Two groups of students, typically, have difficulty with titrations, the first time around. Those that act too rashly and overshoot the endpoint, and those over-cautious ones who take forever to finish. I have a nautical analogy that seemed to have helped my groups in my lab recently.
At sea, move freely. Into port, move cautiously. At the dock, slowly finish.
When at sea one moves with direction, but moves freely. Once one gets close to port, one slows down, and observes closely. And at the dock, one moves slowly to finish the journey (see figure 1). Here, and in the attached PowerPoint, we will walk through the three situations in a typical lab where students will 1) pipet a known amount of a stock solution into a volumetric flask, 2) dilute the solution in the volumetric flask, then 3) use a solution from a volumetric flask, pour it into a buret and then use it to titrate a solution of unknown concentration. Each step, as you will see below, has the At Sea/Into Port/At the Dock sequence.
Figure 1: Ship analogy1
This first situation is one where the student will pipet 25.00 mL of stock solution and drain it into a 100.0 mL volumetric flask (we will dilute this solution and use it in the next task discussed below). To make the images easier to see, a deep blue copper (II) sulfate solution in 2.0 M ammonia is used here and in the next task. In figure 2-A, a 20 mL pipet bulb is deflated and ready to pull up the solution. Moving freely at sea, 20 mL of solution is pulled into the pipet (figure 2-B). Since the bulb is not adequate to draw the whole 25 mL at one time, the student will place their thumb over the top of the pipet and deflate the bulb again. Now we are coming into port. In fact, one needs to slowly move past the dock, that is, draw the liquid past the mark, before slowly backing it in (figure 2-C). A rolling index finger is useful for this. Once the solution is at the mark, the student puts the pipet well into the volumetric flask, and being back out to sea— is moving freely as the solution drains (figure 2-D).
Figure 2: Transferring a stock solution into a volumetric flask
Dilution in a Volumetric Flask
The solution pipetted above is ready to be diluted from 25 mL to 100 mL in a volumetric flask (figure 3-A). The student will use an Erlenmeyer flask or beaker along with a small funnel to assist in filling the volumetric flask. They will then start adding water to the volumetric flask (figure 3-B). The solution will be mixed at sea (figure 3-C). The student will continue to add freely from the Erlenmeyer or beaker until the volumetric flask is filled to the base of its neck (figure 3-D). Finally, they will take the wash bottle or disposable pipet (as illustrated) and steer the flask into port cautiously by adding water to near the dock, the mark on the neck of the flask. Dropwise, the student carefully finishes the journey to the mark.
Figure 3: Stepwise dilution in a volumetric flask
In this titration, standard HCl(aq) is titrated with NaOH(aq). A volume of 25 mL of HCl(aq) has been pipetted to an Erlenmeyer flask and diluted using the same procedure shown above for the copper (II) sulfate solution. Phenolphthalein indicator has been added to the HCl(aq). NaOH(aq) has been loaded into a buret, and the initial volume recorded.
Figure 4: The view of the buret "out to sea"
Swirling the Erlenmeyer flask does require a bit of technique, but if they can get that down, students can see the color change dissipate as the NaOH(aq) is allowed to move freely at sea (into the Erlenmeyer flask) for a significant part of the titration (figure 4). Once the color change persists, they can slowly take it into port (figure 5). One method for adding NaOH while swirling is to make a 180° turn with the buret valve, then a quick swirl. Repeat, repeat, repeat.... When near the dock, dropwise care is necessary.
Figure 5: Ehlenmeyer flask used during titration
I realize that a nautical analogy may not work in all circumstances but the idea is highly adaptable. Perhaps a freeway - surface street - driveway analogy would be better. Make up your own, or have your students do so. You can use any closer to home storyline that you feel comfortable with. You might use “Freeway – move freely. Surface streets – move cautiously. Driveway – finish slowly.” Heating to a temperature, tuning an instrument or cleaning glassware are all tasks that an analogy like this could work for.
PowerPoint Explanation of the Ship Analogy for Chemistry Tasks:
Notes and References
For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).
For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.
Other Safety resources
RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies