Question

PROCEDURE:

Your instructor will demonstrate how to set up, calibrate, and use a pH meter. Be sure to observe the following precautions when using the pH meter.

   1. The glass electrode is rather fragile and expensive. It can be broken easily, or it can be scratched. Be very careful when handling it that no hard object hits or even rubs against the tip of this electrode. Glass electrodes are frequently protected with a plastic shield. Do not try to remove this shield.

   2. The glass electrode should not be left standing in a dry condition. Immerse the electrodes on the pH meter in distilled water when they are not being used.
  
   3. Wash the electrodes with distilled water from a wash bottle. A piece of filter paper can be used to absorb the excess water that clings to the electrodes, but never wipe them with a paper or cloth towel.

   4. The switch on the lower right-hand corner of the pH meter should be on "zero" and not on "measure" when the electrodes are not immersed in a solution.

  
I.   Please note the questions at the end of the report. You should read these prior to beginning the experiment, so you can answer them in a timely fashion.

Titration of NaOH vs. HCl

II.   Use the common buffer solutions from the side table to calibrate your pH meter. Rinse the electrode with distilled water and immerse the tips of the washed electrodes into the buffer solution. Adjust the "calibration" knob so that when the right-hand knob is on "measure," the pH indicated on the meter is the same as that stated on the bottle of buffer solution. Retain this buffer solution until you are finished with the pH meter, and then discard it. It is advisable to check the calibration of the pH meter after each titration but not during a titration.

III.   Pipette exactly 25 ml (your instructor will demonstrate the use of a volumetric pipette) of the approximately 0.1 M HCl into a 250 ml beaker. Dilute with distilled water to a volume of about 100 ml, and add 1 drop of phenolphthalein indicator.

   Fill a burette with the standardized sodium hydroxide solution, and be sure all air bubbles are removed from the stopcock and tip of the burette. Immerse the electrodes of the pH meter into the acid solution, and set up the burette over the beaker so that the sodium hydroxide can be added without moving the beaker of electrodes. Measure the pH of the acid solution, then add an increment (0.2 to 0.5 ml) of sodium hydroxide solution. Be sure to record the reading of the burette before and after each addition of NaOH. Stir the solution, and again measure the pH. Repeat these operations until the pH of the solution is about 3.5. Now add the sodium hydroxide one (1) drop at a time, and measure the pH of the solution after each addition to the base. Continue this procedure until a pH of about 10 is reached, then add a larger increment of sodium hydroxide solution and again measure the pH.

IV.   Record in an orderly manner all the pH readings and the volume of sodium hydroxide added for each reading. Record your data in two vertical columns, pH in one column and milliliters of NaOH added in the other. Indicate in your table of data when the indicator showed a faint pink color.

V.   On the graph paper, plot pH as the ordinate and milliliters of NaOH as the abscissa for all of your data. Connect all the points on your graph.

Titration of NaOH vs. H3PO4

I.   Wash off the electrodes of the pH meter, and recheck the calibration of the glass electrode by using the buffer solution. Again wash the electrodes with distilled water.

II.   Pipette exactly 25 ml of the approximately 0.05 M H3PO4 into a 250 ml beaker, and dilute with water to 100 ml. Set up the beaker and burette containing sodium hydroxide solution as in the previous titration, and measure the pH of the solution after each volume of sodium hydroxide solution is added. Increments of sodium hydroxide between 0.2 and 0.5 ml can be added except when the pH is between 3.5 and 5.5 and between 7.5 and 10. In these two ranges, no more than 2 or 3 drops of sodium hydroxide should be added for each increment.

III.   Again, tabulate all your data in a systematic manner, and plot your data on the graph paper as in the previous titration.

POST LAB QUESTIONS:

1) How many milliliters of sodium hydroxide were required to reach the equivalence point in your HCl titration?
_____________________

2) Does the equivalence point in the HCl titration, as determined potentiometrically (from your graph), agree with that observed with the indicator?
(Yes or No) _________________________

3) What is the precise molarity of the hydrochloric acid solution? Show the stoichiometery calculation.

4) How does the titration curve of the weak acid-strong base compare with that of a strong acid-strong base?

5 a) Write the equation for reaction that takes place from the beginning of the phosphoric acid titration until the first equivalence point is reached.

5 b) Write the equation for the reaction that takes place between the first and second equivalence points.

6 a) How many milliliters of NaOH were required to reach the first equivalence point for phosphoric acid?
                                           _________________

b) How many milliliters of NaOH were required to reach the second equivalence point? ______________

7) Explain why the volume of sodium hydroxide required to reach the first equivalence point for phosphoric acid should be equal to the volume of sodium hydroxide required to go from the first equivalence point to the second.

CONSIDERATION:

Suppose you had a solution that contained H3PO4 and some dissolved NaH2PO4. Explain how this solution could be analyzed for both the H3PO4 and the NaH2PO4 by an acid-base titration.

**I am currently making up acid-base titration using pH meters lab in Chemistry 2 today and the post lab will be due in two days. I am hoping for some insight on how I should set this up as I was sick all last week and got a bit behind. I know how to complete all the work such as doing the actual experiment but I just want to have extra help so I can get this all done quickly as I have more work to do as well; espeically the Considerations Question.Thanks a ton!**

Answer 1

POST LAB QUESTIONS:

1) How many milliliters of sodium hydroxide were required to reach the equivalence point in your HCl titration?
Answer:

Let us calculate it as

M1V1 = M2V2

Molarity of HCl = M1 =

Volume of HCl = V1 =

molarity of NaOH = M2

Volume of NaOH = V2 = ?

So we need to have the molarity of standard NaOH solution

Volume of NaOH = 25 X 0.1 / Molarity of NaOH

We can obtain the value of volume of NaOH from the graph

2) Does the equivalence point in the HCl titration, as determined potentiometrically (from your graph), agree with that observed with the indicator?
(Yes or No)

Answer: Your answer should be yes, it should match the value

3) What is the precise molarity of the hydrochloric acid solution? Show the stoichiometery calculation.

Answer: The precies molarity is as given in the experiment = 0.1 M

4) How does the titration curve of the weak acid-strong base compare with that of a strong acid-strong base?

Answer: the graph of the two differs as

a) The strong acid and strong base titration curve is shows equivalent point at pH= 7.

b) In case of weak acid and strong base, the titration curve shows equivalence point pH > 7.

Let us consider weak acid as HA

Strong Base as BOH

So at equivalenct point

HA + BOH --> AB + H2O

The salt formed will gets hydrolyzed as

A-B+ + H2O -> AH + BOH

so the base will give hydroxide ion and will raise the pH of the solution more than 7.

5 a) Write the equation for reaction that takes place from the beginning of the phosphoric acid titration until the first equivalence point is reached.

The reaction will be:

H3PO4 + NaOH --> NaH2PO4 + H2O

5 b) Write the equation for the reaction that takes place between the first and second equivalence points.

The reaction will be

NaH2PO4 + NaOH --> Na2HPO4 + H2O

6 a) How many milliliters of NaOH were required to reach the first equivalence point for phosphoric acid?
      Answer: We need concentration of phsophoric acid for this

b) How many milliliters of NaOH were required to reach the second equivalence point? ______________

7) Explain why the volume of sodium hydroxide required to reach the first equivalence point for phosphoric acid should be equal to the volume of sodium hydroxide required to go from the first equivalence point to the second.

Answer: One mole of phosphoric acid will dissociate as

H3PO4 --> H⁺   + H2PO4^-

H2PO4^- --> H⁺ + HPO4^-2

So each equivalent of phosphoric acid will dissociate first to give one equivalent of proton

Then the anion will further dissociate to give one more equivalent of proton

so for each equivalent point same amount of NaOH will be used