Calculator
Standardizing a Solution
of Sodium Hydroxide
6
It is often necessary to test a solution of unknown concentration with a solution of a known,
precise concentration. The process of determining the unknown’s concentration is called
standardization.
Solutions of sodium hydroxide are virtually impossible to prepare to a precise molar
concentration because the substance is hygroscopic. In fact, solid NaOH absorbs so much
moisture from the air that a measured sample of the compound is never 100% NaOH. On the
other hand, the acid salt potassium hydrogen phthalate, KHC8H4O4, can be measured out in
precise mass amounts. It reacts with NaOH in a simple 1:1 stoichiometric ratio, thus making it an
ideal substance to use to standardize a solution of NaOH.
OBJECTIVES
In this experiment, you will


Prepare an aqueous solution of sodium hydroxide to a target molar concentration.
Determine the concentration of your NaOH solution by titrating it with a solution of
potassium hydrogen phthalate, abbreviated KHP, with an exact molar concentration.
Figure 1
MATERIALS
LabQuest interface
Vernier pH Sensor
weighing dish (Dixie cup)
solid potassium hydrogen phthalate
solid sodium hydroxide
500 mLvolumetric flask
250 mL Erlenmeyer flask
balance (± 0.01 g)
50 mL buret
Parafilm
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magnetic stirrer
stirring bar
wash bottle with distilled water
1 or more funnels
ring stand
Electrode/utility clamp
2 - 250 mL beaker
100 mL graduated cylinder
2 scoops
buret clamp
6-1
Calculator 6
PRE-LAB EXERCISE
1. Calculate the mass of sodium hydroxide needed to prepare 500. mL of a 0.10 M solution.
2. Calculate the mass of KHP (KHC8H4O4) needed to react completely with 25 mL of a 0.10 M
NaOH solution. Consider the reaction equation to be as shown below.
HP– (aq) + OH– (aq) → H2O(l) + P2– (aq)
PROCEDURES Instructions in italics will be provided in a separate handout.
1. Obtain and wear goggles and apron.
2. Measure out approximately100 mL of distilled water into a 250 mL Erlenmeyer flask (use
the lines on the flask to measure).
3. Decide which partner will perform the tasks described in #3-5.
4. Measure out the mass of NaOH that is needed to prepare 500 mL of a 0.10 M solution and
add it to the flask of distilled water, breaking up the NaOH pieces if necessary and/or using a
funnel. Swirl the flask to dissolve the solid or add parafilm to seal the top, then shake.
CAUTION: Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing.
Add sufficient distilled water to create 500mL of 0.10M NaOH.
5. Measure out the mass of KHP that will completely neutralize 25 mL of 0.10 M NaOH
solution. Dissolve the KHP in about 100 mL of distilled water in a 250 mL beaker (use lines
on beaker to measure). Place the beaker of KHP solution on a magnetic stirrer and add a
stirring bar.
NOTE: Do NOT print the instructions that are in italics. Instead, print out the file “pH Sensor
Instructions LQ”, which you will use as a reference for all titrations.
6. Set up a ring stand, buret clamp, and buret to conduct a titration (see Figure 1). Rinse and
fill the buret with the NaOH solution, make sure there are no bubbles in the buret. Allow
titrant to flow into a waste beaker.
7. Rinse the pH sensor with distilled water using water bottle, allowing the waste water to go
into the waste beaker. Use a nickel as a cap to protect the buffer solution from spilling.
8. Use an electrode support to suspend the pH Sensor on a ring stand as shown in Figure 1.
Position the pH Sensor in the KHP solution and adjust its position so that it is not struck by
the stirring bar. Add sufficient water to make sure the pH sensor is immersed in the solution.
Gently stir the beaker of solution.
9. Connect a pH Sensor to Channel 1 of the LabQuest interface. Turn on the LabQuest. The
screen should show CH 1: pH.
10. Set up the data-collection mode.
a.
b.
c.
d.
6-2
To select MODE, tap the “Mode” box on the right side of the screen
Select EVENTS WITH ENTRY from the SELECT MODE menu.
Set NAME to Volume and UNITS to mL
Select OK to return to the Main screen.
Advanced Chemistry with Vernier
Standardizing a Solution of Sodium Hydroxide
11. Start the graph by pressing the play button
a. Before you have added any titrant, press “keep” and type in “0” as the buret volume in
mL. Press OK to save this as the first data pair for this experiment.
b. Add the next increment of titrant (enough to raise the pH about 0.15 units). When the pH
stabilizes, press “keep” and enter the current buret reading as precisely as possible. Save
this as the second data pair for this experiment.
c. Continue adding the titrant in increments that raise the pH by about 0.15 units and enter
the buret reading after each increment.
d. When the pH values begin increasing more rapidly (e.g. pH of about 3 for strong
acid/strong base or about 6 for weak acid/strong base), change to 1-3 drop increments.
Enter a new buret reading after each increment. As you reach the equivalence point, 1
drop increments will provide you with the most accurate data.
e. After a pH value of approximately 10 is reached, again add larger increments that raise
the pH by about 0.15 pH units, and enter the buret level after each increment.
f. Continue adding titrant until the pH value remains constant.
12. Press the stop button when you have finished collecting data.
13. Use your titration data to find the equivalence point, which is the largest increase in pH
upon the addition of a very small amount of NaOH solution. A good method of determining
the precise equivalence point of the titration is to take the second derivative of the pHvolume data, a plot of 2pH/vol2.
(Note: An alternate method is to transfer your data to Logger Pro first, then use Logger Pro
functions to derive the second derivative of the pH-volume data.)
a.
b.
c.
d.
Select the Data Table from the icons at the top of the screen.
Tap the word “Table” at the top of the screen and create a new calculated column.
Set the equation to 2nd Derivative. It should be all the way at the bottom of the list.
Analyze the data table. The point where it goes from positive to negative is the equivalent
point.
15. Attach your USB stick to the LabQuest and save your data to the USB. To do this press
“File”, then save and tap the USB icon to save your data there.
16. Dispose of the reaction mixture down the sink. Rinse the pH Sensor with distilled water in
preparation for a second titration.
17. Repeat the titration with a second KHP solution. Analyze the titration results in a manner
similar to your first trial and record the equivalence point.
18. Transfer the data to a computer using Logger Pro software. Print two (or three) copies of your
best graph of pH vs. volume of NaOH (one for you, one for your partner, one for your
instructor). Include an embedded graph of the 2nd derivative.
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6-3
Calculator 6
DATA TABLE
Trial
Equivalence point
(mL)
1
2
DATA ANALYSIS
6-4
1.
Calculate the number of moles of KHP used to neutralize the NaOH solution.
2.
Calculate the molar concentration of the NaOH solution that you prepared.
3.
Compare the actual molarity of your NaOH solution with your goal of 0.10 M.
Advanced Chemistry with Vernier