Strong Acid-Stong Base Titration – GLX
Introduction:
One method a chemist can use to investigate acid-base reactions is a titration. A pH
titration is performed by adding small, accurate amounts of standard base or acid ("standard"
simply means that the concentration is known) to an acid or base of unknown concentration. The
pH is recorded methodically and is plotted vs. the volume of base added to the acid solution.
The result of this plot is an "S" shaped curve. The inflection point of this curve (middle of the
"S") is indicative of the endpoint. This point occurs when the acid and base in solution are
stoichiometrically equivalent. An endpoint can be very useful in determining the concentration
of an acid or base.
Purpose:
To titrate a dilute HCl solution with a dilute NaOH solution and to plot a titration
curve to determine the endpoint and find the concentration of an unknown acid
solution.
Objectives:
After completing this experiment the student will be able to:
a.
Set up a pH meter for titration.
b.
Titrate a strong acid with a strong base standard.
c.
Plot titration data.
d.
Determine the endpoint of the titration.
e.
Determine the concentration of an unknown acid.
f.
Describe how a pH meter can be used to determine the concentration
of an unknown acid.
Equipment/Materials:
Van Provides:
dilute solution of HCl (unknown conc.)
(labeled Unk A and Unk B)
0.10 M NaOH (standard solution)
magnetic stirrer and stirring bar
50 mL buret
buret clamp
Kimwipes
DI water and wash bottles
Ring stands
GLX with power adaptors
Teacher Provides:
250 mL beaker for titration
2 100 mL beakers (one for wash and
one for procuring base std.)
25, 50 or 100 mL graduated cylinder
(for measuring acid)
Safety Considerations:
Always wear apron and goggles in the lab.
Avoid contact of solutions with skin and clothing. If there is contact, wash with water
immediately.
Acids and bases can cause instantaneous, permanent eye damage.
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Procedure:
1. Plug in GLX with the power adaptor. Attach the pH sensor in Channel 1. Do not remove
the buffer solution yet.
2. Turn on the GLX using the
.
3. At the Main Menu Screen, select Data Files by highlighting this box and pushing
4. Use the arrow keys to move up
and right
5. Arrow down
to Titration in the lab list.
6. Once Titration is highlighted, press
7. Press the Home Key
8. Press
.
to highlight Flash.
to Open this file.
to return to the Main Menu Screen.
to select Table. You should see a table with a pH column and a volume (mL)
column. If this is shown, press
to return again to the Main Menu Screen.
Procedure (titration):
1. Obtain a beaker for wash water and a wash bottle containing distilled (or deionized) water.
2. Always rinse the electrode with distilled water into a beaker labeled as "Wash Beaker"
(NOT into a container of buffer) between solutions.
3. Measure 25.00 mL of unknown HCl into a 250 or 400 mL beaker and add enough distilled
or deionized water so that the liquid is 1 to 1-1/2 inches deep. (Use the graduated cylinder
for the acid measurement.)
4. Place the beaker on the magnetic stirrer and add a stirring bar.
5. Rinse a 50 mL buret three times with about 5 mL of NaOH standard solution.
6. Fill the buret past the 0.00 mL mark with the NaOH standard solution and deliver the excess
into a waste beaker while ridding the buret tip of air bubbles. Adjust the NaOH standard
solution to the 0.00 mark of the buret.
7. Remove the buffer solution container. (Place it in a 50 mL beaker so that it will not spill.)
Keep the ‘O’ ring and buffer lid on the electrode. Slide them up on the electrode so that they
will not be in the way during titration.
8. Rinse the buffer off of the electrode with distilled water into the waste container and blot.
Immerse the electrode in the HCl solution and turn on the stirring bar at a very slow rate (2
or 3). Higher speeds may cause the stirring bar to hit the electrode and damage it. Keep the
electrode in the solution but above the stirring bar. (NOTE: Electrodes are very expensive
so please be careful.)
9. On the GLX, use arrow keys to move right to highlight the Meter box.
10.A pH value should be visible. Record this initial pH on the data table provided with your lab.
The NaOH volume will be 0 mL at this point.
11. Begin adding the NaOH slowly, stopping at frequent intervals to record the volume used and
the pH. Collect data at intervals of 0.2 pH units or 1 mL, whichever occurs first. Be
particularly alert as you approach pH 4 as you will find the pH rising quite fast at this point .
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12. Continue the titration until the titrant in the buret reaches 50.0 mL or a pH of 12 is
maintained (whichever occurs first).
13. Repeat as instructed by your teacher.
Procedure (Clean-up):
1. Rinse off the pH electrode and replace it in the buffer solution.
2. Wash the buret completely (including the tip) with deionized water at least three times.
Leave the buret open and inverted in the buret clamp so that it will drain optimally.
3. Remove the magnetic stir bar from the titrated solution using the retriever bar. Rinse off
with deionized water and dry.
4. Wash all used glassware and rinse with deionized water.
5. Wash your hands well before leaving the lab.
Procedure (graph construction)
On the GLX be sure that you are on the Main Menu Screen by pressing
1.
Press
to select Table.
2.
Press
again to Edit Cell. Type in the initial pH, then press
.
. The Edit box
should move to Volume. Type 0 for the initial volume, then press
.
3.
Continue typing in the data points from pH and Volume from your Data Table. NOTE: If
you have a large number of data points, you may want to leave off several points in the
beginning where the pH change was minimal Be sure to press
4.
When you have finished entering all your data, press
Screen.
after the final volume.
to return to the Main Menu
5.
Select
Graph, then Autoscale and you should see a graph of your data.
6.
To print this graph plug USB cable into USB port on GLX. Do not go to print until after
you have plugged the GLX to it. Press F4
Graphs, Print, Print again.
7.
Determine the endpoint of the titration (inflection point in the graph).
8.
Determine the molarity of the acid solution. (Average all results if more than one
titration is performed.)
Calculations:
1.
2.
3.
4.
5.
6.
From the graph find the volume of base required to reach the endpoint.
Find the moles of base. [vol. of base (L) x conc. base (mol/L) = moles]
Write and balance the equation for the reaction of HCl with NaOH.
Calculate the moles of acid required to react with the calculated moles of NaOH.
Determine the concentration of acid in moles/L.
Repeat for all trials and average the results.
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Questions:
Explain how each of the following would affect your results.
1.
The buret is not rinsed with NaOH prior to filling.
2.
The buret tip is not filled at the start of the titration.
3.
The volume of unknown acid used is 30 mL instead of 25 mL.
4.
The buret is not read at eye level. (Explain.)
5.
The base is added too rapidly in the region of rapid pH change.
6.
An excessive amount of distilled water is added to the acid in Step 7.
7.
The original volume of base is not at 0.00 mL and a student assumes it is.
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pH TITRATION CURVE
Data Table
Name(s): ____________________________________
Date: _______________
______________________________________
Period: ______________
Identity of Unknown Acid: (circle one)
Buret
Reading
0.00 mL
pH
Unknown A
Buret
Reading
PH
Unknown B
Buret
Reading
pH
Use either the attached graph paper or the graphing calculators to construct your titration curve.
Perform calculations either on the back of this sheet or on a separate sheet, depending on your
instructor’s instructions.
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