2CO3, is the sodium salt of carbonic acid (soluble in water)
• Crystalline heptahydrate, which readily effloresces to
form a white powder, the monohydrate
• Pure sodium carbonate is a white, odourless powder that is
hygroscopic (absorbs moisture from the air)
• Has an alkaline taste
• Forms a strongly alkaline water solution
• Sodium carbonate is well known domestically for its
everyday use as a water softener
•To determine the
concentration of a hydrochloric acid solution through a titration
•The primary standard must be an base
•In the present experiment, anhydrous sodium carbonate will be used
Na2CO3 +2HCl ßà 2NaCl + H2O + CO2
1 mole 2 moles
2 moles 1 mole 1 mole
Preparation
and Standardisation of Secondary Standard Substances: Anhydrous Sodium Carbonate
Anhydrous sodium carbonate:
Na2CO3
Mol Weight=
105.99
Chemicals required to
prepare 0.1N anhydrous sodium carbonate solution
Anhydrous sodium carbonate
Preparation of
Anhydrous Sodium Carbonate
Step 1. Weight out approximately 2 grams of solid sodium
carbonate on a clean, dry watch glass
Step 2.Place watch glass in an oven at a temperature above
100 C (to ensure all the water is evaporated out) but less than 270oC (to
ensure the carbonate does not decompose). A temperature of approximately 260oC
is often used
Step 3. After 30 minutes, remove the watch glass from the oven
and transfer it immediately to a desiccator to prevent the sodium carbonate
from re-absorbing water from the atmosphere
Step 4.Allow the sodium carbonate to cool in the desiccator
before re-weighing it
Step 5. Repeat this heating-cooling-weighing process until a
constant weight is achieved (indicating that all the water has been driven off)
Preparation of
Anhydrous Sodium Carbonate
Step: 1. Weigh the required quantity of anhydrous sodium
carbonate
Step: 2.Transfer into a 100 ml volumetric flask
Step: 3.Make up the mark with distilled water
Standarisation of
0.1N Hydrochloric Acid Using Anhydrous Sodium Carbonate
Step: 1. Fill the burette with 0.1N Anhydrous Sodium
Carbonate
Step: 2. Pipette out 10 ml of prepared 0.1N solution of
hydrochloric acid into conical flask
Step: 3. Add 2 drops of methyl orange as indicator
Step: 4. Titrate the hydrochloric acid solution against anhydrous
sodium carbonate until red to yellow
Step: 5.Repeat the titration until consecutive concordant
readings are obtained
Step: 6. Calculation:
Formula: N1V1=N2V2
Where,
N1 = Normality of Primary standard substance
V1 =
Volume of primary standard substance taken for titration
N2 =
Normality of secondary standard substance (Changed one)
V2 =
Volume of secondary standard substance consumed
N1V1/n1 = N2V2/n2
Step: 7. Report: The exact normality of the secondary
standard substance was found to be:--------- N
Preparation
and Standardisation of Secondary Standard Substances: Sodium Thio Sulphate
Why sodium thiosulphate
is considered as secondary standard substances?
• Sodium thiosulphate contains water of crystallisation, so
it efflorescent in nature
Preparation and
storage of:
Sodium thio sulphate: Na2S2O3 .5H2O
Mol Weight=
248.184
Chemicals required to
prepare 0.1N sodium thio sulphate solution: sodium thio sulphate, anhydrous
sodium carbonate
Preparation and
Storage of Sodium Thio Sulphate: 0.1N
1. Weigh the required quantity of sodium thio sulphate into
a 100 ml volumetric flask
2. Dissolve in small volume of distilled water free from
carbon di oxide
3. Add 0.2 g of sodium carbonate
4. Make up the mark with distilled water
Storage: Store in
a air tight container in a cool dry place
The stability of thio
sulphate solution:
1. The ordinary distilled water contains excess of carbon di
oxide, this may cause a slow decomposition to take place with the formation of
sulphur
S2O3 2- + H+ =
HSO3 - + S
2. Decomposition can also be caused by bacterial action: Thiobacilluc
thioparus
Standardisation of
0.1 Sodium Thio Sulphate
• Standarisation of sodium thio sulphate: Can be standarised
with the titration of the iodine when unmeasured excess of potassium iodide is
added to known volume of an acidified standard potassium dichromate solution:
6KI+14HCl+ K2Cr2O7→2CrCl3+3I2 +7H2O+8KCl
2Na2S2O3 + I2 → Na2S4O6 + 2NaI
•Each mole of
dichromate results in
the production of
three moles of iodine ⇒ meqK2Cr2O7 = M/6
Preparation of
Potassium dichromate solution: 0.1N
Chemicals required to
prepare 0.1N Potassium dichromate solution
Potassium dichromate, Potassium iodide, Hydrochloric acid
and Starch
Preparation and
Storage of Potassium Dichromate: 0.1N
1. Weigh about 0.49 g of potassium dichromate into a 100 ml
volumetric flask
2. Dissolve in small volume of distilled water
3. Make up the mark with distilled water
Storage: Store in
a air tight container in a cool dry place
Standardisation of
Sodium Thio Sulphate
1. Load a burette with standardised sodium thiosulphate
solution
2. Pipette 10-ml aliquot of prepared standard potassium dichromate
solution into 100 ml flask
3. Add 10 ml of 10 % KI solution and 10 ml of 1 M
HClsolution
4. Cover the flask with a stopper and put into dark place at
10 min
5. Titrate the sample solution with thiosulfate until the
solution becomes pale yellow
6. Introduce 5 drops of starch indicator, and titrate with
constant stirring to the disappearance of the blue colour
7. Read the burette mark
8. Calculate precision normality (molarity) of the sodium thiosulphate
standard solution
Preparation
and Standardisation of Secondary Standard Substances: Potassium Permanganate
Introduction:
•Potassium permanganate is a dark purple or brownish black
powder or dark purple or almost black crystals
•It is soluble in cold water and freely soluble in boiling
water
•It is a strong oxidising agent
•It decomposes on contact with certain organic substances
The permanganate ion is strong oxidizing reagent. The
half-reaction is:
MnO4- + 8H+ + 5e- ßà Mn2+ + 4H2O E0 = 1.51 V
Why potassium permanganate
is considered as secondary standard substances?
• Because it contains reduced products like manganese oxide MnO2
•The concentration of KMnO4 changed after preparation
because it dissociated through reducing agents such as ammonia and organic
substances in water
•keep it at least 7-10 days after preparation at dark place
and in a dark bottle
Preparation of
Potassium Permanganate
Potassium
permanganate: KMNO4
Mol Weight=
158.03
Preparation and
Storage of Potassium Permanganate: 0.1N
• Dissolve about 0.32 g of KMnO4 in 100 mL of deionized
water using a large beaker
• Cover that beaker with a watch-glass and heat to boiling using
a hot plate in the hood
• Keep the solution at a gentle boil for about 1 hr
• Let the solution stand overnight
• Remove Manganese di oxide by filtering through a filter
crucible
• Transfer the solution to a clean amber glass- stoppered
bottle
• Store in the dark when not in use
Precaution during
preparation of potassium permanganate solution:
•Before filtration, the reagent solution is allowed to stand
for about 24 hrs or
•Heated for a brief period to hasten oxidation of the
organic species generally present in small amounts in distilled and deionised
water
•Paper cannot be used for filtering because permanganate
ions react with it to form additional manganese dioxide
Chemicals required to
prepare 0.1N Potassium permanganate
Potassium permanganate
Amber bottle for storage
Standarisation of
Potassium Permanganate
• To determine the concentration of a potassium permanganate
solution through a titration: The primary standard must be a reducing agent
• In the present experiment, Sodium oxalate is used as
primary standard substance, as reducing agent
• The titration follows oxidation-reduction reaction in an acidic
media
5C2O 2- + 2MnO41- + 16 H+
→2Mn2+ +8H2O+10CO2
Over all Equation:
Na2C2O4 + H2SO4 = H2C2O4 + Na2SO4
2KMnO4 + 5H2C2O4 + 3H2SO4 = K2SO4 + 2MnSO4 + 10CO2 + 8H2O
Preparation
of Sodium Oxalate
Introduction:
Sodium oxalate, Na2C2O4, is a strong electrolyte that
dissociates completely in water according to the following equation:
Na2C2O4 Ã
2 Na+ (aq) + C2O4 (aq)
Sodium oxalate:
Na2C2O4
Mol Weight= 134
The half-reaction for oxalate as a reducing agent is
C2O4 2- - 2e- →2CO2
The equivalent mass of Na2C2O4 = 2(22.99) +2(12.01)
+4(16.00)/2 = 67.00
To prepare 0.1N
concentration :
Equation to calculate normality or molarity of a solution
Weight of the substancex1000
Normality/
Molarity= ------------------------------------------------------------------
Equivalent
weight/ Mol weight x volume required
Chemicals required to
prepare 0.1N sodium oxalate solution
Sodium oxalate
Preparation of Sodium
Oxalate
Step: 1. Weigh the required quantity of sodium oxalate
Step: 2.Transfer PHT into a 100 ml volumetric flask and
dissolve with distill water
Step: 3.Make up the mark with distilled water
Standarisation of
0.1N Potassium Permanganate Using Sodium Oxalate
Step: 1.Fill the burette
with 0.1N potassium permanganate solution
Step: 2.Pipette out 10 ml of prepared 0.1N solution of sodium
oxalate into conical flask
Step: 3. Add 7 ml of sulfuric acid into conical flask and
heat to 70 o C
Step: 4.Titrate the sodium oxalate against potassium permanganate
solution until pink is discharged
Note: Potassium permanganate acts as self-indicator
Step: 5.Repeat the titration until consecutive concordant
readings are obtained
Step: 6. Calculation:
Formula: N1V1=N2V2
Where,
N1 =
Normality of Primary standard substance
V1 =
Volume of primary standard substance taken for titration
N2 =
Normality of secondary standard substance (Changed one)
V2 = Volume of secondary standard
substance consumed
N1V1 = N2V2
Step: 7. Report: The exact normality of the secondary
standard substance was found to be:--------- N
Precaution during
standarisation of potassium permanganate solution:
Sufficient sulphuric acid is added during standarisation
because potassium permanganate oxidation ability is better in acidic media and
to keep ion concentration constant thorough out the standarisation process
MnO - + 8 H+ + 5e→ Mn2+ + 4
H2O
Effect of temperature:
•Near 70oC to be maintained throughout the titration because
the oxidation of sodium oxalate is rapid enough at such temperature
•Below 60o C the reaction is very slow
•Formation of brown colour during the titration is caused
by:
Insufficient acid
Applying too high temperature
The use of a dirty flask
Summary:
• Primary standard substances-
The reagent which is ready to be weighed and used to prepare a solution with
known concentration (standard)
• Secondary standard:
is a standard that is prepared in the laboratory for a specific analysis. It is usually standardized against a primary
standard
• Standardization: The concentration of a secondary standard substance
is determined by titration with a known concentration of primary standard
substance
• Equation to calculate normality or molarity of a solution
Weight of
the substancex1000
Normality/ Molarity = -------------------------------------------------------------------
Equivalent weight/ Mol weight x volume required
• Sodium hydroxide is considered to be secondary standard
substance as it absorbs moisture from the atmosphere can be standardised by
potassium hydrogen phthalate
• Hydrochloric acid concentration various as it is a fuming
liquid can be standardised by anhydrous sodium carbonate
• Sodium thio sulphate is considered as secondary standard
as it is efflorescent in nature: as it contains 5 moles water of
crystallization
• Sodium thio sulphate is standarised by using potassium di chromate
• Potassium permanganate is considered to be secondary
standard substance as it gets decomposed by organic matter to manganese di
oxide
• Potassium permanganate can be standarised by using sodium oxalate in presence of acidic media and at a temperature of 60 -70 o C
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