Can I make a pH meter??

[rskibum]

So I've been measuring my pH with this cheesy kit with a fish in a labcoat on it, and I thought it was right at 8.2 every time. If i remember right the bottle says cresol red, somewhere?

After reading your article on alkalinity I got a sample of my water and did a ph titration, using a meter at work, and 0.1N HNO3. I have all the numbers, but haven't plotted one of those fancy looking curves, Yet!

An another note, I remember not understanding titration curves I think I get it NOW!!!

Anyway, My pH turned out to be 7.92 with the probe. I tested it with another kit, a Tetra kit (green / blue indicator), and could tell it was between 7.7 and 8.0

So my whole point is this

I want to get a electronic pH meter for home, legally, w/o spending much $$, So I was looking on E-bay, but it seems like a hassle, and new ones WOW!

But I was thinking, basically it's just an ohmmeter, and the probe changes resistance based on the concentration of protons? So, why couldn't I use a normal radio shack ohmmeter hooked up to a probe, I could measure resistance at say 7 and 14 (ph) and then calculate a curve based on that. Then just read the sample??

So then I was thinking, maybe I could make a spectrophotometer too, if I hooked up the same ohmmeter to a photoelectric eye from radio shack something that would change resistance with light, then I would need a light source, flashlight if it had the right spectrum, I could probably figure that out with experimentation,, BUT how would I filter out the light to just the wavelength I wanted?? AHH maybe something colored with just the right color, and then place it in the light path!!, but where would I come up with that?? how do they (engineers) do that in say a spec 20??

Any comments on the above ramblings would be appreciated!!

Ryan

 

[randy holmes-farley]

pH meters actually read mv, not ohms. That said, it can probably be done with a very high impedence mv meter, but I couldn't tell you exactly how.

 

[Chucker]

Call it a gut feeling, but I'd bet you'd blow $100 worth of time, money, and frustration to DIY an $80 item.

 

[Anonymous]

hi.
The difficult part of a pH meter is not the meter itself (as Randy said, it just a voltmeter), but the pH probe. The pH probe is essentially a battery that generates electricity, and the potential is dependant on the concentration of proton in the solution. The meter just read the potential, and display it in pH (instead of mV) unit.

A TDS meter or conductivity meter is essentially a ohmmeter. However, for accurate conductivity measurement, AC must be used because of the gradient near the surface of the probe. Almost all ohmmeter are DC-based, so if you use one to measure conductivity, you will see that it does not give you a stable reading. Unlike the pHmeter, the probe is just two pieces of wire with known distance apart, but the meter is more sophicated.

The principle of "how it works" is simple to duplicate, even for a spectrophotometer. However, if you really want to make a worthwhile instrument, you will find that a lot effort is needed to get it to give you precise and accurate measurement.

 

[rskibum]

Tanks guys,

I don't know why I always want to re-invent the wheel. I don't really want to be an instrument designer, I just want to make accurate observations of my system!

Thanks for helping me out!!