The Great Chemist
ALFRED NOBEL
CHEMISTRY
LABORATORY - I
-1-
WORK SHEET
Titration 1 : Standardization of AgNO3
Standard Sodium chloride Vs AgNO3
Sl.No
Vol.of
Sodium
chloride
V1 (mL)
Burette reading (mL)
Initial
Final
Volume of
AgNO3
V2 (mL)
Concordant
value
Calculation
Volume of Sodium chloride
V1 = ------- mL
Strength of Sodium chloride
N1 = -------N
Volume of AgNO3
V2 = …… mL
Strength of AgNO3
N2 = …?….N
V1 N1 = V2 N2
N2 = V1 N1 / V2
= [------- (V1) X ------ (N1)] / ------(V2)
Strength of AgNO3
N2 = ----------- N
-2-
Ex.No.
Date:
DETERMINATION OF CHLORIDE CONTENT OF WATER
SAMPLE BY ARGENTOMETRIC METHOD
Aim
To determine the chloride content of water sample by argentometric
method (Mohr’s method).
Principle
Chloride is present in water usually as NaCl, MgCl2 and CaCl2.
Although chlorides are not harmful as such, their concentrations beyond 250
ppm impart a peculiar taste of the water, thus rendering the water
unacceptable for drinking purposes.
Further, existence of unusually high concentration of chloride in a
water sample indicates pollution from domestic sewage or industrial waste
water. Salts like MgCl2may undergo hydrolysis under the high pressure and
temperature prevailing in the boiler, generating hydrochloric acid which
causes corrosion in boiler parts. Chlorides in the form of MgCl2 and CaCl2
cause various disadvantages in household and industrial use of water.
By argentometric method chloride ions in water sample, which is
neutral or slightly alkaline can be determined by titrating it against standard
silver nitrate solution using potassium chromate as an indicator. The pH
should be between 7 and 8.
At higher pH
Ag+ + OH-
Ag OH
and at lower pH K2CrO4 indicator is converted to K2Cr2O7
2 K2CrO4 + 2 HCl
2 KCl + K2Cr2O7 +H2O
-3-
WORK SHEET
Titration 2 :
Standard AgNO3 Vs Distilled water
Vol.of
Sl.No
Distilled
Water (mL)
Burette reading (mL)
Initial
Final
Volume of
AgNO3
V1 (mL)
Concordant
value
Titration 3:
Standard AgNO3 Vs given Water Sample
Vol.of
Sample
Sl.No
Water (mL)
Burette reading (mL)
Initial
Final
-4-
Volume of
AgNO3
V2 (mL)
Concordant
value
As KHCrO4 is weak in nature, concentration of CrO42- decreases and
therefore, higher concentration of Ag+ is needed for the solubility product of
Ag2CrO4 to be exceeded.
Ag+ reacts with Cl- and CrO42-, when AgNO3 solution is released from
the burette sample solution which has ClAg+ + Cl-
AgCl (K sp = 3X10-10)
White ppt
2Ag+ + CrO42-,
Ag2 CrO4 (K sp = 5X10-12)
Red ppt
Red colour formed because of formation of silver chromate, disappears
as the solution contains high concentration of ClAg2 CrO4 +2Cl-
2AgCl +CrO42-
As the concentration Cl-ion decreases, the red colour disappears more
slowly and when all the chloride is precipitated, a faint reddish or pinkish
tinge persisting the white precipitate even after brisk shaking.
Procedure :
Titration 1 : Standardization of AgNO3
Transfer 20 mL of standard NaCl in a conical flask and add about 3-4
drops of potassium chromate indicator. Then slowly add standard solution of
AgNO3 solution from the burette and the value of titrant is noted as end
point, when yellow colour changes to red colour. The titration is repeated
until a concordant volume V2 is obtained.
-5-
WORK SHEET
Calculation :
Volume of AgNO3 consumed by given sample V2 =……….. mL
Volume of AgNO3 consumed by dis.water
V1 =……….. mL
Net volume (V2 – V1) = ………. mL (V2 )
Volume of given sample
V1 = 50mL
Strength of given sample
N1 = …?…….N
Volume of Net AgNO3 consumed
V2 = ………...mL
Strength of AgNO3 consumed
N2 = …………N
V1 N1 = V2 N2
N1 = V2 N2 / V1
= [------- (V2) X ------ (N2)] / ------(V1)
N1 = ---------- N
The amount of chloride in the given sample = …………(N1) X 35.5 X 1000
= ------------ ppm
Precautions
1. The whole apparatus must be washed with distilled water.
2. The volume of the indicator should be same in all titrations.
3. The reaction mixture should be briskly shaken during the titration.
-6-
Titration 2 : Distilled water Vs AgNO3
Transfer 50 mL of standard distilled water in a conical flask and add
about 3-4 drops of potassium chromate indicator. Then slowly add standard
solution of AgNO3 solution from the burette and the value of titrant is noted
as end point, when yellow colour changes to red colour. The titration is
repeated until a concordant volume V1 is obtained.
Titration 3: Estimation of chloride present in the given sample.
Transfer 50 ml of given water sample in a conical flask and add about
3-4 drops of potassium chromate indicator. Then slowly add standard
solution of AgNO3 solution from the burette and the value of titrant is noted
as end point, when yellow colour changes to red colour. The titration is
repeated until a concordant volume V2 is obtained.
Result
The amount of Chloride present in the given water sample
= -------------- ppm.
Viva questions
1. Name the sources of chlorides in water
2. Why does AgNO3 combine first with chloride ions in the water and not
with K2CrO4?
3. What is the action of chloride on pathogens?
4. How is the pH adjusted when the alkalinity is not being measured?
5. What is the effect of temperature on the determination of chloride content
in water?
6. What is the principle of Mohr’s method?
-7-
WORK SHEET
Observation
Sl. No
Volume of NaOH added
(mL)
-8-
Observed Conductance (ohm-1)
Ex.No.
Date:
CONDUCTOMETRIC TITRATION
(STRONG ACID Vs STRONG BASE)
Aim
To find out the strength of acid (HCl) (approximately N/10), by
titrating it against sodium hydroxide solution conductometrically.
Principle
When a strong acid like HCl is titrated against a strong base like NaOH,
the neutralization reaction takes place.
HCl +NaOH
NaCl +H2O
This neutralization reaction can be followed by conductometric method.
During the course of titration, the conductivity decreases due to the
replacement of H+ ions by Na+ions.
After the neutralization, the
conductivity increases steeply due to the excessive presence of OH- ions.
Procedure
0.5 N NaOH solution is taken in the burette. 50 mL of the given HCl is
taken in a 100 mL beaker. Conductivity cell containing two platinum
electrodes, is placed in HCl. The cell is connected to the conductivity meter.
Now add 1mL, 2mL, 3mL and 4mL of NaOH from the burette and stir the
contents of the beaker thoroughly. Note the conductance readings after each
addition.
-9-
WORK SHEET
A graph is plotted with observed conductivity against volume of NaOH
added. The shape of the curve will be as shown in the figure.
Calculation :
V 1N 1 = V 2N 2
Volume of NaOH V1 = ---------- mL (from graph)
Strength of NaOH N1 = ---------- N
Volume of HCl
V2 = --------- mL
Strength of HCl
N2 = -----?----- N
N2 = (V1 X N1) / V2
= [……..( V1) X ……..( N1)] / …….( V2)
Strength of unknown HCl (N2) = ------------- N
Amount of HCl present in the whole of the given solution
= [---------------- (N2) x ------------- (Eq.wt) x 50] / 1000
= ------------------- g
(Equivalent Weight of HCl = 36.5)
-10-
Result
(i)
(ii)
The strength of the given acid = ----------------- N
Amount of HCl present in
the whole of the given solution = ------------g
-11-
WORK SHEET
Observation
Volume of
NaOH
solution
added (mL)
pH
∆pH
∆V
∆pH/∆V
Average
volume of
NaOH (mL)
Where, ∆V = difference in two consecutive volumes of titrant added in mL
∆pH = difference in two consecutive reading
-12-
EX.NO.
Date:
DETERMINATION OF STRENGTH OF
HYDROCHLORIC ACID BY pH TITRATION
Aim
To find out the strength of given hydrochloric acid solution by titrating
it against sodium hydroxide (0.1N) using pH meter.
Principle
When an alkali is added to an acid solution, the pH of the solution
increases slowly, but at vicinity of the end point, the rate of change of pH of
the solution is very rapid. From the sharp break in the curve, we can find out
the end point, from which the strength of HCl can be calculated.
Procedure
First standardize the pH meter using different buffers of known pH,
then wash the glass electrode and reference electrode with distilled water
and then with the acid solution. Take 20 mL of HCl solution in a 250 mL
beaker. Add sufficient distilled water (100mL) so that the glass electrode as
well as the reference electrode is completely dipped.
Note the pH of the pure acid solution. Now add 1 mL of 0.1 N NaOH
from the burette in the beaker. Stir the contents well. Note the pH of the
solution. Now keep on adding NaOH solution from the burette and the note
the pH of the solution, up to 9- 10 mL of the NaOH. Near the end point add
very small amount of sodium hydroxide, because change in pH will be very
much appreciable when the acid is neutralized, further addition of such a
small amount as 0.01 mL raises the pH about 9 to 10.
-13-
WORK SHEET
Graph
Calculation:
Volume of NaOH required for complete neutralization of HCl = --------- mL
Volume of NaOH
V1 = ------------- (mL) from graph
Strength of NaOH
N1 = ------------ N
Volume of HCl
V2 = ------------- (mL)
Strength of HCl
N2 = ------?------- N
Therefore Strength of HCl N2 = [-------- (V1) x -------- (N1)] / ---------- (V2)
= ------------N
Amount = Normality x Equivalent Weight (HCl)
Therefore Amount of HCl present in the whole of the given solution
= [---------------- (N2) x ------------- (Eq.wt) x 20] / 1000
= ------------------- g
(Equivalent Weight of HCl = 36.5)
-14-
Plot a graph between pH and Volume of NaOH added. From this graph,
determine the volume of NaOH required for the complete neutralization of
HCl.
Result
(i) Strength of the given hydrochloric acid solution
= ---------- N
(ii) The amount of hydrochloric acid present in the whole
of the given solution = --------- g
Viva questions
1. What do you mean by pH?
2. What is the effect of temperature on pH?
3. What are the composition of glass electrode and calomel
electrode?
4. What is the effect of dilution on pH of an acidic solution?
5. What is the pOH of pure water at 25o C?
6. What chemicals would you use to make a buffer of pH 10?
7. What is a combined electrode?
8. Why is hydrogen not generally used in pH measurements?
9. What is the significance of pH titration?
10. What are the industrial applications of pH metric measurements?
11. What is the pH value of lime juice, blood and de-ionized water?
12. What is function of calomel electrode?
-15-
WORK SHEET
Observation
Sl.No
Volume of NaOH added (mL)
-16-
Observed Conductance (ohm-1)
Ex.No.
Date:
CONDUCTOMETRIC TITRATION OF
MIXTURE OF ACIDS
Aim
To find out the strength of mixture of acids (hydrochloric acid and
acetic acid) of approximate strength N/10 each, by titrating it against sodium
hydroxide solution conductometrically.
Principle
The type of titration is just a combination of two separate titrations,
viz. HCl against NaOH and CH3COOH against NaOH. By adding to the
mixture, the conductivity of sodium ion decreases due to the replacement of
H+ ions forms the strong acid. The conductivity of the solution increases as
the weak acid is converted into salt and finally rises more steeply as excess
of alkali is added.
A graph is plotted with conductivity against volume of alkali added. It is
observed that there is a rounding off at both the end points. Usually,
extrapolation of the straight lines of the three branches would lead to a
definite location of the end points. It must be noted that the first end point
will be that of hydrochloric acid (strong) while the second will be that of
acetic acid (weak).
-17-
WORK SHEET
Calculation :
For HCl: (Strength of unknown HCl)
V1N1 = V2N2
Volume of NaOH V1 = --------- mL ( from graph )
Strength of NaOH N1 = ---------- N
Volume of HCl
V2 = ---------- mL
Strength of HCl
N2 = [--------( V1 ) X--------(N1)] / --------(V2)
Strength of unknown HCl N2 = --------- N
Amount of HCl present in the whole of the given solution
= [---------------- (N2) x ------------- (Eq.wt) x 50] / 1000
= ------------------- g
(Equivalent Weight of HCl = 36.5)
For CH3COOH: (Strength of unknown CH3COOH)
Volume of NaOH
V3 = -------- mL (from graph)
Strength of NaOH
N3 = ---------- N
Volume of CH3COOH V4 = --------- mL
Strength of CH3COOH N4 = [--------(V3) X-------- (N3)] / --------(V4)
Strength of unknown CH3COOH N4 = --------- N
Amount of CH3COOH present in the whole of the given solution
= [---------------- (N4) x ------------- (Eq.wt) x 50] / 1000
= ------------------- g
(Equivalent Weight of CH3COOH = 60.05)
-18-
Procedure
Prepare an exact N/10 Solution of sodium hydroxide. Rinse and fill the
burette with the alkali solution. Now take 10 mL of the mixture of HCl and
CH3COOH in a 200 mL beaker. Add about 100 mL of distilled water.
Immerse the cell in this solution and determine the conductance of the
solution. (The first end point will lie near about 5 mL, while the second will
be near about 10 mL of sodium hydroxide solution).
Now add 1mL, 2mL, 3mL and 4mL of NaOH from the burette and stir
the contents of the beaker thoroughly. Note the conductance readings after
each addition. Now, add NaOH at an increment of 0.2 mL up to 6 mL and
then again between 9 mL and 11 mL.
After adding 11 mL of NaOH solution, add 1mL in each addition till you
have added 18 mL, note the conductance of the solution after each addition
thoroughly stir the contents of the beaker.
Result
The strength of each acid in the given mixture is
a. (i) Strength of hydrochloric acid = ---------------- N
(ii) Amount of HCl present in the whole of the given
solution = ---------------g
b. (i) Strength of acetic acid
= ---------------- N
(ii) Amount of CH3COOH present in the whole of the given
solution = ---------------g
-19-
WORK SHEET
Observation
Sl.No.
Sl.No.
Vol.of
Stock
Solution
Vol.of
HNO3
(mL )
Vol.of
Potassium
Thiocyanate
(mL)
Concentration in ppm
Vol.of
Distilled
water (mL)
Total
Volume
(mL)
Absorbance at 480 nm
-20-
Ex.No.
Date:
ESTIMATION OF FERRIC ION BY SPECTROMETRY
Aim
To estimate the amount of ferric ion present in the given sample using
spectrophotometer.
Principle
When a monochromatic light passes through a homogeneous coloured
solution, a portion of incident light is reflected, a portion is absorbed and the
remaining is transmitted.
Io = Ir + Ia + It
Where Io, Ir, Ia, It are the intensities of the incident, reflected,
absorbed and transmitted light, respectively, Ir is usually eliminated and
hence, Io = Ia + It
The mathematical statement of beer- Lambert’s law is given by
Log Io / I = ε Ct
A = ε Ct
Where, log Io /I = A
Io = intensity of incident light
It = intensity of transmitted light
C = Concentration of the solution in moles /L
t = thickness of cell in cm
ε = molar absorption coefficient
A = absorbance or optical of solution density
-21-
WORK SHEET
Graph
-22-
When a ray of monochromatic light passes through an absorbing
medium, its intensity decreases exponentially as the concentration of
absorbing substance and the length or thickness increase independently.
Keeping the path length constant (t=1 cm), the variation is with reference to
only concentration, C.
Fe3+ion do not give any colour. However, it develops a red colour when
it reacts with potassium thiocyanate solution.
Fe3+ + 6 KSCN → [Fe (SCN) 6]3- + 6K+
Further, this colour is in the blue region (λ- 480 nm). Spectrophotometer
has a wide range of adaptability that allows selection of monochromatic light
of any wavelength in the visible spectrum.
The light source is an ordinary light bulb and monochromatic light is
obtained by using either a prism or a diffraction grating. The monochromatic
light is then passed through the filter and is directed through a cell
containing the sample. The light that penetrates hits the photoelectric cell
and the output of this can be seen in the display.
-23-
WORK SHEET
-24-
Estimation of Ferric ion in the given sample:
The given sample solution is taken in 100mL SMF and 3 mL of strong
HNO3 (1:1 V) and 5mL of potassium thiocyanate solution are added and
made up to the mark using distilled water. The absorbance of the test
solution can be calculated by performing the same procedure as standard.
Result
The amount of iron present in the given test sample = ------------ ppm
-25-