The purpose of this article is to show how a microtitration can introduce Honors Chemistry students to titrations.
A microtitration is pretty much like it sounds—a titration on a microscale. The unit of volume is the “drop” (d), not mL or L. Using a glass medicine dropper, 1 mL ≈ 20 drops; a thin-tipped Beral pipette will deliver > 40 d·mL-1. Microtitrations are an ideal way for students to see what titrations are all about. Without the fancy glassware, students focus on the chemistry and on the logic behind the procedure. Proper use of burets and pipettes can wait ‘til the next titration. Microtitrations are super-fast. A novice can complete several trials in 15 minutes or so. This means that the experiment can form a part of a larger lesson on titrations.
How I use microtitrations:
- The first titration—an acid-base titration—that students complete in Honors Chemistry is on a microscale. I have provided a student-ready handout in the supplemental materials. They titrate NaOH(aq) with HCl(aq).
- Students dispense solutions using plastic Beral pipettes or medicine droppers. To ensure that a drop of acid has the same volume as a drop of base, identical pipettes must be used.
- Rather than using a beaker or flask as a receiving vessel, microtitrations can be carried out using the (cut-off) bottom of a clear plastic 2-L soda bottle. The bottom 1.5” (or so) of a 2-L soda bottle can be removed using a sharp knife, or an (old, but still sharp) knife that has been heated in a Bunsen burner flame.
- Stirring is accomplished using a plastic fork, “liberated” from the school cafeteria or local fast-food establishment, with all but one tine removed.
- To transfer a tiny—but adequate—amount of indicator, place a decent-sized drop of indicator on a watch glass. Immerse the tip of the stirrer in the indicator and transfer the small amount to the solution in the well. Wipe the stirrer before transferring the indicator to the next well.
- Provide a square of white paper to use as a reflective background. This facilitates observation of the endpoint.
And this is pretty much it. Students dispense solutions dropwise, keeping track of the quantity of drops.
Students may ask, 'What is the volume of a drop?' Luckily, if identical droppers are used, the “drop” units cancel out in the stoichiometric calculations: The drop is a unit of volume, don’t forget, and so mol·L–1· d is equivalent to moles.
Take a look at this sample calculation to see how “d” cancels out:
Summary and Benefits of Microtitrations
- Very fast; a microtitration can form part of a larger lesson
- Reasonable—but not super-great—precision
- Since each student works alone, he/she is busy counting drops. (This means that the lab will be quiet. Don't get used to it.)
- Small amounts of reagents required; this saves money and reduces chemical waste (if applicable)
- For teachers whose chemistry lab is less-than-well equipped, microtitrations can suffice
- Beral pipets are inexpensive; clear 2-L soda bottles are free. Both can be washed and reused
After this introductory, get-your-feet-wet, microtitration, my student complete two macro titrations.
- Determination of concentration, mol·L–1 and %(m/v) and %(v/v), of acetic acid in vinegar;
- Determination of the molar mass of an unknown solid acid.
Bonus: I begin AP Chemistry with a redox microtitration, which introduces—quickly—the need for the Oxidation Number concept—and onward to Redox and Electrochemistry.