Potentiometric Titration of H3PO4 and Calculation of K 1, K 2 & K 3 (Adapted from: Sawyer, Heineman, & Beebe, Chemistry Experiments for Instrumental Methods, Methods, MIT Department of Chemistry Lab #5: Potentiometric Pot entiometric titration, titration, and LaSalle University Department of Chemistry) Pre-Lab Reading (in Harris)
Chapters 8-10 – 8-10 – pages pages 162-235 Background
In the Lab 3 analysis, a nalysis, a strong base (NaOH) was used to t itrate a weak acid (acetic acid or tartaric acid) and a visual visual indicator determined the endpoint endpoint of the titration. For this analysis, the same strong base (NaOH) will be used to titrate an unknown concentration of a weak p olyprotic acid (H3PO4) and a mixture containing unknown concentrations of both a strong acid (HCl) and a weak poylprotic acid (H3PO4). The endpoints for these titrations titrations cannot be easily detected using a visual indicator. However, it is possible to determine the endpo ints by monitoring the pH of the solution as a function of titrant titrant volume. These titrations can be used to calculate the dissociation dissociation constants (K 1, K 2, and K 3) of the weak acid and the molarities of both acids. Introduction
The relative acidities of acids and bases are co mmonly expressed in terms of pK a = -log10K a, + where K a is the dissociation constant for the reaction HA H +A For H3PO4, a triprotic acid, there are three successive dissociation constants: -
-
H3PO4 + OH = H2PO4 + H2O H2PO4- + OH- = HPO42- + H2O 23HPO4 + OH = PO4 + H2O
-
+
K 1 = [H2PO4 ][H ]/[H3PO4] K 2 = [HPO42-][H+]/[H2PO4-] 3+ 2K 3 = [PO4 ][H ]/[HPO4 ] -
During the titration of the unknown concentration o f H3PO4, the addition of OH will only slightly increase the pH of the solution until most of the H 3PO4 has been changed into H2PO4-. Further addition of OH will result in a sharp r ise in the pH of the solution, signaling the first equivalence point. -
-
2-
Additional OH will then react with the second hydrogen ion, converting H2PO4 to HPO4 and once again only a small change c hange in the pH of o f the solution upon addition add ition of base will be observed (i.e., a buffered solution). As the conversion is completed, there will once aga in be a sharp rise in pH, signaling the second equivalence point. -
3-
In contrast, the third hydrogen ion reacts o nly partially with OH , yielding PO4 and the pH of the solution will rise only gradually with the continued add ition of OH-. For HCl, a monoprotic acid, there is a single dissociation constant: -
-
HCl + OH = Cl + H2O
-
+
K a = [Cl ][H ]/[HCl]
During the titration of the unknown mixture of HCl and H3PO4, the HCl proton and the first first H3PO4 proton will both react with the OH simultaneously (HCl actually reacts first but no distinct endpoint is is observed) and the sharp rise rise in pH equals the first first equivalence point. The
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volume of base used to get to the first equivalence point represents the sum of the HCl a nd H3PO4 concentrations. -
-
2-
Additional OH will then react with the second hydrogen ion converting H2PO4 to HPO4 as before with a sharp rise in pH signaling signaling the second equivalence eq uivalence point. The volume of base used to get from the first equivalence point to the t he second equivalence point represents H3PO4 concentration alone. From this information, information, the concentrations of both both HCl and H3PO4 may be calculated. Equipment Set-up
The set-up set-up for this lab’s titration is shown below:
Procedure
1) Take a 25 mL aliquot of the H3PO4 solution (‘S (‘Sample 1’) (record the sample number in your lab book) and dilute it with distilled distilled water to 100 mL. Transfer to an appropriately sized beaker (250 mL) NOT an Erlenmeyer flask. 2) Add the stir stir bar making sure it will not strike the delicate delicate electrode surfaces. Keep the solution stirred consistently throughout the titration and do not use excessive stirring, which will generate bubbles under the electrode. 3) Record the pH of the solution solution before adding any of the titrant. titrant. The first first few additions of titrant (˜0.100 M standardized NaOH) may be rather large, 3-4 3-4 mL. Readings of pH and volume are taken after each add ition. Allow several seconds for the pH to stabilize before recording each value.
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4) When the pH begins to change rapidly (e.g. 0.5 pH units units per addition), reduce the size size of each addition so that accurate read ing can be recorded. In the neighborhoo d of the endpoints the additions should be reduced to 0.1 mL. 5) The titration should be continued continued until a pH of 12 is obtained. Rinse Rinse and store the electrodes in distilled water when not in use. 6) Take a 25 mL mL aliquot aliquot of H3PO4 plus 1 mL of an unknown concentration of HCl (sample 2) and dilute to 100 mL with distilled distilled water and repeat the titration procedure as before. Analysis st
For each set of data; plot p lot pH vs. milliliters milliliters of o f NaOH added, and also a lso plot the 1 derivative plot (ΔpH/ΔmL vs. median mL). Determine the end points. Calculate molarity and the number of grams of H3PO4 and HCl in your samples. Calculate the dissociation constants K 1, K 2, K 3 for phosphoric acid. Be sure to take into account increased volumes when calculating concentrat ions, remembering remembering the initial aliquot sample. Do not forget to include your unknown sample numbers and NaOH concentration in your report. REMEMBER: use the Correction Factor for the volumes of NaOH delivered determined determined in Experiment 1! Questions (Answer these separately after the conclusion )
1. Comment on the statement that t hat a buffer is a mixture of a co njugate acid and base. 2. Why is a saturated soluti so lution on of potassium pot assium acid acid tartrate (KHC4H4O6) acceptable as a pH standard? Is it a buffer? 3. How accurate is K 1? K 2? K 3? Why? 4. Why is the last equivalence po int in the H3PO4 titration not as sharp as the first two? Lab Report
A full report is required for this lab.
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