Understanding the nature of pH has bedeviled beginning (and not-so-beginning) chemistry students for nearly as long as chemistry has had students. It all seems so arbitrary, being the base-10 log of the inverse of hydrogen ion concentration and with a measurement range of 0 to 14. Add to that the electrochemical reactions needed to measure pH electronically, and it’s enough to make your head spin.
Difficulties aside, [Markus Bindhammer] decided to tackle the topic and came up with this interesting digital pH meter as a result. Measuring pH electronically is all about the electrode, or rather a pair of electrodes, one of which is a reference electrode. The potential difference between the electrodes when dipped into the solution under test correlates to the pH of the solution. [Markus] created his electrode by drawing molten antimony into a length of borosilicate glass tubing containing a solid copper wire as a terminal. The reference electrode was made from another piece of glass tubing, also with a copper terminal but filled with a saturated solution of copper(II) sulfate and plugged with a wooden skewer soaked in potassium nitrate.
In theory, this electrode system should result in a linear correlation between the pH of the test solution and the potential difference between the electrodes, easily measured with a multimeter. [Marb]’s results were a little different, though, leading him to use a microcontroller to scale the electrode output and display the pH on an OLED.
The relaxing video below shows the build process and more detail on the electrochemistry involved. It might be worth getting your head around this, since liquid metal batteries based on antimony are becoming a thing.
Was once a working chemist. The whole definition of pH and the general use of the “explaination” that “it’s easier this way” has always been frustrating. Anyone taking chemistry learns about concentrations of stuff. So saying [H+] = 10^-7 should not be confusing you’ve been using exponents for years already. How on earth it is “easier” to shorten that to “7” is beyond me. Like, are you paying to print stuff by the character or what?
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As a later in life ham radio lover the whole dB thing and the argument that “it’s easier to add dB than multiply stuff” is likewise ridiculous. I can add. But I can multiply also. I’m even pretty good at it after (checks notes…) let’s say numerous decades. It isn’t that hard. I’m not saying logs and dB themselves are useless, just that the argument that it’s easier the weirdass backwards archaic way never made sense to me. And seems to have confused the heck out of a lot of others as well.
Oh yeah also kinda a nitpick but definitely illustrates the point. pH doesn’t go from 0-14, strong acids at 10M are pH -1. Again dumb archaic and confusing. At least to me so much more obvious to just say [H+]=10M and leave it at that.
thanks. the folly of teaching a subject from the shortcuts used in practice for mental math rather than first principles.
The log(10) comes from the Nernst equation, a fundamental chemical thermodynamical relationship. For convenience, the natural logarithm is usually converted to the base-10 logarithm. I don’t understand what’s wrong with that. If we are going to start with units, the SI units and a metric system would be a better place to start :D
Sometimes the logarithmic scale better matches up with human experience. We don’t really sense much of anything linearly. and we often get multiplicative ratios showing up.
In the dB terms: can you imagine how much people freak out about little things like Wi-Fi signal strength if the terms were absolute? “I can transmit at 1W of power in the US. Why does my device only transmit at 200mW?”
Well, I will humbly disagree with you on both pH and dB.
I have dealt with a number of systems where pH was very much a desirable measure. For example, when a wide tolerable range is in place and buffered systems both come to mind. Add to that cases where the grunt work is being done by trained trades people rather than chemists (think manufacturing, plant systems [boilers, condensers, lubrication systems, etc], etc) where straightforward numbers matter, and if out of range specified procedure are used to correct, or the chemist comes in.
dB has the historical definition in telephony, but that is, mostly, as far out of use as copper wires for telephones. The convenience there, aside from graphical computation and computational nomograms, is a reasonable model to how the human ear perceives sound is logarithmic with intensity.
Moving wider, it is a dead natural fit for passive filters (20dB/decade for first order), graphically combining filters and transmission lines, and a whole host of other use cases.
Another instance where pH being measured as 0-14 (preferably staying close to 7 in this case!) is by far the easiest way of dealing with it is for those in the aquarium hobby.
I don’t know much about PH or chemistry. My brother had a PH meter with a small glass sphere and apparently it was very expensive, and I think he had to replace it every 2 to 3 years. He used it for fertilizer concentrations in his greenhouse. Linearity is not a problem these days, as a simple uC can easily linearize things. Any Idea about accuracy and longevity of this thing?
A while back I discovered that the magnitude of brightness of stars are not powers of 10. The burden of historical cruft.
https://en.wikipedia.org/wiki/Magnitude_(astronomy)
A non linear function is a non linear function. A uC can’t linearize a non linear function, but can easily calculate any function value. The accuracy can be very high if you calibrate it accordingly (use a common glass electrode ph meter as a counter probe) and do the regression analysis properly. The wooden stick as a porous stopper is not ideal. This must be removed from time to time and fresh copper sulphate solution refilled. As mentioned in the video, the elegance of the antimony electrode lies in its ability to be miniaturized. They have already been used in vivo in medicine to measure the pH value of stomach acid.
And add temperature. Industrial PH probes we use also have temperature compensation. Apparently that has a big effect on the measurement. I’m not sure how he dealt with that issue.
If you had watched the video, you would know.