Have you learnt find out how to use a pH meter accurately? The pH meter is perhaps the least understood and most abused piece of kit within the lab.
Why such neglect?
In all probability as a result of the pH meter is an secret agent from chemistry—and solely a uncommon biologist likes chemistry.
A uncared for pH meter means much less dependable experiments, poor reproducibility, and your time wasted.
Allow us to, then, get previous our prejudice and discover ways to use a pH meter in 4 easy steps. We’ll additionally be taught fairly a bit in regards to the design and performance of pH meters.
How Does a pH Meter Work?
We will solely do a half-baked job of taking care of any piece of kit until we perceive the way it works. So how does a pH meter work?
They’re sophisticated, however you don’t want an engineer’s understanding of them to take care of and use them. Take a look at the illustration under in Determine 1.
In a nutshell, the glass probe of the pH meter incorporates two electrodes:
- A reference electrode.
- A conductor electrode.
The reference electrode by no means comes into contact with our check options (until the pH meter is of the one junction sort—see later). It has a secure and fixed electrical potential.
The conductor electrode is often immersed in saturated (3M) potassium chloride. It does come into contact with our check options by way of a porous membrane through which it’s housed.
Two extra essential factors:
- The potassium chloride answer is conductive and has a impartial pH (7.0).
- The porous membrane is selective in the direction of hydrogen ions.
When the conductor electrode is dipped into our check answer, ion trade takes place by way of the porous membrane.
That’s to say, as a result of our check answer and the saturated potassium chloride have a special pH, and since the porous membrane permits the passage of hydrogen ions by way of it, hydrogen ions transfer to or from the check answer till equilibrium is achieved.
Bear in mind, pH is a measure of hydrogen ion focus.
This accumulation or trip of hydrogen ions from the conductor electrode adjustments its chemical potential.
The chemical potential of the conductor electrode is measured and in comparison with the chemical potential of the reference electrode.
The distinction in chemical potential between the 2 electrodes is named their potential distinction.
And the potential distinction might be transformed to pH utilizing the Nernst equation. (Thank goodness we don’t have to do that bit!)
Informed you—sophisticated stuff!
So let’s transfer on to find out how to use a pH meter.
4 Steps For How To Use a pH Meter
1. Take Care of the pH Meter Probe
The pH meter probe ought to by no means dry out.
Ought to the probe dry out, it is going to break or no less than develop into unreliable. I as soon as had a run of unsuccessful yeast transformations as a result of the pH meter was improper by 0.2. Ouch!
You may refill the pH meter probe with saturated potassium chloride. However be sure you verify that the pH meter nonetheless works by measuring the pH of a number of reference options that the pH of.
To stop the answer contained in the probe from leaking and/or evaporating, the probe needs to be saved in an answer of 3M potassium chloride (KCl).
It ought to not be saved in H2O, and particularly not distilled H2O, as a result of we want the conductive KCl answer. Dialyzing all of it away shouldn’t be good.
Tris Base and Proteins Might Shorten the Lifespan of Your pH Meter
There are two forms of pH meters:
- Single junction.
- Double junction.
Double junction variations are designed such that our check answer by no means comes into contact with the answer through which the reference is immersed.
Key truth: the answer through which the reference electrode is immersed is silver chloride (AgCl), not KCl.
For single junction variations, this isn’t the case. Our check answer will come into contact with AgCl.
One other key truth: tris base reacts with silver cations (Ag+) to provide junction-blocking precipitates. So, actually, do a number of proteins.
A blocked junction means a poorly pH meter.
So, double-check what sort of pH meter is in your lab earlier than utilizing it.
Briefly—don’t use single-junction pH meters with tris-based buffers or protein options.
2. Calibrate Your pH Meter Usually
You calibrate the spectrophotometer every time you measure the optical density of a microbial tradition through the use of a clean.
You calibrate a spectrophotometer earlier than each measurement, actually.
The identical needs to be performed with a pH meter.
Calibrating a pH meter is completed utilizing calibration options which have an accurately-known pH. The method needs to be carried out each time you employ the instrument or no less than each morning the pH meter shall be used.
The calibration options are often pH 4.0, 7.0, and 10.0. You may even purchase little calibration tablets that dissolve in a recognized quantity of water to make the calibration options (Determine 2).
Calibrate the pH meter with no less than two totally different options encompassing the ultimate desired pH of your options. Higher but, calibrate it with all three!
3. Use a pH Meter Appropriately
For the reason that probe ought to at all times be moist, whenever you pH an answer you’ve two selections:
- Hold the probe submerged in your answer throughout pH-ing.
- Take the probe out, wash it in deionized water, blot it with paper, put it within the storage answer, and wash once more earlier than placing it again into your answer.
The primary selection is okay.
Care should be taken whereas utilizing a magnetic stirrer to not smash the probe. A medical pupil in our lab smashed a probe to items throughout their first day. Not an auspicious begin.
4. Beware! Bodily Chemistry Exists
At all times pH your options on the temperature at which you propose to make use of them.
As a result of pH is temperature-dependent. Moreover, diluting sturdy acids into water is (often) significantly exothermic. Your answer will often heat up by a couple of levels. This can change its pH.
And what did I simply say? pH is temperature-dependent.
So, as soon as your check answer cools, it is going to have a special pH than the one you measured. So be affected person and permit it to chill (or actively cool it with some ice) earlier than scribbling the pH worth on the falcon tube.
Strive to not use concentrated acids to pH your options—they have an inclination to vary the pH of most options instantly. So instantly that you could overshoot simply.
And all of us love beginning over once more once we are already sweating in latex gloves and simply need to go residence.
At all times add diluted acid to your check answer and never the opposite method round. Dilution of acids is strongly exothermic as already talked about. Diluting the acid into your water-based answer, which is current in giant extra, means there’s loads of water to absorb the warmth that’s liberated.
Including drops of water to sturdy acid might generate sufficient warmth to boil the water and spray it over you.
(Editor’s word: please at all times add all the things to water.)
Physics Additionally Exists
Depart simply sufficient spare quantity for including acid or base to your answer, until you need to begin over again!
Assist! What to Do When Crystals Kind on the pH Meter
These crystals are potassium chloride and so they type when the potassium chloride answer evaporates.
It’s a typical sufficient incidence, so don’t panic. Right here’s what to do:
- Gently scrape off any exterior crystals.
- Soak the probe in heat water to dissolve the crystals.
- Refill the probe with 3M KCl.
- Verify the operate of the probe by measuring the pH of calibration options.
There, that wasn’t so scary, was it?
Use a pH Meter in Abstract
Hopefully, after studying this text, the internal workings of a pH meter are rather less of a black field to you. And also you now have loads of suggestions and methods on find out how to use a pH meter accurately.
No excuses for letting it get all crusty now.
Even higher—you are actually geared up with some sensible data to pH your options like a professional!
Like what you’ve learn? Have I missed some essential factors out? Tell us within the feedback under. We do pay attention!
Initially printed March 2013. Reviewed and republished, July 2022.
- pH Measurement Handbook. Thermo Scientific. Accessed June 2022