F.6/7 Chemistry Practical: An Analysis of Bath Salt
Objective: To determine the quality of a bath salt by determining the mass of sodium carbonate and
the mass of sodium hydrogen carbonate in a sample of the bath salt.
Group size: Individual
Introduction
Bath salts are basically composed of the carbonate and hydrogencarbonate of sodium with
some water of crystallization. Poor quality bath salts may be just washing soda, Na2CO3.2H2O, in
which case they are rather hard on the skin. Good bath salt contains mainly sodium sesqui
carbonate, Na2CO3.NaHCO3.2H2O.
In this experiment a solution of bath salt is prepared. This bath salt is titrated with standardized
hydrochloric acid. Two different pH indicators are used. One for the determination of the end-point
for
(1)
Na2CO3 + HCl ----> NaHCO3 + HCl
Another pH indicator is for the determination of the end-point for
(2)
NaHCO3 + HCl ---> NaCl + H2O + CO2
Calculation of number of mole of sodium carbonate and sodium hydrogen carbonate is thus
possible.
-------------------------------------------------------------------------------------------------------------------Theory:
(The following questions should be useful to you.)
1. Which pH indicator is suitable to determine the first end-point? Which pH indicator is suitable to
determine the second end-point?
2. Let m = mass of the bath salt sample
C = concentration of HCl in mol dm-3
V1 = titre for the first end point
V2 = titre for the second end point
MC = formula mass of sodium carbonate
MH = formula mass of sodium hydrogencarbonate
If m (g) of bath salt is dissolved in water and the solution is made up to 250 cm3 and 25.0 cm3 of
this solution is pipetted into a flask and titrated with hydrochloric acid of molarity C, work out
formulae for calculating
(i) mass of sodium carbonate
(ii) mass of sodium hydrogencarbonate and
(iii) mass of crystallization of water and other impurity
-----------------------------------------------------------------------------------------------------------------------Chemicals:
300 cm3 of standardized hydrochloric acid (0.10 M - 0.13 M)
Screened methyl, bath salts, orange indicator solution, mixed indicator of thymol blue and cresol
red.
Additional Materials:
50 cm3 burette, 25 cm3 pipette, pipette filler, stand and clamp, balance, readable 0.01 g
The pH indicators
The first end point:
Phenolphthalein has the pH range 8.3 to 10.5. It may be employed to detect the first
equivalence point. However, it is not very accurate as it starts to lose colour too early. A mixed
indicator of thymol blue (8 to 9.6) and cresol red (7.2 to 8.7) is much better. This is blue at 8.4,
violet at 8.3 and rose at 8.2. One takes the end point at the change violet to blue.
The second end point:
Va03_bathSalt /p. 1
Methyl orange changes its colour over the pH range 3 to 4.5. Hence it is good to be employed to
detect the second equivalence point. Screened methyl orange indicator gives a better result. A
sample of bath salt is accurately weighed and dissolved to make a known volume of solution in this
practical. It is then titrated with standardized hydrochloric acid. If the mixed indicator is used, the
end point is obtained in which all carbonate ions are reacted to form hydrogencarbonate ions while
the original hydrogencarbonate ions are not affected. Hence the mass and thus percentage of
sodium carbonate in the sample can be calculated.
Procedure
1.
Weigh accurately 2.0 - 2.5 g of the bath salts by using a weighing bottle (accuracy: 0.01 g).
2.
Transfer most of the salt into a clean beaker and reweigh the bottle.
3.
Add de-ionized water to dissolve the salt and make the solution up to 250 cm3 by using a
volumetric flask.
4.
Rinse and fill a burette with the standardized hydrochloric acid.
5.
6.
7.
8.
9.
10.
Pipette 25 cm3 of the bath salt solution into a clean conical flask. Add 4 drops of a mixed
indicator of thymol blue and cresol red to the flask. Titrate the solution against the acid.
As the colour turns yellow, you obtain the first end-point (V1).
Repeat step 5 to get two concordant data. Fill your data into Table A.
Repeat step 5 but add 4 drops of screened methyl orange (a mixture of bromocresol green
and methyl orange) to the flask instead of the mixed indicator of thymol blue and cresol.
Titrate it against the standardized hydrochloric acid.
As the colour turns blue-green, you obtain the second end-point (V2).
Repeat step 7 - 8 at to obtain two sets concordant results (max. 4 runs). Enter your data into
Table B.
Data and Results:
Brand of the bath salt:
Mass of weighing bottle + bath salt
Mass of weighing bottle
Mass of bath salt
Concentration of standardized hydrochloric acid
Colour change of the first end point:
Colour change of the second end point:
Table A:
Run
Trial
1
Final burette reading/cm3
Initial burette reading/cm3
Volume of HCl used/cm3
Average titrant:
Table B:
Run
Final burette reading/cm3
Initial burette reading/cm3
Volume of HCl used/cm3
Average titrant:
Trial
1
=
=
=
=
2
3
2
3
Calculation and Conclusion:
Calculate percentage of sodium carbonate and sodium hydrogencarboante, by mass, of the sample.
Discussion
Va03_bathSalt /p. 2
1.
Estimate percentage error for your results.
2.
Is the sample of bath salt a good one? Give reasons for your answer.
3.
Another method for titrating bath salt is the use of a 'double indicator', that is, the use of
phenolphthalein and methyl orange to indicate the first and the second equivalent points,
respectively. Methyl orange is added to the titrating mixture after the first end point. Discuss
the advantage(s) of this method and the method employed in this practical.
Hand in your report to Mr. Lam before 4:30 p.m.
Va03_bathSalt /p. 3