Looking for ideas how to use a pH meter with youngsters

I am trying to think of fun ways to use an inexpensive pH meter in home experiments with a hope of getting youngsters interested in science.  I will describe what I have imagined thus far, and I welcome suggestions. 

When I took college chemistry, many years ago, a pH meter was an extremely exotic and expensive piece of equipment.  If one were an ordinary freshman or sophomore in our chemistry lab, the pH meter was out of reach.  There was only one of them and there were sixty students in the lab.  I have to imagine that the only students who ever got to actually touch the pH meter were in 400-level courses.

So imagine my reaction to see that nowadays, a workable and fairly accurate pH meter can be purchased for a mere $8 (blog article).  A person could purchase several of them, cross-check their results, and treat them almost as a disposable item.

Which brings me to today’s topic.  What kinds of home experiments might a person carry out with such a pH meter, having invited a few youngsters to the house, with a goal of perhaps triggering an interest in science among one or another of the youngsters?

One of the challenges in this is to try to figure out “to what extent will I try to explain what is really going on in detail?” and “to what extent will I duck the question of what pH really means in detail?”  If have a few would-be scientists in front of me, all of whom are around 12 years old, should I launch into an hourlong discussion of the negative of the base-10 logarithm of the proton concentration in the aqueous solution, and what protons are, and what it means for a proton to disassociate from whatever it was previously bound to, and how this ties in with broader and cruder approximate terms like “acid” and “base”?

On  my first day of chemistry lab (this is some decades ago) the activity of the day was that each student would make up the two or three “reference solutions” that the student would use again and again over the course of the semester.  What I sort of figured out after a while was that on the first day of lab, each of us was doing the equivalent of carving our own measuring stick from a piece of wood, marking it with our initials, comparing it with a standard measuring stick from somewhere else in the lab, and preserving our own newly carved measuring stick to use, in our subsequent seventeen lab sessions, to measure the lengths of things.

What I am leading up to is that with any pH meter, whether it be the thousand-dollar meter that would get used in a serious lab, or the $8 meter that you can get by mail order, the meter is worthless, completely worthless, unless the user is willing to be diligent about “reference solutions” and “calibration procedures”.  On the day that this $8 meter showed up, what I found in the package were three little sachets, each containing a bit of white powder to be used in making up three “reference solutions”.  And what I would need to do, for the next year or so, is preserve these three reference solutions in a safe place.  And from time to time, I would make use of the reference solutions to do recalibrations of this $8 pH meter.

Part of what I am getting at is that even if you decide to throw money at the problem and buy the $1000 pH meter instead of the $8 meter, you would still need to maintain reference solutions and carry out recalibrations.

So back to our two or three 12-year-olds who might join us with our home experiments to try to learn a bit of science.  As a prelude to the eventual use of the pH meter, do we spend maybe an hour talking about “reference solutions”?

Here is one guess about how to proceed.  See what you think of it.

Hello Anna and Bill.  Thank you for visiting.  Today if we are very lucky we will make a volcano erupt, here in the kitchen.  I will describe a bunch of pretty boring things but I hope you can somehow manage to keep your interest up while I talk about these things.  And as I say, after we talk about the boring things, maybe we will manage to make our volcano erupt.

(We point to a papier-mâché volcano that rests on a big square of corrugated cardboard on the kitchen table.  It has a plastic cup molded into the cone at the top center of the volcano.  Dear reader you know exactly where I am going with this.  We will pour some baking soda into the cup and then we will pour some vinegar or other acid into the cup, and a dramatic froth will spill out and run down the face of the volcano, hopefully not straying past the edge of the corrugated cardboard.)

We point to three cups that are standing in a row, nearby on the table.  The cups are numbered.  At first glance each cup has water in it.

(Dear reader you know exactly where I am going with this. Two of the cups are just water.  But one of the cups is a mild acid.   The student will dip the probe of the pH meter into each cup, write down the pH number in a log, and then we try to guess what will happen if we pour a bit of the liquid into the cup of the volcano.)

The sort of dumbest way I could try to be a teacher with this would be to just say “well, there is this thing called pH, and sorry we won’t really learn today what it really means in any detailed way, but it allows us to sometimes guess what might happen when we mix something with something else.  And now we will use this meter to measure the pH of the water in the three cups, and then we will pour a bit of the water into to the volcano one by one.”

Back to the setup.   If we use vinegar as our acid, it will have a strong smell.  This will give everything away.  I was sort of thinking let’s use ascorbic acid.  When I was a teenager working in a restaurant we would lightly sprinkle some ascorbic acid powder on the newly chopped salad, to try to keep it from turning brown over the next few hours.  So maybe try to buy some ascorbic acid powder and use that.

What just popped into my head is that I guess I could just go down to the local drug store and buy some vitamin C tablets and crush them and use that to make a clear odorless liquid that is an acid.

Anyway, what sequence of events will we use?  I guess the way to go is, we measure and write down the pH numbers.

Again I cheat.  I give away the answer to the youngsters.  A smaller pH number is what is going to make the volcano erupt.  Shame on me.

Then I ask the youngsters to point to a cup that does not have the “smaller pH number”  (It is just tap water.)  And we say “okay, let’s see if this will make the volcano erupt!”  And everybody stands around the volcano with great anticipation.   And we pour a bit of the water into the volcano, and … nothing happens.

What a letdown.  But we sort of knew nothing was going to happen, because I cheated and told them that if the pH number is not smaller, nothing will happen.  We go back and consult our notes.  This water had a pH of 7 or so.

Okay, then the youngsters, knowing perfectly well where we are going with this, they select the next cup.  And yes, everybody knows it is the acid one.  And we say “okay, let’s see if this will make the volcano erupt!”  And everybody stands around the volcano with great anticipation.   And we pour a bit of the water into the volcano, and … a rather dramatic eruption happens.  Froth all over the table, spilling onto the floor.

We go back and consult our notes.  This water had a pH of 6 or so.

Maybe I should not cheat?  Maybe I should not tell them in advance “smaller pH means it will make the volcano erupt”?  But the problem is, if the first cup we pick is the one that makes it erupt, then there is no drama from trying it and having it fail to erupt.

There is yet another possible outcome for all of this, which is that maybe ten minutes into our two-hour program, one of the 12-year-olds interrupts me and starts talking.  The youngster says:

Okay you are going to mix an acid with a base and get a vigorous chemical reaction.  You are going to use this pH meter to measure the acidity of these liquids.  And we will waste all of this time pretending that you don’t know perfectly well which one is the acid.  (Precocious youngster makes a show of picking up each cup and smelling it, and says “well you didn’t use vinegar.  I suspected you were going to use vinegar.”)

And then the youngster says “let’s get this over with.  We will measure the three pH values and pick the smallest number and pour it into the volcano.”

But now back to the original goal of picking a plan for the evening with the youngsters.

It seems to me that if we want this to be a good learning opportunity, there is no choice about it.  Before we stick the probe into the three “unknown” cups of water, we need to carry out the calibration procedure (you can read the procedure here).  We click a button on the meter to say “let’s learn or re-learn what voltage counts as pH 6.86” and then we do it again so the meter will learn or re-learn what voltage counts as pH 4 and then the same for pH 9.18.  (Where did these three numbers come from?  See previous blog article.)

Then having completed the calibration procedure, we return our attention to the three seemingly identical three cups that might make the volcano erupt.  And it erupts.

And then at this point maybe the youngsters are glancing at their wristwatches and mentioning that after all, this is a school night, and maybe they need to head home now.  Indeed had I been more alert I would have seen the glancing at wristwatches after just the first 20 minutes or so of this hours-long activity.

Bit if against all odds the youngsters are still showing interest in what is going on, we continue.

Pour some water from the kitchen faucet and measure its pH.  Measure the pH of some fresh-squeezed lemon juice.  Repeat this for some fresh-squeezed orange juice.  (The lemon will be stronger acid than the orange.)  But then maybe we planned ahead and picked up some alkaline sports water and staged it nearby.  We invite the youngsters to notice that on the label it proudly proclaims that it has a pH of 9.  We stick the probe of the pH meter into this sports water to see if their claimed pH is accurate.

Oh and then I guess at this stage we do the “reveal”.  We pull out the bottle of vitamin C tablets and we explain that this is how we made the liquid that actually made the volcano erupt.

What do you think?  Can you suggest something better in terms of what to do with a few youngsters in a home experiment with a pH meter?