cheap pH sensor idea

[Mosaic]

Hello All:
I need to use a uC to monitor a narrow band of pH. Perhaps in the 5 to 8 band with an accuracy of 0.25 , nothing lab grade.

It's for hydroponic applications.

I am thinking to attempt to make a zinc /carbon cell with a TL082 amplifier /buffer to feed the voltage into the uC.

I am not sure if I would see any negative voltages, but I may need a +/- 12V supply for the op amp. then I need to have the uC read a possible -ve voltage. Any help is appreciated.

 

[aljamri]

Hi,

Although I'm late, but as this is the region where I'm working, I'm interested if you achieved any progress. All those years I've got theoretical idea to make Home Made sensor replica, but for vibration and pH sensors I've got no clue.

Again I'm ready to help as I've got a lot of old pH sensors (different makes), and ready to do whatever test you suggest and share the results.

 

[cowboybob]

What are you growing that will tolerate a pH of 5?

 

[jpanhalt]

Blueberries? (https://shanna11.hubpages.com/hub/Best-Plants-for-Acid-Soil)

Seriously, since I am a wannabe chemist, I think this is a great idea to pursue. How far have you gotten with the idea? What cells have you looked at?

Sorry I missed the original post back in August, but that was around the time of my anniversary, and the emotion must have overcome me.

John

 

[KeepItSimpleStupid]

How is $8.00 for a commercial one for soil. https://www.amazon.com/dp/B0087ZSIN...e=asn&creative=395105&creativeASIN=B0087ZSINQ

 

[Mr RB]

Don't they have to be used quickly and carefully cleaned etc, from what I know they can't be left in the liquid due to issues with corrosion and salts buildup damaging the sensor or accuracy. So a mcircontroller based one would only be as good as any other device you temporarily dip and test?

 

[aljamri]

Don't they have to be used quickly and carefully cleaned etc, from what I know they can't be left in the liquid due to issues with corrosion and salts buildup damaging the sensor or accuracy. So a mcircontroller based one would only be as good as any other device you temporarily dip and test?

that's right, it has to be always in wet medium with specific pH. For accurate pH measurements, temperature has to be considered.

We have old pH calibrator, it has two options to use, one of them is to inject mV into the circuit instead of the pH sensor. What is the relation, I don't know ?

 

[jpanhalt]

Are you (Mr RB, aljamri, and maybe others) talking about the ion-selective glass electrodes one sees in a typical pH meter?

I believe the OP was referring to a slightly different approach in which the voltage of an electrochemical cell operating as a battery is affected by the pH. That is where the "zinc/carbon cell" came into play. There is no ion-selective membrane in that approach. It appears he is not interested in pursuing that approach, though.

John

 

[aljamri]

Are you (Mr RB, aljamri, and maybe others) talking about the ion-selective glass electrodes one sees in a typical pH meter?

YES, that's what we are using to measure the drinking water pH in Water Blending Stations.

 

[cowboybob]

Are you (Mr RB, aljamri, and maybe others) talking about the ion-selective glass electrodes one sees in a typical pH meter?

I believe the OP was referring to a slightly different approach in which the voltage of an electrochemical cell operating as a battery is affected by the pH. That is where the "zinc/carbon cell" came into play. There is no ion-selective membrane in that approach. It appears he is not interested in pursuing that approach, though.

John

I agree w/ jpanhalt.

Don't know of a reliable way to measure pH other than the ion-selective glass electrode method, especially when dealing with any solution "hovering" around 5-8 pH. There is, however, a probe that uses an ion-selective FET and, although it can be damaged by ammonia, is a non-glass bulb alternative. But you have to buy the whole probe assembly, making this NOT a "cheap pH sensor" alternative.

Any probe not electrically isolated from the tested medium would, perforce, be introducing an electrical potential into the fluid. This cannot help but cause some manner of electrolysis, thereby disrupting the ratio of H cations and skewing the pH (immediately surrounding the probe) and, thus, skewing the readout value, rendering the reading meaningless.

At least that's my understanding of the process.

 

[Mosaic]

Hello all:

I did some more thinking on this....rather than zinc carbon , perhaps 308 s/s TIG rod and carbon maybe a more stable approach regarding combating galvanics etc. My application will cause the sensor to see liquid every 8 hours for about 2 minutes. So electrolytic corrosion and concurrent drift should be minimized.

Carbon is quite noble and s/s should give a decent lifetime given the lowish voltages generated. Since the chemical composition of the nutrients would be reasonably stable as they are refreshed every few days I hope to establish a correlation between pH and galvanic voltage via empirical experiments.

@ KISS those meters appear to be based on galvanics as well. But they need strong voltages ( probes with very dissimilar electronegativity) to drive the deflection meter. Thus they are at risk for significant galvanic corrosion over time. I don't see why doing this with more noble materials driving a decent TL072 opamp buffer can't provide a uC with useful information on pH.

 

[jpanhalt]

...perhaps 308 s/s TIG rod

I am not sure what you mean? All of the TIG welding electrodes I have are tungsten. Some have a little thorium in them. Do you mean a filler rod that is used for welding 308 SS?

Do you get a measurable voltage in a cell with carbon? Will the very small amount of chromium potentially released from a SS electrode be a concern? Are highly alloyed electrodes a potential source of error as the constituents of the alloy may corrode (oxidize) at different rates and affect surface properties.

I would tend to use something that is non-toxic (even though the amount is very low), common, and easily replaced. Aluminum might work, and if you have access to welding supplies, filler rod for TIG or MIG might be an easy source.

What are others who use this method using for electrodes?

John

 

[Mosaic]

yes u r right 308 s/s filler rod.
Since the s/s rod is exposed to air 99% of the time, i don't think corrosion is an issue. s/s is a food grade item and non toxic, for casual contact.

I haven't tried to check the p.d. yet...mebbe this weekend.

 

[jpanhalt]

In a battery, like you are planning to make, oxidation occurs at the anode in direct proportion to the current produced (Faraday). I think you will be better off getting a working model based on a conventional battery using a simple anode, such as zinc or aluminum. SS, even 308, may prove to be complicated. What effect will the oxidized chromium on the surface have on the potential you measure? How will that change with time?

The saving grace is that with modern devices, you will be able to measure the cell voltage with at least (hopefully) negligible current flow.

John

 

[alec_t]

I haven't tried to check the p.d. yet...mebbe this weekend.

Keep us posted. I'm interested to know if these electrodes work.(Don't fancy the expense and fuss of the normal glass electrodes.)

 

[Mosaic]

Will do, I also have some new 14 ga nichrome wire on hand.... might make a decent electrode as well.. Now to find my micropencil leads to use as the carbon rod.