Phosphoric acid (H3PO4)(H3PO4) size 12{ \( H rSub { size 8{3} } ital "PO" rSub { size 8{4} } \) } {} is a chemical that is commonly found in everyday products such as soft drinks and cleaning agents. It is called a polyprotic acid because it can donate more than one proton ( H+H+ size 12{H rSup { size 8{+{}} } } {} ion) per phosphoric acid molecule. The released protons combine with water to form hydronium ions (H3O+)(H3O+) size 12{ \( H rSub { size 8{3} } O rSup { size 8{+{}} } \) } {}.

Phosphoric acid releases its protons in a step-wise manner:

H3PO4+H2O↔H3O++H2PO4−H3PO4+H2O↔H3O++H2PO4− size 12{H rSub { size 8{3} } ital "PO" rSub { size 8{4} } +H rSub { size 8{2} } O↔H rSub { size 8{3} } O rSup { size 8{+{}} } +H rSub { size 8{2} } ital "PO" rSub { size 8{4} } rSup { size 8{ - {}} } } {}Ka1=7.5×10−3Ka1=7.5×10−3 size 12{K"" lSub { size 8{a1} } =7 "." 5 times "10" rSup { size 8{ - 3} } } {}(1)

H2PO4−+H2O↔H3O++HPO42−H2PO4−+H2O↔H3O++HPO42− size 12{H rSub { size 8{2} } ital "PO" rSub { size 8{4} } rSup { size 8{ - {}} } +H rSub { size 8{2} } O↔H rSub { size 8{3} } O rSup { size 8{+{}} } + ital "HPO" rSub { size 8{4} } rSup { size 8{2 - {}} } } {}Ka2=6.2×10−8Ka2=6.2×10−8 size 12{K"" lSub { size 8{a2} } =6 "." 2 times "10" rSup { size 8{ - 8} } } {}(2)

HPO42−+H2O↔H3O++PO43−HPO42−+H2O↔H3O++PO43− size 12{ ital "HPO" rSub { size 8{4} } rSup { size 8{2 - {}} } +H rSub { size 8{2} } O↔H rSub { size 8{3} } O rSup { size 8{+{}} } + ital "PO" rSub { size 8{4} } rSup { size 8{3 - {}} } } {}Ka3=4.2×10−13Ka3=4.2×10−13 size 12{K"" lSub { size 8{a3} } =4 "." 2 times "10" rSup { size 8{ - "13"} } } {}(3)

For example, reaction (2) will not occur until reaction (1) is complete.

The KaKa size 12{K rSub { size 8{a} } } {} values listed after each reaction are called acid ionization constants. They indicate the relative ease with which each reaction occurs. A small KaKa size 12{K rSub { size 8{a} } } {} value shows that a reaction does not occur easily. The KaKa size 12{K rSub { size 8{a} } } {} value for phosphoric acid’s second donated proton is much smaller than for the first donated proton, while the third KaKa size 12{K rSub { size 8{a} } } {} is five orders of magnitude smaller than the second.

To determine the amount of acid in an unknown sample, you will need to add a known amount of base until the acid and base are neutralized. This technique is known as titration, and it is widely used in chemistry and other natural sciences.

During a titration, the pH of the solution is constantly monitored while the known acid or base (called the titrant) is slowly added to the unknown solution. The pH of the unknown solution will stay fairly constant until the moles of titrant added equals the moles of unknown acid or base. When the moles of acid and base are the same, further additions or titrant will cause a dramatic change in pH until the pH eventually stabilizes. A graph of pH versus added titrant is called a titration curve, and the point at which the pH changes drastically is called the equivalence point.

The titration curve for a polyprotic acid will have more than one equivalence point. As the added base completely removes each proton from the acid, the pH will jump significantly. Figure 1 shows the titration curve for ascorbic acid, a polyprotic acid also known as Vitamin C:

Instructions for MicroLab Titrations Experiment.

Open the MicroLab Program by clicking on the Shortcut to MicroLab.exe tab on the desktop.

On the “Choose an Experiment Type” Tab, enter a name for the experiment, and then double click on the MicroLab Experiment icon

Click “Add Sensor”, Choose sensor = pH/D.O.

Choose an input, click on the red box that corresponds to the port that your pH meter is connected to.

“Choose a Sensor”, click radial button that says pH. Click next.

Click “Perform New Calibration”

Click “Add Calibration Point” place the pH meter in pH= 4.0 buffer solution, while swirling wait until the value is constant and then enter 4.0 into the “Actual Value” box in MicroLab and hit “ok”.

Again, Click “Add Calibration Point” place the pH meter in pH= 7.4 buffer solution, while swirling wait until the value is constant and then enter 7.4 into the “Actual Value” box in MicroLab and hit “ok”.

Under Curve Fit Choices , click on “First order (linear)” and then “Accept and Save this Calibration”, when prompted to “Enter the units for this calibration”, leave as (pH) click ok, save as your name-experiment-date. Click finish.

Click “Add Sensor”, Choose sensor = keyboard, change the label from Kbd to “Burette Reading (mL)”, click next.

“Enter the prompt to use in the Keyboard dialog box” put, “Enter Volume”, Enter units = mL. Click finish.

In the sensor area, left click on the pH icon and drag it to the Y-axis over “data source two”, also click and drag to column B on the spreadsheet and also click and drag to the digital display window.

In the sensor area, left click on “Burette Reading (mL)” and drag to the X-axis over “data source one”, also click and drag to column A on the spreadsheet and also click and drag to the digital display window.

Obtain, in a dry beaker, 30 mL of a phosphoric acid solution. Rinse your 10 mL pipette with this solution and pipette 10 mL into a 250 mL beaker. Use a graduated cylinder to add 50 mL of distilled water. Rinse and fill your 25 mL burette with 0.4 M NaOH. The initial burette reading should be 0.

Place the beaker on the magnetic stirrer and add a stir bar. Position the burette ready for titration. Insert the pH probe. Turn on the magnetic stirrer and adjust the stirring rate to moderate speed (without splashing or hitting the probe).

When ready to obtain data, click start.

When prompted, enter the initial burette reading. The program will automatically take a pH reading. Add an amount of NaOH so that it gives an approximate 0.2 pH rise. Once the pH has stabilized in the digital window, enter the burette reading value that corresponds to the pH. Repeat.

Note that “Burette Reading (mL)” is not the same as Volume NaOH unless you start at 0.00 mL. Students can convert Burette Readings to volume NaOH by using the formula feature. Click on “Add Formula” and then click on the “Burette Reading (mL)” and subtract the initial burette reading. Label it as “Vol NaOH added” Click and drag it to the X-axis of your graph and to column C in the spreadsheet.

First and Second derivative plots can be made by clicking on “analysis” in the Graph Window. Select the desired plot.

1. Calculate the concentrations of Na2HPO4Na2HPO4 size 12{ ital "Na" rSub { size 8{2} } ital "HPO" rSub { size 8{4} } } {} and NaH2PO4NaH2PO4 size 12{ ital "NaH" rSub { size 8{2} } ital "PO" rSub { size 8{4} } } {} to produce 100 mL of the buffer solution with the pH = 6.91. Show your calculations to your TA before proceeding.

2. Prepare your buffer solution from 0.1 M Na2HPO4Na2HPO4 size 12{ ital "Na" rSub { size 8{2} } ital "HPO" rSub { size 8{3} } } {} and 0.1 M NaH2PO4NaH2PO4 size 12{ ital "NaH" rSub { size 8{2} } ital "PO" rSub { size 8{4} } } {} solutions.

3. Insert the pH probe in your buffer solution and wait until the reading becomes stable and write down the value in your report form. Don’t worry if the pH reading isn’t exactly 6.91. The important thing is that there isn’t a drastic change in pH upon addition of acid or base.

4. Pour 50 mL of the buffer solution into another beaker. Add 1 mL of 0.1 M NaOH to the first beaker and mix the solution with a glass rod. Wait until the pH reading becomes stable and write down the value in your report form. Add 1 mL of 0.1 M HCl to the second beaker and mix the solution with the glass rod. Insert the pH probe into the second beaker. Wait until the pH reading becomes stable and write down the value in your lab report form. If the pH of your buffer solution changes by more than 0.3 pH units, you will need to redo the calculations and re-prepare the buffer solution in order to get an acceptable result.