Potentiometric Determination of Ka by patrickoquinn


									                   PROCEDURE FOR DETERMINING Ka and

Determining Mass of Weak Acid and Preparing Titration Flask
1. Obtain a 0.200 to 0.300 gram sample of a mono-protic weak acid.
2. Determine the mass of a 250 ml wide mouth Erlenmeyer flask to a precision of
   +/ – 0.001 grams and record on Data Sheet.
3. Determine the mass of the flask and weak acid sample.
4. Calculate the mass of the weak acid and record on Data Sheet.
5. Dissolve the weak acid by adding 20 mls of 95% ethanol to the titration flask and swirl until
   weak acid dissolves.
6. Add approximately 70 mls of de-ionized water to titration flask.

Calibrating a pH Meter
1. Calibrate pH meter with standard solutions (pH 4.00 (minimum) and pH 10.00 (maximum).

Titrating an Unknown or Known Weak Acid
1. Set-up lab titration equipment using 50 ml buret, ring stand, magnetic stirrer, and pH meter.
2. Clean the 50 ml buret rinse with three 20 ml aliquots of de-ionized water.
3. Condition (rinse) buret with three 10 ml aliquots of standard NaOH solution.
4. Fill the buret to exactly the 0.00 ml mark with standard NaOH solution and record
    buret reading on titration Data Sheet. All buret reading must be read to +/ – 0.01 mls.
5. Record the molarity of the standard NaOH titrant solution on the Data Sheet.
6. Place pH meter electrode in flask, wait 15 seconds read the pH, and record on Data Sheet.
7. Add standard NaOH solution in 1.0 ml increments, wait 15 seconds read pH, and record
    on Data Sheet. Continue this procedure until a pH of 4.5 is reached.
8. At a pH of 4.5, add standard NaOH solution in 0.20 ml increments, wait 15 seconds read
    pH, and record on Data Sheet. Continue this procedure until a pH of 10.0 is reached.
9. At a pH of 10.0, add standard NaOH solution in 1.0 ml increments, wait 15 seconds read
    pH, and record on Data Sheet.
10. When TEN pH readings of 12 or more are recorded the titration is complete.

Calculating Unknown or Known Molars Mass and Ka of Weak Acid
1. See sample calculations and graph for calculation of Ka and molar mass of weak acid.
2. Plot a titration curve showing mls NaOH added (x-axis) versus pH readings (y-axis).
3. Use inflection point derivative calculation to determine the mls of NaOH add to reach the
   titration end point. The end point is the mls of NaOH add to completely neutralized the
   weak acid.
4. Calculate pH at equivalence point by substitute the mls NaOH added to reach end point into
   the equation of titration curve and calculate “y” which is pH at equivalence point.
5. Calculate moles of weak acid titrated by multiplying volume NaOH add to reach end point
   times molarity of standard NaOH. The molar mass of the weak acid can be calculated by
   dividing mass of weak acid by mole of weak acid.
6. Determine volume of NaOH added to reach one-half titration end point and calculate pH at
   that point.
7. Use Henderson Hasselback equation to determine pKa and calculate the Ka of weak acid.
8. Identify the name of the weak acid by using Ka, molar mass, textbook tables, and internet.
                            EXAMPLE CALCULATIONS:

1. Use Excel or TI-86 to determine the polynomial (trinomial) equation that best fits the
   titration curve from your experimental data.

2. Take the derivative of the equation: Nx z  derivative = Z*Nx (Z – 1)
   Polynomial:    – 0.0003x3            + 0.0231x2         – 0.2901x             + 5.5031
   1st derivative: – 3*0.0003x (3 – 1) + 2*0.0231x (2 – 1) – 1*0.2901x (1 – 1)   + 0*5.5031

   1st derivative – 0.0009x2            + 0.04462x1        – 0.2901.             +0

   2nd derivative – 2*0.0009x (2 – 1) + 1*0.04462x (1 – 1) – 0*0.2901

   2nd derivative – 0.018x                  + 0.0462           –    0

3. Set 2nd derivative equal to 0 and solve for x (mls of NaOH added to reach equivalence point)
   – 0.0018x + 0.0462 = 0
   – 0.0018x = – 0.0652
   x = 25.66 mls NaOH

4. Substitute known x into polynomial and solve “y” (pH at equivalence point)
   y = – 0.0003x3 +0.0231x2 – 0.2901x + 5.5031
   y = 8.20 pH

4. Determine Ka of Weak Acid (HA) using Henderson Hasselback Equation
   General Equation: HA (aq) + H2O (l)  H3O +1(aq) + A – 1(aq)
   Henderson Hasselback Equation: pH = pKa - log [Acid]/[Conjugate Base]
   If [Acid] and [Conjugate Base} concentrations are equal (assume 0.05M) then:
   log [0.05]/[0.05] or log (1)      or   0 and equation become pH = pKa
   At equivalence point the concentration of the [Weak Acid] and [NaOH] are zero.
   At one half the equivalence point (1/2 added NaOH) the concentrations of the
   [Weak Acid] and [Conjugate Base] are equal.. Based on attached graph and
   equivalence point determined by derivative 25.66/2 = 12.83 mls NaOH
   Substitute 12.83 into polynomial and solve for “y” which equals pH at one-half the
   equivalence point. Based on graph y = 4.951 = pH = pKa
                                  – 4.951                      –5
  Ka = – log(pKa) or Ka = 10                  Ka = 1.12 x 10

To top