Notes on Procedure for Experiment No. 10B
Prepare 250 mL of 0.1 M HCl by diluting with distilled water 25 mL of 1 M HCl to 250 mL in a small
plastic bottle. Shake the solution until it is well mixed. Into each of three 200 mL tall-form beakers, weigh 100
to120 mg of Na2CO3. Add a stir bar. Obtain about 100 mL of boiled water in a clean 100 mL graduated
cylinder and add 20 to 25 mL to each flask containing Na2CO3. Set up your hot plate and set it on 4 to start
heating. Get two 50 mL burets and prepare them for titration, as was done in Experiment 10A. Rinse and fill
one of the burets with your ~0.1 M HCl. Add 1 or 2 drops of phenolphthalein solution to a beaker containing
Na2CO3 solution; read the initial volume of the HCl solution, and start the titration. When the initial pink color
of the phenolphthalein just disappears, you are a little more than half way to the end point. Add 1 to 2 drops of
bromcresol green indicator to the solution and continue the titration. You can quickly add about ¾ of the titrant
volume that was required to change the phenolphthalein from pink to colorless, but then more slowly approach
the bromcresol green end point, which is yellow. Unfortunately, the carbon dioxide that is produced during the
titration results in the formation of a H2CO3/HCO3- buffer. As a result, it is almost impossible to titrate to an
indicator end point in the presence of such a buffer because the pH change is so gradual that it is difficult to
determine accurately when the color has changed. Titrate until there is a slight change in the original blue color
(preferably to a greenish tinge). When you then boil the solution, it should return to blue. If you added enough
titrant that the solution turned yellow, then hopefully it will return to blue upon subsequent heating (or at least
to green). If not, then you have overshot the end point too far, and you will have to repeat the titration. If you
have not added too much titrant, it will probably require only another drop or two to complete the titration.
Record the final volume and the final pH. Perform two more good titrations. Use the Vernier pH probe to
monitor the pH of the solution during each titration, as was done in Experiment 10A.
The titration to determine total alkalinity [2 x moles of Na2CO3 + moles of NaHCO3] is similar to that
for the standardization of the HCl described above, except that you will pipet 2 mL of the unknown into each
beaker, add 20 to 25 mL of boiled water and 1 to 2 drops of phenolphthalein solution. Titrate with the
standardized HCl solution. This time, however, the phenolphthalein end point will be less than half way to the
bromcresol green end point. From the results of the first titration, you will know the approximate total volume
of titrant needed for the succeeding titrations.
To determine the amount of NaHCO3, you will again pipet 2 mL of the unknown into each beaker, add
10 to 15 mL of water and 1 to 2 drops of phenolphthalein solution. Rinse and fill the second buret with your
standardized NaOH solution that was saved from Experiment 10A. Add about 15 mL of the NaOH solution
and 4 to 5 mL of 10% CaCl2 solution to the beaker containing the unknown and mix thoroughly. Then titrate
with the HCl solution until the pink color just disappears. The mixture will look milky because of the white
CaCO3 precipitate that forms. This is a difficult end point to determine accurately because it is more difficult to
judge the disappearance of color from a solution than its appearance. For this titration, however, if you have
overshot the end point, all is not lost. You can add more NaOH solution and titrate again to a new end point.
Do not forget to read the new final volumes for the NaOH and HCl solutions. Ideally, you will reach a point
where, if you add a drop of HCl solution, the pink color disappears, or a drop of the NaOH solution and the
pink color returns, another drop of acid and the pink color again disappears, etc. Perform two more titrations.
Monitor the pH during each titration using the Vernier pH probe, recording the pH at each end point.
Save the remaining standardized NaOH solution for use in Experiment 28 at the next lab
meeting. You may dispose of any unused HCl solution. Rinse the burets with distilled water. Return the stir
bars and the glassware to where they belong.
Locations of supplies for this experiment:
BALANCE ROOM:
Three small cross-shaped stir bars (check out)
Sodium carbonate (Na2CO3)
UNDER THE WINDOW:
1 M HCl solution
One 250 mL plastic bottle
Two 50 mL burets
Three 200 mL tall-form beakers
One 2 mL pipet
One pipet pump
AT THE LAB BENCHES:
Phenolphthalein solution in dropping bottles
Bromcresol green solution in dropping bottles
RIGHT-HAND FUME HOOD:
Waste jug
Data Recording:
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Fill in information for item 1.
Items 2 through 5 do not apply, since you did not prepare a standard solution of Na2CO3.
For item 6, record the mass of Na2CO3 (you might want to record the moles of Na2CO3 )
Fill in the information for items 7 through 10.
Page 234:
Items 1 through 3 for a solid unknown do not apply.
Fill in the rest of the items. Hopefully, you will not be refilling a buret during a titration.
Page 235:
Fill in all of the items on this page, except for items 11 and 12.
Items 2 through 10 should each have three columns, one for each titration.
Items 9 and 10 are concentrations (molarities) of NaHCO3 and Na2CO3.