Chemistry 102 ______________________________________________________________________________ EXPERIMENT 9 STANDARDIZATION OF A SODIUM HYDROXIDE SOLUTION BY pH TITRATION 2015 www/proffenyes.com 1 PURPOSE: 1. To standardize a solution of 0.10 M sodium hydroxide (NaOH) solution with potassium hydrogen phthalate (KHC 8 H 4 O 4 ), by pH titration. 2. To determine the Acid Ionization Constant (Ka) for the Hydrogen Phthalate Ion (HP - ) PRINCIPLES: Most shelf reagents, such as 0.10 M sodium hydroxide, could not be kept at a previously determined accurate concentration, because the concentration of these reagents change in time, due to exposure to the environment. In order to obtain a reagent of accurately known concentration, expressed to four significant figures, the concentration of the reagent must be determined by reacting it with a known amount of another reagent. The entire procedure by which the molarity of a solution of one substance (NaOH) is obtained from an accurately known amount of another substance, commonly referred to as a primary standard, is called standardization. The preferred method commonly used for the standardization of NaOH is an Acid – Base titration with potassium hydrogen phthalate (KHC 8 H 4 O 4 , thereafter abbreviated as KHP) used as a primary standard. Potassium hydrogen phthalate (KHP) is a soluble salt, and is completely dissociated in aqueous solution. KHP(aq) K + (aq) + HP - (aq) Hydrogen phthalate ion The hydrogen phthalate ion, HP - is a weak acid and it undergoes partial ionization: HP - (aq) + H 2 O(l) H 3 O + (aq) + P 2- (aq) The addition of NaOH to this equilibrium system will cause the OH - ions to combine with the hydronium (H 3 O + ) ions to form water. OH - (aq) + H3O + (aq) H2O(l) The decrease in the concentration of hydronium (H 3 O + ) ions will cause the equilibrium system to shift to the right and, as a result more of the weak acid (hydrogen phthalate ion, HP - ) will ionize. Successive additions of NaOH will continually remove H 3 O + ions, shift the ionization equilibrium of the weak acid (hydrogen phthalate ion, HP - ) to the right and force the weak acid into complete ionization. The situation can be summarized in the equilibrium table below: Equation: HP - (aq) + H 2 O(l) H 3 O + (aq) + P 2- (aq) Stress: decreases Shift: New Equilibrium: decreased decreased decreased increased
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PURPOSE: PRINCIPLESproffenyes.weebly.com/uploads/2/5/2/3/25237319/... · 2019-11-28 · 1. Standardize a solution of 0.10 M sodium hydroxide (NaOH) solution, as performed in this
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STANDARDIZATION OF A SODIUM HYDROXIDE SOLUTION BY pH TITRATION
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Cap the vial and reweigh it on the analytical mass. Read and record this mass. The accurate mass of KHP transferred from the vial into the beaker is the difference
between the mass of the vial before transferring some of the solid sample into the
beaker (Mass of vial + KHP) and the mass of the vial after transferring the solid
sample into the beaker (Mass of vial – KHP)
ONLY THE CAPPED VIAL WILL BE PLACED ON THE
ANALYTICAL BALANCE, BUT NOT THE FLASK.
THE TWO MASSES OF THE VIAL (BEFORE AND AFTER
TRANSFER) MUST BE RECORDED IN FOUR SIGNIFANT
FIGURES.
THERE IS NO NEED TO RECORD THE MASS READING FROM
THE CENTIGRAM BALANCE.
2. Add about 100 mL of D.I. water to the beaker containing the KHP.
You may use the 100 mL mark on the beaker to estimate the volume of D.I. water
added. No need to use a graduated cylinder.
3. Place a stirring magnet in the beaker.
4. Place the beaker on a stirring plate and set the stirring plate so as to provide a gentle and
uniform mixing of the KHP solution. Mix the solution gently until the KHP is completely
dissolved. Stirring is important because in the next step the pH will tend to drift, unless a
completely homogeneous solution of KHP is achieved.
III. Preparation of the buret for the titration
1. Obtain a 50.0 mL buret and clean it thoroughly with deionized water.
2. Rinse the buret with three portions of about 5 mL of the sodium hydroxide solution,
coating the barrel each time before emptying out the solution.
3. Fill the buret with the sodium hydroxide solution a little above the “0” line.
4. Open the stopcock and drain the sodium hydroxide solution in order to completely fill the
tip of the buret and flush out any air bubbles caught in the tip.
5. Check the stopcock for leaks.
6. Set the level of the titrant (NaOH solution) at the 0.00 mL mark.
Record the initial buret reading (0.00 mL preferred). It is not absolutely necessary
to refill it to exactly 0.00 mL; however it is necessary to record exactly the
STANDARDIZATION OF A SODIUM HYDROXIDE SOLUTION BY pH TITRATION
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5. Recall that all buret readings should be read and recorded to the nearest 0.01 mL(two
decimals).
At the beginning of the titration, while you are adding 1 mL of NaOH, the pH will change very little (less than 0.3 pH units)
When the pH begins to increase by more than about 0.3 pH units, decrease the
portions of NaOH that you add to about 0.2 mL.
When the pH begins to increase again by less than about 0.3 pH units after the addition of NaOH, increase the portions of NaOH that you add to about 1 mL of
NaOH,
Steps throughout the
pH titration
Change in pH
(Δ pH)
Additions of
NaOH
Before the
Equivalence Point
Less than 0.3 units About 1mL
Around the
Equivalence point
More than 0.3 units About 0.2 mL
Passed the
Equivalence point
Less than 0.3 units About 1 mL
Continue the titration until the pH is about 11.5 – 12.0
6. Repeat steps 1 through 4 with four more samples of solid KHP.
You are required to perform at least five titrations in order to be able to evaluate
your experimental data and obtain an accurate value for the molarity of your
sodium hydroxide solution.
VI. Interpretation of experimental data.
1. Plot graphs of pH versus the volume of NaOH used for all your titrations
Please see sample graph on page 19.
Clearly label each graph.
Determine from each graph the volume of NaOH required to reach the equivalence point and indicate it on your graph (See Figure 1 on page 3)
Indicate on your graphs the volume of NaOH and the corresponding pH at the half-equivalence point (See Figure 1 on page 3)
2. Attach ALL your graphs to your Report Form.
Note:
If you chose to plot in Excel, it is possible to plot all five titration curves on
one single graph. Please see sample graph on page 19.
STANDARDIZATION OF A SODIUM HYDROXIDE SOLUTION BY pH TITRATION
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Guidelines for an accurate standardization 1. A proper standardization requires a minimum of three trials that agree within plus or
minus 0.5% of each other.
It is very unlikely that all five trials will satisfy this requirement.
A data evaluation process that follows this experiment will assist you in determining which trials should be kept and which trials should be rejected.
The procedure used for this data evaluation requires that more than half of the values obtained must be included in the calculation of the Mean Molarity.
This implies that you are not allowed to discard more than two molarities if you
have performed five titrations.
2. A quick check of the precision of your titrations (reproducibility of the titrations) is to
calculate the ratio of g KHP/mL NaOH at the estimated equivalence point to four
significant figures.
This ratio should vary only in the last significant figure. During the titration the estimated
equivalence point can be detected without a graph, by noticing a sudden and significant
increase in the pH value.
3. Under ideal circumstances, the standardization process should be repeated until it is
confirmed the g KHP/mL NaOH ratio varies only in the last significant figure.
Bibliography:
1. Nivaldo J. Tro, “Chemistry: A Molecular Approach”, Third Edition
2. R.A.D. Wentworth “Experiments in General Chemistry”, Sixth Edition
3. James M. Postma & all, “Chemistry in the Laboratory”, Seventh Edition