Lab Equipment: pH measurement with paper and meters [Part 1]


Uploaded by NurdRage on 13.04.2010

Transcript:
Greetings fellow nerds.
When doing chemical reactions, sometimes the acidity or the basicity is important.
This is usually defined as pH
and measuring it can be very useful for getting a reaction right.
Now there are a few ways to measure pH.
The cheapest, simplest and most reliable is pH paper.
On the left here I got blue litmus paper,
which if dipped into an acidic solution will turn pink or red.
Now I don't have it here,
but there is also a red litmus paper that turns blue if dipped into basic solutions.
Litmus paper is good for simple acidic or basic measurements.
But for a more precise determination we have this more advanced pH paper on the right.
It consists of multiple strips of pH indicators that have different transition points.
Just apply a small drop of the solution you want to measure to the strip.
Dipping a strip is actually not the best way to use pH paper
because you might contaminate your sample with the indicators used in the paper.
Anyway, wait for the color change,
and then compare the color to a reference chart to find the pH.
For things like fieldwork and quick measurements, this is great.
For even greater precision,
an expensive but high-tech solution is to use a pH meter.
I got this expensive laboratory model,
but you can get cheaper ones off eBay for about forty dollars.
It's not as good as lab grade stuff but it's still very useable.
Another company that sells a broad range of pH meters at various price points is omega engineering.
I put the link in the video description.
Now let me show you its components.
This blue cylinder is the pH meter which houses all the electronics to measure pH.
And this plastic tube mounts the glass electrode.
It's currently in a potassium chloride storage solution.
Electrodes must be stored in a 4 molar solution of potassium chloride
to keep the probe surface hydrated and active.
Do not let them try out,
and do not store them in distilled water,
as this leaches out the ions in the glass that make it work.
Now let me show you the actual glass electrode.
And here it is, this glass bulb is what measures pH.
If you look inside there is a metal wire,
this wire is the inner reference electrode that measures the potential across the glass.
Now to measure a voltage you need two electrodes
so if I can find it... here... there it is.
Can you see that little nub there?
That is the outer reference electrode.
This electrode works with the inner reference electrode
to measure the potential across the glass and determine the pH.
For the glass electrode to work
you must immerse the electrode all the way up to this reference electrode.
Otherwise you'll get erratic and irreproducible readings if you don't.
Now I’m going to dip the electrode into some distilled water
to wash off the storage solution.
So let's start taking measurements.
But before we can do that we have to calibrate the electrode.
To do a calibration you need to purchase these calibration buffer solutions.
Buffer solutions are carefully prepared samples that have an exactly known pH.
They are very cheap and a must have for any chemist serious about the quality of their pH measurements.
You can buy them from omega engineering as well.
So here is sample of the calibration solution.
This particular one has a pH of 7.01 at 25 Celsius,
which is the current temperature of my lab.
I’m putting it in this beaker 'cause the vial can't hold the electrode by itself.
Now I’m dabbing off the excess water.
Be careful not to touch the glass bulb itself,
you don't want to contaminate or scratch the bulb.
Now simply insert the electrode into the calibration solution
and give it a few minutes to equilibrate.
As you can see the electrode has drifted quite a bit since it's last use so we need to reset it.
On an expensive meter we just press the right commands.
On these smaller ones there is usually an adjustment screw or similar that we can use.
Let me adjust this one here...
There we go we've now calibrated this pH point to its proper reading,
but what about other points?
Let me get a different pH calibration buffer solution and see if the measurement is correct.
This solution has a pH of 4.01.
As you can see here the pH meter says it's off.
The slope of the pH response is not correct.
On a more advanced instrument we could calibrate this new point
and the onboard computer would automatically readjust the slope.
But for this cheaper instrument we don't have that kind of ability.
If we readjusted this point we would misadjust the previous point.
Now this error is acceptable for many experiments as the change in pH is more important,
but in part two of my video on pH measuring tools,
I’ll show you how to use a calibration curve to compensate for this error.
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