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Standardization of hydrochloric acid
BACHELOR OF SCIENCE TECHNOLOGY CHEMISTRTY (Kyambogo University)
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KYAMBOGO
UNIVERSITY
FACULTY OF SCIENCE
Ordinary diploma in Science Technology Chemistry
CHEMISTRY PRACTICAL REPORT 1 / YEAR I SEMESTER II
Saturday, February 10, 2018
MAKASI GEORGE
17/U/16852/STC/PD
Sign
……………………………..
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STANDARDIZATION OF HYDROCHLORIC ACID
Aim
The purpose of this experiment is to standardize a solution of hydrochloric acid using two
primary standards; sodium carbonate and borax. And also to introduce one of the methods used
to prepare standard solutions.
Introduction
This experiment involves the standardization of a solution of hydrochloric acid using two
primary
standards,
sodium
carbonate
and
borax
(sodium
tetraborate
decahydrate,
Na2B4O7.10H2O).
Standardization of an unknown solution involves reacting the solution with another solution
whose concentration is already known very accurately. For this experiment, a known quantity of
hydrochloric acid called the aliquot is measured and neutralized with say; sodium carbonate
whose concentration is known very accurately.
Adding an indicator such as methyl orange indicator provides a visual indication (a color change)
when the equivalence point is reached i.e when just enough of the standard solution has been
added to the unknown solution to neutralize it exactly. By determining how much of the salt
solution is required to neutralize the hydrochloric acid, we can calculate a very accurate value for
the concentration of the hydrochloric acid.
The results obtained will be used to determine the suitability of the two salts (sodium carbonate
and sodium tetraborate decahydrate) as primary standards.
Theoretical Background
An acid –base titration depends on the reaction between an acid and a base to produce a salt. The
reaction is normally a quantitative one and is carried out in aqueous solution. An acid can be
defined as a substance which, when in aqueous solution, has a tendency to donate a hydrogen ion
(proton) to a water molecule.
HA(aq) + H2O(l) → A+(aq) + H3O+(aq)
Where HA is the acid
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HA and A- from a conjugate pair in which A - is the base. Similarly H2O and H3O+ are also a
conjugate pair in which H3O+ is the acid.
A base in this case, is defined as a substance which, when in aqueous solution, has a tendency to
accept a proton from water.
B(aq) + H2O(l) → BH+(aq) + OH-(aq)
Therefore water acts both as an acid and as a base. Salts derived from a combination of strong
acids-weak acid also function as bases due to hydrolysis, which results in the release of OH ions.
Examples are sodium carbonate and borax.
The reaction between an acid and a base is a neutralization reaction. Which takes place between
H3O+ and OH- ions
H3O+ (aq)
+ OH-(aq) → 2H2O(l)
The endpoint which is the point at which all the acid or base has been completely neutralized by
an equivalent amount of base or acid can be marked by a suitable visual indicator such as methyl
orange or phenolphthalein.The choice of the indicator is dictated by the type of acid and base
used in the titration.
A primary standard solution is defined as a solution of known concentration (C) which does not
change with time and the concentration is defined by the equation
C = n/v
Where n = amount (in moles) of solute contained a volume V (expressed in dm3) of solution.
In this experiment
We draw from the above principles, the experiment uses two primary standards i.e. Sodium
carbonate and sodium tetraborate decahydrate in an acid-base titration with methyl orange as
indicator.
For instance, Borax (Na2B4O7 .10H2O) is used as a primary standard since it does not decompose
under normal storage, it is readily obtained in a very pure form (99.999% purity), it reacts with a
known stoichiometry and can be weighed and used directly.
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In water, the salt dissociates:
Na2B4O7.10H2O → 2Na+ + B4O72- + 10H2O
and the anion is hydrolyzed:
B4O72- + 7H2O → 4H3BO3 + 2OH-
The liberated hydroxide ions above, can be titrated with a strong acid and at the point of their
neutralization, the solution in the flask will contain the very weak acid H 3BO3, which will
dissociate according to:
H3BO3 + H2O → H2BO3- + H3O+. Ka = 5.8 x 10-10.
Thus at the neutralization point (or equivalence point: the point when all the liberated OH- have
been converted to H2O) the solution will be slightly acidic (pH ~ 4.8; the actual pH depending on
volumes and amounts used). The indicator used to identify the strong base-strong acid
equivalence point must therefore change color over the pH range 4.8 ± 1 and as such methylorange indicator is used. Methyl indicator is an acid –base indicator and its structure is as shown
below.
Now using both sodium carbonate and borax as primary standards;
Reaction equation between hydrochloric acid and sodium carbonate
Na2B4O7 ⋅ 10 H2O (aq) + 2 HCl(aq) → 2 NaCl(aq) + 4 H3BO3(aq) + 5H2O(l)
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Reaction equation between hydrochloric acid and borax
Na2CO3(aq) + 2 HCl(aq) → 2 NaCl(aq) + H2O(l) + CO2(g)
Since water is sometimes contaminated with acidic or basic impurities, the indicator blank on the
water is obtained. It is then subtracted on the obtained titres in order to obtain corrected values of
the volumes obtained.
Obtaining the actual volume of (sodium carbonate or borax) required to completely neutralize
the acid, averaging it and employing the values in a set of calculations and analysis, can enable
us obtain with precision the concentration of Hydrochloric acid and standardize it.
Requirements
Conical flasks
Measuring cylinders 10ml and 100ml
Burette
10cm3 pipette
Retort stand
Distilled water
0.1 M hydrochloric acid
250cm3 volumetric flask
Methyl orange indicator
Pasteur pipette
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Experimental setup
0.1M hydrochloric acid in a burette
Retort stand/ burette stand
250ml conical flask
Sodium carbonate/ borax with methyl orange indicator
Procedure/Method
About 1.3249g anhydrous sodium carbonate was weighed into a clean 250cm 3 volumetric flask
and 100cm3 of distilled water added, the flask was stoppered and shaken until all the solid had
dissolved. Distilled water was then added to the mark. The flask was labelled clearly.
4.7673g of pure borax was weighed into another clean 250cm3 volumetric flask and 100cm3 of
distilled water added, the flask was stoppered and shaken until all the solid had dissolved.
Distilled water was then added to the mark. The flask was labelled clearly.
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Titration with sodium carbonate
10cm3 of the stock carbonate solution was pipetted into a clean 250cm3 conical flask, and 3 drops
of the methyl –orange indicator added. The acid solution was transferred into the burette and the
meniscus was adjusted to a suitable starting point.
A piece of white paper was then placed under the flask. While swirling, the acid solution was run
into the flask until the indicator showed a distinct orange color. The procedures were then
repeated and values read recorded in a suitable table.
Titration with Borax
10cm3 of the sodium tetraborate decahydrate solution was pipetted into a clean 250cm 3 conical
flask, and 3 drops of the methyl –orange indicator added. The acid solution was transferred into
the burette and the meniscus was adjusted to a suitable starting point.
A piece of white paper was then placed under the flask. While swirling, the acid solution was run
into the flask until the indicator showed a distinct orange color. The procedures were then
repeated and values read recorded in a suitable table.
Indicator blank
The indicator blank is found by measuring out accurately a volume of which equals the total
volume of the solution in the flask at the end point. To this volume the same number of drops of
the indicator as used in the titration was added. The water was then titrated against the same acid.
The acid solution was run into the flask until the indicator showed a color change, the volume
read and recorded in a suitable table.
Results
a. Hydrochloric acid and Sodium Carbonate
Molarity of sodium carbonate solution = 0.05M
Capacity of the pipette used = 10.0cm3
Number of drops of methyl-orange = 3 drops
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Run
1
2
3
4
Final burette reading /cm3
4.40
8.80
13.60
18.60
Initial burette reading /cm3
0.00
4.40
9.00
13.60
Volume used/cm3
4.40
4.40
4.60
4.40
Indicator Blank/cm3
0.20
0.20
0.20
0.20
1
2
3
4
Final burette reading /cm3
30.70
41.50
25.80
36.50
Initial burette reading /cm3
20.00
30.70
15.00
25.80
Volume used/cm3
10.70
10.80
10.80
10.70
Indicator Blank/cm3
0.20
0.20
0.20
0.20
Corrected average titre = 4.27cm3
b. Hydrochloric acid and Borax
Molarity of borax solution = 0.05M
Capacity of the pipette used = 10.0cm3
Number of drops of methyl-orange = 3 drops
Run
Corrected average titre = 10.57cm3
Analysis, Calculations and Evaluation of data
a. Using Sodium Carbonate as primary standard;
The balanced equation for the reaction is;
Na2CO3(aq) + 2 HCl(aq) → 2 NaCl(aq) + H2O(l) + CO2(g) --------------------------------(i)
The average titre can be obtained:
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Total volumes used / number of runs; (4.40 + 4.40 + 4.60) / 3 = 4.46cm 3; Indicator blank (0.2
+ 0.2 + 0.2) / 3 = 0.2cm 3. Thus the corrected averaged value = volumes used – indicator
blank; (4.47 – 0.2) = 4.27cm3
To obtain the concentration of Hydrochloric acid, we need to first obtain the number of moles
of sodium carbonate that reacted;
Please note that a 0.05M sodium carbonate solution is used in the first experiment, this
means every 1000cm3 of water contain 0.05 moles of the salt. As such 1cm 3 of water
contains;
(0.05 / 1000) x 1moles of salt; hence a volume of 10cm 3 of sodium carbonate will contain
(0.05 / 1000) x 10 moles of sodium carbonate; 0.0005 moles of sodium carbonate.
Equation (i) shows that 1 mole of sodium carbonate reacts with 2 mole of hydrochloric acid;
as such the mole ratio is 1:2, base to acid respectively. To obtain the number of moles of acid
in 4.27cm3 simply multiply number of moles of base x 2; (0.0005 x 2) = 0.001 moles of
hydrochloric acid reacted.
The concentration of acid is obtained as follows.
If 4.27cm3 contains 0.001 moles of hydrochloric acid; then in 1000cm3 will shall have (0.001/
4.27) x 1000 = 0.23M. The concentration of Hydrochloric acid while using sodium carbonate
as primary standard is exactly 0.23M.
b. Using Borax as primary standard;
The balanced equation for the reaction is;
Na2B4O7 ⋅ 10 H2O (aq) + 2 HCl(aq) → 2 NaCl(aq) + 4 H3BO3(aq) + 5H2O(l) -------- (ii)
The average titre can be obtained:
Total volumes used / number of runs; (10.70 + 10.80 + 10.80) / 3 = 10.76cm3; Indicator blank
(0.2 + 0.2 + 0.2) / 3 = 0.20cm 3. Thus the corrected averaged value = volumes used – indicator
blank; (10.76 – 0.20) = 10.57cm3
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To obtain the concentration of Hydrochloric acid, we need to first obtain the number of moles
of sodium tetraborate decahydrate that reacted;
Please note that a 0.05M sodium tetraborate decahydrate solution is used in the second
experiment, this means for every 1000cm3 of water contain 0.05 moles of the salt. As such
1cm3 of water contains; (0.05 / 1000) X 1moles of salt; hence a volume of 10.56cm 3 of
sodium tetraborate decahydrate will contain (0.05 / 1000) X 10 moles of sodium carbonate;
0.0005moles of sodium tetraborate decahydrate.
Equation (ii) shows that 1 mole of sodium tetraborate decahydrate reacts with 2 moles of
hydrochloric acid; as such the mole ratio is 1:2, base to acid respectively. To obtain the
number of moles of acid in 10.56cm3 simply multiply number of moles of base x 2; (0.0005 x
2) = 0.001 moles of hydrochloric acid reacted.
Obtaining the concentration of acid is obtained as follows.
If 10.57cm3 contains 0.001moles of hydrochloric acid; then in 1000cm3 will shall have (0.001
/ 10.57) x 1000 = 0.094M. The concentration of Hydrochloric acid while using sodium
tetraborate decahydrate as primary standard is exactly 0.09M
Discussion of Results
It was found that the concentration of the given hydrochloric acid was 0.09M while using
sodium tetraborate decahydrate as primary standard and 0.23M while using sodium carbonate as
primary standard. This result is acceptable and reliable especially for sodium tetraborate
decahydrate because no great error was made in the experiment and since 0.1M hydrochloric
acid is provided for this experiment. Sodium carbonate shows a relatively big difference in the
concentration of the acid as compared to sodium tetraborate decahydrate.
However these values are what is expected for this experiment, since one of the goals of the
experiment was to determine the suitability of a salt over another in the standardization of
hydrochloric acid i.e. one salt giving a much more precise value to the other. Obtaining a 0.09 M
concentration with sodium tetraborate decahydrate and 0.23M while using sodium in this
experiment demonstrates the differences in the salts as primary standards. It can be observed that
sodium carbonate is a not entirely suitable for standardization of the acid because there is a
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discrepancy in the values (0.23M and 0.1M) while sodium tetraborate decahydrate gives a more
precise and accurate value. One of the reasons for this could be that primary standards is are
expected to be pure reagents which do not gain any weight by absorbing moisture. Hence any
error during weight measurement is prevented.
Borax is a hydrate and it gives consistent results. The .10H2O hydration number will remain
constant and will not fluctuate to 11 or 99 within normal temperature and humidity ranges. This
means that per mass the same amount of sodium ions are dispensed every time and gives the
same results time after time and experiment after experiment even with different batches of
powder. Also the tetraborate conjugate base is very weak, which is ideal for a titration agent as
well. This demonstrates that borax is a much more suitable primary standard for the
standardization of Hydrochloric acid.
Because there is a discrepancy in values particularly with sodium carbonate, there must be some
errors.
Error Analysis
Sodium carbonate is hygroscopic hence it could have absorbed water molecules from the
atmosphere during weighing thereby increasing its weight.
The greatest error in the experiment was that when determining the end-point of titration, it was
easy to get over the end-point with a few drops. Although the color was still orange, the amount
of hydrochloric acid used was different. To improve the experiment and reduce this error, it was
suggested to repeat the titration for some more times in order to help adjusting the average
volume of hydrochloric acid to a more accurate number or to add in hydrochloric acid drop by
drop when it was near to the end-point. Thus even a few over added drops can be avoided.
Markings on a burette can be easily misread. One way to misread the volume is by looking at the
measurement on an angle. From above, it can seem like the volume is lower, while from below,
the apparent volume looks higher. This could have generated an error in readings and thus
recording of wrong values, this error can be progressively carried on to the analysis and thus
yield wrong results.
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Errors in concentrations directly affect the measurement accuracy. Errors include using the
wrong concentration to begin with, which can occur from chemical decomposition or
evaporation of fluids. The solutions may have been prepared incorrectly or contaminants could
have been introduced into the solution.
Conclusion
The Concentration of hydrochloric acid was found to be 0.23M and 0.09M for sodium carbonate
and borax respectively, which do not differ greatly to the expected concentration. The
concentration of the analyte is also found to be dependent on the purity of the standard solution
used. The results obtained are acceptable and thus the objective of the experiment fulfilled.
Recommendations
Accurate measurements are essential when it comes to a standardization experiment, as such all
equipment such as the electric balance and all volumetric glassware should be calibrated for
correct measurements, to assess and reduce on random errors.
Ensure the primary standards used are in their most pure states especially with regard to sodium
carbonate. Sodium carbonate will rapidly absorb water from the air. If not actively weighing it,
this primary standard should be stored in a desiccator.
References
http://www.foragetesting.org/lab_procedure/sectionB/3/part3.1a.1.htm
https://www.pharmaguideline.com/2010/09/preparation-and-standardization-of-1m.html
https://www.quora.com/What-is-the-standardization-of-hydrochloric-acid-with-sodiumcarbonate
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