b. Diprotic Acid Titration
- Page ID
- 309473
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Students should operate in pairs with one student recording the volume titrant and pH of the solution in a Google Sheet while the second runs the virtual lab. You should switch roles between experiments. Each group is assigned an unknown by their instructor and the can obtain the unknown by going to the bottom of the stockroom where they can obtain 100 mL of 1M unknown (figure \(\PageIndex{1}\). Even though we could safely titrate any concentration in the virtual lab we want you to dilute all stock solutions to 0.1 M.
We will assume Ka1 > 1000Ka2 and the protons are removed stepwise, that is, we convert the acid to the acid salt, and then convert the acid salt to the base (review nomenclature section 2.5.5 and section 16.1.3 if you are not clear on what an acid salt is). The first equivalence point involves the removal of the first proton from the diprotic acid and the formation of the acid salt.
\[\underbrace{H_2A(aq)}_{acid} + NaOH(aq) \rightleftharpoons \underbrace{NaHA(aq)}_{acid \; salt} + H_2O(l)\]
Since this salt is an acid, it too has a titratable proton and the second equivalence point deals with the removal of the second proton, making the salt out of the acid salt.
\[\underbrace{NaHA(aq)}_{acid \; salt} + NaOH(aq) \rightleftharpoons \underbrace{Na_2A(aq)}_{salt} + H_2O(l)\]
Before proceeding you should quickly review section 17.3.5 Polyprotic Titration curves where you can see the curve for the triprotic acid Phosphoric acid.
Assignment
Before starting you titration in the virtual lab you need to design your experiment and figure what volumes of analyte and titrant you should use. The virtual lab gives you the concentration of each reagent in the stock room, which is what we are calling the Stock solutions. These are 1M or greater and although you could titrate 1 M solutions in a virtual lab we want you to dilute them to 0.1 M like you would in the real lab. So Question 1 deals with diluting the stock analyte and titrant solutions to safe concentrations to perform the titrations. The second question is central to the design of the titration as you need a lot of data around the equivalence point.
In this lab you need to titrate 25 mL of the analyte and make pH measurements every 1 mL except when you are near each of the two equivalence points, where you need to take make a measurement every 0.1 mL for the range from one mL below to one mL above the equivalence point.
Question 1: Dilution of Stock Solutions
You have stock 1 M NaOH and 1 M unknown solutions and need to make 100 mL of each solution to use in the titration. Indicate the quantity of reagent (NaOH or unknown) that would need to be added to a 100 mL volumetric flask so the final solution would be 0.1 M, and then add that to a 250 mL Erlenmeyer flask. Record this value in the Google Sheet
Question 2: Predict the Two Equivalence Points
Since you know the concentration of the acid and base you can predict the volume of base required to titrate off the first proton, and then the second proton. Insert these values into the Google Sheet, and you can change them later if you discover they are wrong, but you need to know these values in order to make small measurements around the equivalence points. You will not be required to do derivative plots in these labs as you "know" the equivalence point.
Figure \(\PageIndex{1}\): Copy and Paste Caption here. (Copyright; author via source)
3. Calculate Ka1 and Ka2 from the titration curve (using the value of half equivalence).
Virtual Lab
This virtual lab should load the "Unknown Diprotic Acid" lab, if it says "Default" you need to refresh the page.
Figure \(\PageIndex{2}\): Be sure the correct lab loads