Chemistry Lab Report #2
Titration
08. 04. 2012
Zeynep GÜNLER
Merve Kasımoğulları
Nurefşan Efeoğlu
Purpose
The aim of this experiment was an introduction to the pH indicator -litmus paper-. Also, the
purpose of the titration experiment was to understand how to use volumetric concentration of
acids and bases. Which colour does the litmus paper get when a base or an acid is dropped on
it?
Hypothesis
Blue litmus paper turns red under acidic conditions and red litmus paper turns blue under
basic (alkaline) conditions.
Experiment: Titrating Sodium Carbonate with Hydrochloric Acid
Introduction
Sodium carbonate reacts with dilute hydrochloric acid in two stages.
STAGE 1
Na2CO3
+
HCl
NaCl
+
NaHCO3
STAGE 2
NaHCO3
+
HCl
NaCl
+
CO2
+ H2O
+
2HCl
2NaCl
+
CO2
+ H2O
The overall reaction is
Na2CO3
The end points for the two stages can be found using suitable indicators. The reaction can
also be followed using a pH meter.
Indicator Method
1)
Use a graduated pipette and safety filler to put 20.0cm3 of approximately 0.1M
sodium carbonate solution into a small conical flask.
2)
Add 4 drops of phenolphthalein indicator. The solution should be purple-pink.
3)
Titrate with 0.2M HCl from a burette until the last traces of pale pink colour have
disappeared. Note down the volume of acid used.
4)
Add 4 drops of methyl orange. The solution should be a yellow colour.
5)
Continue to add acid from the burette until the solution turns orange-red. Note down
the final volume of acid used.
6)
Repeat steps 1) to 5) to get an accurate measurement of the two end points.
pH Method
1)
Prepare a piece of graph paper for a pH titration. The y-axis is for pH and the x-axis is
for the volume of 0.2M HCl added (go up to 25cm3). Alternatively prepare a table to record
your results.
2)
Use a graduated pipette and safety filler to put 20.0cm3 of the 0.1M sodium carbonate
solution into a small 100cm3 beaker.
3)
Check your pH meter against the standard buffer solution, then rinse it well in distilled
water.
4)
Take the pH of the sodium carbonate solution and plot it on your graph or record it in
your table.
5)
Add 1.0cm3 of the 0.2M HCl from the burette. Mix the solution well with the pH
meter and record or plot the reading.
6)
Continue to add the acid from the burette 1.0cm3 at a time until you approach the
Stage 1 end point (given by your indicator titration). Now add the acid 0.2cm3 at a time until
you have passed the end point. After each addition of acid, mix the solution well with the pH
meter and record or plot the reading.
7)
Once you have passed the first stage, go back to adding 1.0cm3 of acid at a time until
you approach the second end point. Add the acid 0.2cm3 at a time until you have passed the
second end point. Add 1.0cm3 of acid at a time until you have added a total of 25cm3.
8)
Rinse the pH meter, switch it off and replace it in its receptacle. Wash out the burette
with distilled water and replace it in the rack upside-down with the tap open. Empty and clear
away all the other apparatus.
TITRATING SODIUM CARBONATE WITH HYDROCHLORIC ACID
Indicator Titration
VOLUME OF 0.2M HCl ADDED (cm3)
TITRATION Stage 1
(phenolphthalein)
Stage 2
(methyl orange)
1
9.50
18.90
2
9.40
18.80
Average
9.45
18.85
COMMENTS ON THE EXPERIMENT
This is a good way of showing a two-stage reaction and of emphasising the need for an
indicator to change colour at the right pH for an end-point. Using the half “neutralisation”
volume of HCl from the Stage 1 end point (= 4.7cm3 in this case), the pH value is 10.1 (Data
Book value for pKa is 10.3).
Safety & Equipment and Materials
The students are using alkali and should wear goggles.
0.1M Na2CO3(aq) 50cm3 each
0.1M HCl(aq) each 50cm3 each
Phenolphthalein and Methyl Orange 4 bottles
A pH meter 1 per pair
PH 7 buffer
20cm3 safety pipette + filler
White tile
http://www.chemguide.co.uk/physical/acidbaseeqia/phcurves.html
DETERMINING THE MOLAR CONCENTRATION OF VINEGAR BY TITRATION
Objective: Determine the concentration of acetic acid in a vinegar sample
Expressing solution concentration.
Using volumetric glassware: pipet and burette.
Performing a titrimetric analysis.
Background
In a titration, the analyte (the substance whose concentration is unknown and sought in the
analysis) is reacted with a standard (a substance that reacts with the analyte but whose
concentration is known). The analysis uses just enough of the standard to react with all of the
analyte, thereby allowing the amount of analyte present to be determined. In this experiment,
acetic acid (CH3COOH) is the analyte and sodium hydroxide (NaOH) is the standard. The
reaction is:
CH3COOH (aq) + NaOH (aq) --> CH3COONa (aq) + H2O (l)
Titration: an analytical procedure involving a chemical reaction in which the quantity of at
least one reactant is determined volumetrically.
Standard solution: a solution in which the concentration of a solute is precisely known.
Usually it is the volume of the standard solution required to react with a given quantity of an
analyte that is precisely determined during a titration.
Titration endpoint: The quantity of reactant in the standard solution added during the titration
is stoichiometrically equivalent to the quantity of reactant in the analyte at the titration
endpoint.
Expressing Solution Concentration
Moles (solute)
Molarity = ------------------ = M (mole/L)
Litre (solution)
0.493 M NaOH means 0.493 mole NaOH/L moles
Moles = ------ x L
Moles = M x V
In a titration procedure, 40.57 mL of 0.493 M NaOH solution was used. How many mols
NaOH did this volume of NaOH solution contain?
Moles = M x V 0.493 moles NaOH
Moles = ----------------------- x 0.04057 L
Moles = 0.0200 moles NaOH
Volumetric glassware: burette and pipet
Burette reading = 0.76 mL
Determining the Volume of Titrant Delivered in a Titration Final burette reading: 49.37 mL
Initial burette reading: 0.74 mL
Volume delivered: 48.63 mL
Sample Calculation
Calculating the concentration (M) of CH3COOH in commercial vinegar.
From the balanced chemical equation:
Moles CH3COOH (vinegar) = moles NaOH (titrant)
Moles NaOH = M NaOH x V NaOH, L (from titration)
0.493 moles NaOH
Moles Na OH = ------------------------ x 0.04863 L
L
Moles NaOH = 0.240 = moles CH3COOH (vinegar)
Moles CH3COOH (vinegar)
M CH3COOH (vinegar) = ----------------------------Volume (vinegar)
0.0240 moles
M CH3COOH (vinegar) = ---------------- = 0.96 M
0.0250 L
http://web.lemoyne.edu/~giunta/chm151L/vinegar.html
DETERMINATION OF AMMONIA BY TITRATION WITH SULPHURIC ACID
Background
Ammonia is found as ammonium hydroxide in solution and reacts with sulphuric acid
according to the following equation.
Sulphuric acid + ammonium hydroxide water + ammonium sulphate
H2SO4 + 2NH4 OH2 H2O + (NH4)2SO4
Use previously standardised 0.05M sulphuric acid.
All volumetric glassware is used in this procedure
Method
Using a bulb pipette, quantitatively transfer a 10 mL aliquot of the cleaning solution sample to
a 100 mL volumetric flask. Dilute to the mark with distilled water, replace the stopper and
mix the contents by inverting and swirling the flask a number of times.
Using a bulb pipette, quantitatively transfer a 25 mL aliquot of the diluted cleaning solution
sample to a 250 mL Erlenmeyer flask. Wash down the inside of the flask with about 50 mL of
distilled water delivered from a wash bottle.
Add 2 drops of methyl red indicator and mix well.
Carefully fill a 50 mL burette with standard 0.05 M sulphuric acid solution, M (sulphuric).
Note that this solution must be freshly standardised before use.
Titrate the diluted cleaning solution to the first sign of a permanent pink end point (use a
white tile beneath the Erlenmeyer flask during the titration). Record the titre to the nearest
0.01 mL.
Repeat the titration (steps 2 to 5) until three titres are obtained that agree within 0.10 mL.
Average these readings T (sulphuric).
Calculate the concentration of ammonia hydroxide available in the undiluted cleaner sample
C (ammonia), % w/v to 2 decimal places:
2 x M (sulphuric) x T (sulphuric) x Mr (ammonia)
C (ammonia) =
-------------------------------------------------------------------------------25
Where Mr (ammonia) is the formula mass of ammonium hydroxide
http://toolboxes.flexiblelearning.net.au/demosites/series4/412/laboratory/methodsman/MMSO
P-DetmntnAmmoTitSulfAcid.htm
Results
Blue litmus paper turns red under acidic conditions and red litmus paper turns blue under
basic (alkaline) conditions.
Conclusion
We can find out the volume of acid needed to neutralise an alkaline.