Ion-specifc electrode array: control ALL water parameters

[Foxy Brown]

My original post for this was in the DIY forum, but it would be nice if all you fellow chemistry buffs could have a look. Here's the link the the original:

http://www.reefcentral.com/forums/showthread.php?p=17232298&posted=1#post17232298

Hi all, I've been working slowly on a long term project to build a system to measure water parameters, and to link the measurement to a computer controlled dosing rig. The trick is: I don't want to just measure pH, or ORP but EVERYTHING. Before I start, I do hold graduate degrees in both chemistry and engineering, so I know a thing or two on the subject. It's mostly practical fish-keeping experience I lack, as I'm still relatively new.

Here's the idea: We ALL need to measure some basic parameters, and there are others we would all like.

Good: pH, ORP, Salinity (conductivity) Alk (CO3), Hardness (Ca)
Better: Good + NH4, PO4, Mg
Best: Better + Trace elements (I, Sr, etc.)

You can measure all of these with ion-specific electrodes BUT there are usually errors. These errors are due to cross-sensitivity (NH3 with K or NO3 with Cl for example) That tends to limit the accuracy.

What I've been working on is, to pair-up these electrodes so that each pair would measure both the desired ion and its interfering partner. So here's how you start:

First you make a reference electrode (Ag wire coated in AgCl - very easy to make). Then add ORP electrode (Gold or platinum wire) and measure the voltage between the 2 electrodes to have ORP. Same thing for a glass pH electrode. Conductivity is also easy and all 3 above are commercially available.

Now for the ion-specific electrodes, you coat a silver wire with a special solution of PVC containing a few additives (called ionophores - also commercially available), and you measure the voltage vs the REF electrode. You must measure them in pairs though and you can use a simple equation to calculate the concentration of BOTH. The pairs are: (Mg + Ca, NH4 + K, NO3 + I, PO3 + SO4) and then add 1 for HCO3.

Sounds like alot of electrodes yes, but they are only tiny coated wires and when done, you can control ALL water parameters. Conductivity = dose DI water / concentrated salt solution, pH = dose HBO3/NaB4O7 or H2SO4/Na2SO4, ORP = dose ascorbic acid (Vitamin C) / ozone, Mg = dose MgSO4, Ca = dose CaCl2, Alk = dose NaHCO3, NO3 or PO4 = turn on a reactor (Zeolite/GFO) or dose vodka if you prefer, etc... you get the idea.

The interface is easy, you just need to build some FET-input instrumentation amplifiers (easy), hook them to the ADCs of a microcontroller or PLC (very easy), do a bit of programming (less easy but not hard) and use it to turn on dosing pumps. The last step is calibrating everything, which is not a bad as it sounds.

Anyone got any comments or feel like helping out? What I could really use is someone with a good tank to test it on (don't hook-up the pumps, because I don't want to mess with anyone's system), just see if it gets the diagnosis right, and gives the correct response. That, and maybe someone with more programming experience than I have.

Feel free to post comments here, or in the DIY thread linked above.

Thanks!

 

[Randy Holmes-Farley]

You cannot measure alkalinity with an ISE. It is more than carbonate or bicarbonate alone.

 

[Boomer]

Yup, it can only be measured with Closed Cell Alkalinity Titration. It is carried out by the means of potentiometric titration of a weighed sample of seawater in a closed container not affected by the atmosphere condition's. Hydrochloric acid is added to the sample and the change in the emf is measured using a std pH probe, where the temp is also held constant. In short, the Alk is reached when all the HCO3- and CO3-- is converted to CO2. A PC then runs a bunch of math equations which gives the Alk. Two years ago we had a guy here that designed and built one that also auto-added buffer to maintain Alk.


Alk /meter/titration
http://reefcentral.com/forums/showthread.php?s=&threadid=1073780

And measuring Ca++ and Mg++ can get tricky in seawater if you do not know what you are doing. And I do not know of any means to measure phosphate or sulfate with a ISE, as there are no such probes I know of.

 

[Foxy Brown]

Hmmm, well you're right about the lack of commercial probes for phosphate and sulfate, but there are commercial ionophores out there, There have also been successful applications of custom-made probes for biological samples (plants mostly). Making them is actually quite simple, in fact. Of course, you will only dectect inorganic phosphate in any case, but these are the least important to the process as you can just run your GFO reactor continually.

I certainly don't want to sound like I'm contradicting the experts, but I would like to hear more about the Alk problem. I seems to me that if you know CO3 or HCO3 concentration and pH you can back-out the concentration of the other two simply using the pKa equations, which is actually fairly simple algebra. Is there something else I'm missing here?

Please feel free to hit me with the hard science and don't just say "you can't do it", because I have seen papers where they in fact DO perform these measurements in this way. I could send you the PDFs from the journals if you're interested.

Ca and Mg aren't so bad since you will have a pretty rough idea of everything else that's there in large quantities, and you can include them as the matrix in the calibration solutions (ionic strength and so-forth).

 

[Boomer]

Please feel free to hit me with the hard science and don't just say "you can't do it", because I have seen papers where they in fact DO perform these measurements in this way. I could send you the PDFs from the journals if you're interested.

OK, show me a paper for Seawater where a ISE probe is used. All oceanographic institutes and labs use only Closed Cell Alkalinity Titration. Then you need to look at what is seawater Alk. Seawater is NOT FW. Then for SW what are the interfering ions that may affect the ISE probe.

What is Alkalinity
http://www.advancedaquarist.com/2002/2/chemistry

See ref listed
http://en.wikipedia.org/wiki/Alkalinity

Determination of total alkalinity in sea water
http://cdiac.ornl.gov/ftp/cdiac74/sop03.pdf

High precision measurements of alkalinity and total carbon dioxide in seawater by potentiometric titration — 1. Presence of unknown protolyte(s)?
http://www.sciencedirect.com/scienc...serid=10&md5=3d01c439b6d0d69334d869506f11d026

CO2 in seawater: equilibrium, kinetics, isotopes
http://books.google.com/books?id=g3...ge&q=measuring alkalinity in seawater&f=false

Basic Equations for the Determination of the Total Alkalinity
(TA) and Total Carbon Dioxide (TCO2) in Natural Waters
http://www.hbustos.com/files/AT_OCQUIM.pdf


I seems to me that if you know CO3 or HCO3 concentration and pH you can back-out the concentration of the other two simply using the pKa equations, which actually fairly simple algebra.

And these solve for what ? Carbonate Alk, as Total Carbonate or separately as Bicarbonate or Carbonate Alk and CO2 but not for Total Alk. And pKa's change in SW as a function of Salinity and temp. The pKa1 and pKa2 are not the same at 34 ppt as they are at 35 ppt @ 25 C and as C changes even at the same Salinity the pKa's also change as a function of temp. So, how are you going to make/build a ISE probe that corrects for Salinity, unless a program runs it through at Conductivity reading for Salinity to correct it and calculates new pKa's. And all of these also leave out what ? Borate Alk.

Nobody uses a ISE in seawater for Alk because they do not work right.

 

[Boomer]

Making them is actually quite simple, in fact. Of course, you will only dectect inorganic phosphate in any case

Making and getting them to read right in seawater are two different issues

 

[elijaher]

Are you going to have any addition for sulfate, chlorine, oxygen, co2 and halocarbon probe to measure?

 

[Foxy Brown]

I think you're missing the point completely.

If you know the concentration of H2CO3, HCO3 or CO3 and the pH then you know them all, as long as you have pKa's... that's intro acid-base chemistry. The only thing you wont know is dissolved CO2.

Yes, there are different values for SW and FW... so use the right one.
Yes, pKa varies with salinity and temperature. So what? We know what the salinity and temperature are, so we can account for that.

Yes there are interfering ions that throw off the measurement. That's what all those other probes are for... we measure those too, and because we cross-calibrate for them in the beginning, their effects can be removed. The point is to use the whole array of ISEs, pH, salinity, etc. at the same time exactly for that reason.

It's the same principle as an electronic tongue, and its all very well defined. I wrote my PhD thesis on it... though I admit, not on seawater measurements specifically. There may be something special about seawater that can't be overcome, but you haven't mentioned anything yet.

The point of the whole idea is this...

If you have experiment that's dependent on several variables that you can't control or eliminate, then you measure them instead. The equations are already figured out, as you so kindly linked for me (Thanks by the way! ).

The reason ISEs don't work right, is because those experiments just stick in a probe and read a number. As I stated in the very beginning that's not good enough, but that doesn't mean they just flat don't work... you just have to be smart and use them correctly, taking their limitations into account by measuring the interfering ions too.

 

[Foxy Brown]

Are you going to have any addition for sulfate, chlorine, oxygen, co2 and halocarbon probe to measure?

I already proposed sulfate and chloride (though that one is hidden in there and is not obvious). Dissolved O2 is commercially available, so no problem to add it if desired.

CO2 is the one that seems to be the big debate... and as for halocarbon, well TBH I'm not sure what you mean.

 

[Foxy Brown]

Also, please remember that we don't need most of these to 5 significant figures... 2 or 3 is usually enough. Especially since we wont be able to practically control it better than that anyway unless you have enormous water volumes.

Besides natural seawater isn't exactly the same all the time everywhere anyway, so small fluctuations are normal... the idea is just to keep your system within a preset range of conditions.

 

[Randy Holmes-Farley]

I think you're missing the point completely.

Think so? Bear in mind that Boomer and I have discussed these possibilities over the years with many people. This is neither a new idea, nor one that will be easily implemented. IT IS VERY MUCH HARDER TO ACCURATELY IMPLEMENT THAN YOU ARE SUGGESTING. Companies like American Marine have tried hard (very hard, with our help, in some cases) to come up with calcium, nitrate and alk probes. The first two are commercial, but are really rather hard to implement and are not continuous reading probes, but rather require careful seawater calibration each time you use them.

Let's back up and have you explain how you will get total alkalinity (TA) using ion selective electrodes:

TA = [HCO3-] + 2[CO3--] + [B(OH)4-] + [OH-] + [Si(OH)3O-] + [MgOH+] + [HPO4--] + 2[PO4---] - [H+]

Yes, you can theoretically have a bicarbonate probe (although you'd need to invent one that works in seawater), and perhaps knowing it and pH will give you enough to be very useful, but it will not be alkalinity.

 

[Foxy Brown]

I think you're missing the point completely.

Think so? Bear in mind that Boomer and I have discussed these possibilities over the years with many people. This is neither a new idea, nor one that will be easily implemented. IT IS VERY MUCH HARDER TO ACCURATELY IMPLEMENT THAN YOU ARE SUGGESTING. Companies like American Marine have tried hard (very hard, with our help, in some cases) to come up with calcium, nitrate and alk probes. The first two are commercial, but are really rather hard to implement and are not continuous reading probes, but rather require careful seawater calibration each time you use them.

Let's back up and have you explain how you will get total alkalinity (TA) using ion selective electrodes:

TA = [HCO3-] + 2[CO3--] + [B(OH)4-] + [OH-] + [Si(OH)3O-] + [MgOH+] + [HPO4--] + 2[PO4---] - [H+]

Yes, you can theoretically have a bicarbonate probe (although you'd need to invent one that works in seawater), and perhaps knowing it and pH will give you enough to be very useful, but it will not be alkalinity.

Please don't take offense, as it really is advice and experience I was looking for... but also please don't just dismiss it out of hand. I have 3 patents on things people said couldn't be done.

I would love some specifics about your work... what ionophores did you use? What membrane composition (PVC, Silicone, Urishi, other?) What plasticizers? Did you use internal filling solutions, and did you include anything in the solution to counter interferences? How long ago was this? You know there have been a lot of improvements in ISE and particularly ISFETs in the last few years.

As for the TA calculation, the only ones I haven't already proposed measuring are Si and B. Si is almost inconsequential with respect to the others... The concentrations of all acid-base pairs may be known by measuring 1 member. (For H+ and OH- are both known with 1 pH measurement... the same math works for the others only using Ka instead of Kw.) Adding a commercial BOH4 electrode is not impossible, but I was planning using Borate/Boric acid anyway for adjusting pH (as with most commercial buffers) since the borate buffer system is so well suited to that pH range.

Perhaps calling it carbonate hardness (which seems to be what most reef guys want anyway) would be better than calling is alkalinity... especially since I'm talking about dosing NaHCO3 for it, so KH is really all I want.

Please don't get hung up on the TA problem. Is there any reason that KH isn't good enough for our marine aquarium needs? Has anyone you worked with tried to measure all these parameters together and use and computer algorithm to back-out the interference effects all at once, or was it piecemeal (which I already know wont work)?

OH and FYI, I'm already planning on recalibrating against a reference solution between readings.

 

[Randy Holmes-Farley]

Frankly, it is a research program to develop new ISEs. It is far more complicated than putting an ionophore into a polymer. I think you have a small chance of success using commercial probes, although as I pointed out, companies have worked on this and failed to achieve even a single measurement as you suggest for many (unless you consider the way the Pinpoint calcium and nitrate probes are used to be a success). I give it no chance if you have to DIY individual ISE probes. IMO, that is not a DIY project, even for a chemist or chemical engineer.

That said, while my professional work is designing ion selective polymers for many ions (commercial successes of which include Renagel, Renvela, and Welchol/Cholestagel), that is not what I personally advised on that I referred to above. In my case, it was how to design an alk probe based on measuring CO2 and pH, not with an ISE. The company I discussed this with certainly understood bicarbonate was a good way to go, but did not have access to any ISE that worked for bicarbonate in seawater. I know Boomer has other experiences discussing ISE’s with companies.

As I mentioned, even knowing bicarbonate alone could be perfectly adequate for reefers. There is nothing magic about alkalinity. But I consider that a hard problem all by itself, and do not know how you'd proceed.

 

[Foxy Brown]

Oh I fully agree that it's a real research program to develop ISEs. You need to figure out the best matrix, ionophore, neutral carrier, etc., etc... then optimize the concentrations of each for best response, selectivity, stability etc. Not an easy project... fortunately I have some experience.

My PhD project was developing lab-on-chip chemical microsensors for the US DOE. That project used optical resonance techniques that are FAR more precise than electronic methods. That project was for detecting chemical and biological warfare agents, explosives and such. Of course, at that time I had access to a multimillion dollar fab center. Now I have to work on the macro scale, but the principles are the same. (Wires and op-amps are much easier to work with than lasers and waveguides.) It was actually that work which gave me the idea.

Fortunately there are hundreds of researchers who work on this very thing and publish their recipes. You can even buy the ionophore from places like Sigma-aldrich, so most of the work is already done except for tinkering. I've used such recipes to make electrodes before and they worked just fine. I've even seen them used in blood and intracellular fluids which is at least a problematic as seawater, though again not exactly the same problems.

I also agree that alk is a tough problem. Since alkalinity is a just a measure of buffering capacity - titration makes alot of sense... and I looked at that titatration gadget of yours... it's very nice. But if you're actively controlling pH, then you've just added a technological version of a buffer so that's not so important to measure anymore. What is more practically important for this purpose is the available carbonate for photosynthesis, calcification, etc. But as you'll recognize, it's much more tractable.

As for the rest, I think most people would agree that being able to control pH, ORP, salinity along with Ca, Mg and being able to get regular readings of ammonia and nitrate would already a big win even if ALK or KH had to be an ongoing project.... but from the sound of it your titrator could suffice.

 

[Boomer]

I'll be short You will never get an ISE to work right in seawater for Alk, even bicarb or carb alk There are seawater pCO2 probes but they cost an arm and leg and is how ocean water CO2 is monitored on buoys. Now, if you know the pH and CO2 you can calculate Carbonate Alk.

Now lets pretend one has all these probes and they all work as planned. I'll give you the benefit of the doubt. Who in their right mine is going to spend ~thousands of dollars to monitor their tank with them ?

 

[tonyf]

I'd like to see Foxy-Brown get some support here. In my professional field, there are too many people that say "you can't do that!" And then that's precisely what we go and do ... succesfully.

Let's get this thread out of the box and start imagining how these things might come about. Scientific discovery would still be in Dark Ages otherwise.

Light it up Foxy!

Tone

 

[Randy Holmes-Farley]

I fell quite confident to speak for Boomer as well as myself that we would both be thrilled if this would work, at least if it were affordable, and we will both provide any useful advice we might have that Foxy wants in order to proceed. We are just being realistic based on others attempts to use ISEs in seawater. Some work (like fluoride) and many do not.

In any case, if it is approached as a chemical engineering research program, that is a fine endeavor.

 

[lex211]

I'd like to see Foxy-Brown get some support here. In my professional field, there are too many people that say "you can't do that!" And then that's precisely what we go and do ... succesfully.

Let's get this thread out of the box and start imagining how these things might come about. Scientific discovery would still be in Dark Ages otherwise.

Light it up Foxy!

Tone

+1 on attitude :this would a great development for people like me who have problems with the color transition in test ,as someone stated its going to cost big money:uzi::celeb3:

 

[Foxy Brown]

I don't see why this should cost such big money. I'll admit that initial development wont be exactly cheap (due to the need to buy chemicals and such) but most companies are trying to both recoup their development costs and make a significant profits in a relatively small market, which necessitates very high profit margins.

I don't want to take the project on for profit but for the challenge and the potential reward to the hobby. The actual costs for the materials in the probes are not that high per probe, especially if you look at the amount of active material in membrane. For the meter itself.... the planned design is to connect the electrode and reference each to one gate of a matched quad FET bridge which will act as a zero-drift buffer, and read it out with a 16bit low-drift differential input I2C ADC, and poll it with the microcontroller. That's only about $5 in ICs per probe.

The real trick is in the algorithm that calculates the concentrations based on looking at the whole picture, and the calibration scheme that you have to use which is a bit complex. It doesn't need to LOOK complicated though... "insert probe into bottle 1 and push button, after the beep, rinse with DI water and insert in bottle 2, then push button". The microcontroller's program can take care of the heavy math.

As for seawater probes and CO2, there are many examples. It may be as simple as a pH probe suck in a container of an alkaline solution with a silicone membrane between the solution and the tank water. Since only gas will permeate the membrane, only NH3 and CO2 will affect its pH (and I'm already measuring ammonia). Or, it may be more complex and require the generation of OH electrochemically, and performing a coulometric titration. Take for example this article: http://www.sciencedirect.com/scienc...serid=10&md5=6c777ea0e5a1cf683c7c81747b581283. You might have to change filling solutions occasionally, but that's not difficult.

And just for levity: Do squirrels come out of their trees to poo? I always wondered that.

 

[HighlandReefer]

And just for levity: Do squirrels come out of their trees to poo? I always wondered that.

Now, I can anwer that question, but as far as the rest of the conversations you are having, that's a different story. :lol:

I actually rasied a baby squirel from before its eyes were open when I was kid.

I am enjoying this thread though.