Titration: What do you prefer?

Titration: Option 2

"What are we doing to help kids achieve?"

It is the time of year for acid base chemistry. Acid bases usually means a demonstration or lab involving titrating. Titrating this year for me has an added complication. The complication is that half of the science department has to be completely packed up and out of our rooms, including chemicals and equipment, before school ends. Half of the department is going to be completely remodeled (my half of course). Teaching must still go on in the middle of the madness. Choices must be made during the chaos on the best labs to do and fit in. Here are three options for a titration lab. Which would you do?

 

Option 1

Students are provided with beral pipettes. First they count the number of drops it takes to fill one milliliter in a graduated cylinder multiple times. This allows students to determine the volume of a drop of water. Next they are provided a base of known concentration, usually sodium hydroxide. The acid they titrate is the vinegar in a packet of mustard. The ratio of sodium hydroxide to acetic acid is 1:1. Tumeric is a spice in mustard that is a natural indicator. It turns brick red when it is past the end point. Students can use that information to find the percent of vinegar in mustard.

Pros Students can do multiple runs. Using mustard really holds their interest. It is safe and cost effective. There is no glassware to break. Error is usually within five to ten percent.

Cons If students are going on in chemistry it would be helpful to experience with other forms of equipment.

 

Option 2

Students use expensive 50 mL glass burettes to titrate. First they make a 0.2 molar sodium hydroxide solution. This is titrated with KHP as the acid in an erlenmeyer flask. Students measure out the KHP in grams. It is a one to one ratio with the base. They titrate the base with the acid and indicator (phenolphthalein). They bring the purple solution to the instructor who adds a drop of acid. If it goes clear, they keep the data. If it stays purple, they overshot the endpoint and start over. They need to get two good trials to standardize their base. Next, the standardized base is used to titrate a known volume of HCl with an unknown concentration and indicator. They again need two good trials. Each "good" trial is where the instructor only needs to put in one drop of acid to back titrate. If it stays purple after a drop, students went too far by adding excess base and need to start over.

Pros Students usually get very good data. They gain experience with handling burettes. They become good at the physical process of titrating.

Cons This takes a lot of time, chemicals and equipment. Is the goal to teach the chemistry or make them into chemists?

 

Option 3

Students have two 50 mL glass burettes. One glass burette contains a base that has been standardized by the instructor. Students have been given the data from all of the trials. They must take the data and calculate the molarity of the standardized base. The second burette contains an unknown concentration of HCl. They deliver a volume of the acid from the second burette into an erlenmeyer and add indicator. They then titrate with the base from the first burette. If they add too much base and pass the endpoint, they can add a bit more acid from the second burette. They essentially can go back and forth doing this until the difference is about one drop. If they are careful, they can run multiple trials with just two burettes. They record the volumes delivered from both burettes.

Pros Students get good data. They are not penalized for overshooting the endpoint. They become good at titrating. Two groups can usually do four trials with two burettes in a fifty minute period. Not as many solutions are required as in option 2.

Cons There are only two burettes for four people. One group has too much down time.

In a crazy world, which option would you pick? Is there another option that you prefer? What do you think students really need to know to go on into chemistry? I would love to hear from you....let me know what you think...

 

Join the conversation.

Comments 6

Dan Meyers's picture
Dan Meyers | Wed, 05/02/2018 - 17:42

Hey Chad,

Options 1 and 3 make the most sense. Surprisingly, we do not standardize the base for regular or honors chemistry. IB might but I’m not certain. Our focus is to have students demonstrate proficiency in the titration technique and math. Most of our kids don’t go on to become chemists.

Another thought…our unit progression is Stoichiometry, solutions, equilibrium, acids/bases, energetics, kinetics/gas laws, and electrochemistry (if time). Typically I make the titrations unit 2.5 because kids get experience making the solutions and testing them before going to acids and bases. My honors level will usually add in potentiometric titration when we get to acids and bases. Perhaps this rearrangement may be of help logistically in the future. 

Bob Worley's picture
Bob Worley | Thu, 05/03/2018 - 03:01

This whole area of error analysis is a minefield. Please read https://eic.rsc.org/maths/in-uncertain-terms/2000162.article if you can. Just adding one error to another gives silly results; it is much more complex than this. In the 1970s and 80s, we never bothered. It was hard enough teaching calculations anyway. Putting in correct error analysis was an extra burden. To be frank we only put calculations in to make gradation easier when examining students. Some of the questions asked in exams are highly contrived, and would never be faced even in real analysis. I suggest we leave correct error analysis, ie summing up all the errors, to the Universities and Colleges. At schools we should concentrate on technique and realising that there are errors stated on the equipment we use.

Yes weighing by mass with 2 and 3 decimal place balances is more precise. In fact when I have been teaching technicians, they run a traditional version alongside one of my microscale version  that Chad put up and they get very similar values so much so that one Bulgarian born technician (with secondary degrees in chemistry) exclaimed “They have been telling me lies in Sofia”.

Titrating via mass has appeared in J Chem Ed. (dx.doi.org/10.1021/ed2003466| J. Chem. Educ. 2012, 89, 958−959). Comparing the two methods is a very good excercise.

So here is a historical hypothesis. When titrating was developed (Gay Lusssac, Mohr?), in the 1850s balances reading to milligram amounts (and lower) were rare and tedious to use in the extreme, so they worked on volume. If titration were invented today, would they use volume or balances weighing to 2 to 6 decimal places in grams?

To be discussed over a drink I think.

Chad Husting's picture
Chad Husting | Thu, 05/03/2018 - 09:33

Bob - I love the history part and the drink part.  I am certainly going to take a closer look at titrating by mass.  Thanks again.

Donna Engel | Fri, 05/25/2018 - 12:40

Thank you for the great idea to titrate mustard.  We used tumeric as a natural indicator this year so this was a great way to end the acid/base unit.