please help me on ph meter

[ayien]

hi there! i'm an undergraduate student, working on my final year project on development of a ph meter. i'm asked to develope a ph meter that can measure range of ph from -2 to 19.999 pH. is it possible? i've known some manufacturer like Hanna Instrument claim that their ph meter can measure that range of ph. my problem is, in the mean time i just can explain how the ph range extent from -2 to 16 pH. but how can i prove that theoretically, it can be measure until 19.999 pH? please...anybody got some ideas, how?

 

[jpanhalt]

It's been a few hours since you posted and there are no replys. First, let me say that I cannot answer your question regarding HOW to measure pH over that range. I may be able to give some insight into what that range means. As you no doubt know, pH is the negative log of the hydrogen ion concentration. Thus, for example, a 1 molar (M) concentration of hydrogen ion (H+) is pH 0. A pH of -2 would be a 100 molar concentration. I don't believe that is possible in an aqueous solution, as the concentration of water is only 55.6 M(i.e., 1000/18). In fact, a pH of -2 would imply to me some pretty odd chemistry and/or a very dense solution. That is, at a minimum, you would need 100 moles of a fully ionized acid in a 1-liter volume. Liquid HF, with a formula weight of 20, would have to have a specific gravity of 2 and be fully ionized (which I doubt) to give a pH of -2. Maybe Google can give some insight on what the pH of liquid HF or super acids (see: George Olah, who received the Nobel prize in chemistry) is considered to be.

The other end of the pH scale is a little easier (for me at least) to comprehend. A pH of 20 simply means that there is not a lot of H+ around, i.e., the extent of ionization is very low. Hydrocarbons are said, as I recall, to have pH 's in the range of 20 to 30. Again, such high pH's do not exist in water, but can be approached in other liquids, such as by dissolving sodium (metal) in liquid ammonia.

So, while it is possible to describe what the numbers would mean, it seems unlikely to me that your professor would accept as a final year project a simple definition. Secondly, it would seem a daunting task to try to design a single instrument to measure a dynamic range of 22 orders of magnitude. My first step would be to confer with the professor to be sure I understood the question correctly.

Good luck. John

 

[Tarsil]

hi there! i'm an undergraduate student, working on my final year project on development of a ph meter. i'm asked to develope a ph meter that can measure range of ph from -2 to 19.999 pH. is it possible? i've known some manufacturer like Hanna Instrument claim that their ph meter can measure that range of ph. my problem is, in the mean time i just can explain how the ph range extent from -2 to 16 pH. but how can i prove that theoretically, it can be measure until 19.999 pH? please...anybody got some ideas, how?

Believe me...in real life the pH scale is between 0 and 14. If u have some highly concentrated strong acid sometimes a pH-meter (esp old ones) will give u somwthing like -1.5 or so. But in that case the main ion isn't H3O+ so u can't talk about pH ....u'l talk about p...something. I've only seen that "-" reading on a 30 yo pH.

 

[ayien]

thanx to john n tarsil..now..i think that i might got some ideas how to explain this matter to my professor. just to let u know that i've seen a product specialist from Mettler Toledo..she told me, the range of ph from -2 to 16 are measured in the 24mA current.actually,4mA to 20mA indicate pH 0 to 14.0 to 4mA for -2 and 21 to 24 for 16..

i still hope someone there can come out with a better solution..thank you

 

[jpanhalt]

Hi ayien, I have spent most of my life in a laboratory dealing with equipment sales persons and product specialists. A simple way to cut through the hype is to ask for a demonstration of claims that just don't seem plausible or practical. Failing that, ask for more details, such as what standards the vendor may provide for pH 16 or pH -2, or a copy of the product's user manual for the sensing electrode that woould cover that range. You will probably get referred to a more technical oriented person who will then help separate theory from the practical. My original response was intended only to define what the terms would mean and to give examples. I believe tarsil is completely correct in what the practical application of pH is in aqueous solutions. John

 

[Tarsil]

You should ask for more details. The pH it's useful for indicating acidity in aqueous solutions. I'm a student in Chemical Engineering and I've never heard about a pH lower then 0 and higher then 14. The H+ ion can't exist it's bonded by electrons from a water molecule (=> H3O+) so for talking about pH (as a acidity indicator...actualy about pH3O+) u should have ENOUGHT water molecules for H+ ions. That isn't the case in Highly conc acid solutions. For Acetic Acid solutions u can talk about pCH3COOH2+ function (as an example).
The pH is actualy pH3O+ u could say.

 

[ayien]

ya, i strongly agree with u tarsil n u to john because me myself had calculated the possibility of that mol..but now,how come the manufacturers have come out with the specification that their product (pH meter) can measured that range of pH. i'm sure both of u noticed that.i wonder if there are something about the technical part or programming or just the marketing strategy??? but i'm sure they had done research so that they can come out with this specs otherwise the product would'n be reliable, right? i keep focused on this because actually my project here is sponsored by a private company.they ask me to develope this kind of ph meter. they are choosing this spec (pH range) because that big manufactuer like Hanna produced them.

anyway thank you for ur kindly respond

 

[Tarsil]

Hmm....The pH-meter is composed of a sensor and a high resistance voltmeter or low res ampmeter. The range of ampmeter has to be more then 4-20mA (a standard curent range for industrial unified signal), like 4-24mA. The curent coresponding to 0...14 must be well inside that range so puting a low quality sensor inside a nonaqueous solution of alkali or nonaqueous acid solution (which might give readings outside the 4-20mA range) wont burn the meter. I also think that the precision of the meter is whorse at range limits. Most of the pH -meters won't give readings outside 0-14 range and display Error or something.
The specs, I think, are for "pH-meter's meter" not for the pH-meter.
That's why the specialist from Mettler Toledo told u about 24mA range and so on. Just ask her if the specs are for the meter only!!! not for the sensor/meter/display sistem

 

[ayien]

which one should i use

hi there!! i need suggestion for which microC or microP should i use for my pH meter circuit..actually, i'm not use to this microC or microP. whats the differences? more i read, more i'm become confuse.. my friend is using PIC16FXXX. i want something else... anybody can help me??

Thank you

 

[Nigel Goodwin]

hi there!! i need suggestion for which microC or microP should i use for my pH meter circuit..actually, i'm not use to this microC or microP. whats the differences? more i read, more i'm become confuse.. my friend is using PIC16FXXX. i want something else... anybody can help me??

A micro-controller is a one chip solution, a micro-processor is a many chip solution (and MUCH more expensive).

If your friend is using a PIC, why do you want something different? - PIC's are the most popular choice, and for good reason!.

 

[Tarsil]

hi there!! i need suggestion for which microC or microP should i use for my pH meter circuit..actually, i'm not use to this microC or microP. whats the differences? more i read, more i'm become confuse.. my friend is using PIC16FXXX. i want something else... anybody can help me??

Thank you

uP? It's way to complicated!! Even with a low price uC u can implement lots of functions. Depending on the precision u want to obtain, u can use any uC with a decent onchip ADC or use a specialized ADC chip (more expensive but better precision). U might consider other uC then pic (come much cheaper).
How about the sensor amplifier? U solved that problem? U'l use a voltage or curent reading circuit? Give more details!

 

[ayien]

amplifier..

Hi tarsil!
amplifier?? I got some curcuit but haven't go thru that part yet..anyway..i'm doing voltage correlation with pH. i'm not going to build the electrode.i just bought one from Hanna. when the electrode received, i'll doing test for voltage reading circuit..i thought that's easier..

 

[Tarsil]

Hi tarsil!
amplifier?? I got some curcuit but haven't go thru that part yet..anyway..i'm doing voltage correlation with pH. i'm not going to build the electrode.i just bought one from Hanna. when the electrode received, i'll doing test for voltage reading circuit..i thought that's easier..

I wasn't sugesting to build the electrode . It's almost imposible in nonidustrial conditions. The problem is that I've seen some electrods with built in circuits giving either voltage readings or curent readings in IUS. Prob that the one u bought from Hanna is for lab not auto purposes so it gives voltage readings.
U can make a HIGH impedance curent&voltage amplifier. Get your hands on some constant&bufered pH solutions (like the ones sold by Merk) and measure the voltage given at the output of the amplifier. Get several points and u can aproximate de pH=f(voltage) function. Now imput the voltage from the amplifier/a fraction of it in the ADC chip or directly to the ADC imput of the uC and let the uC do the math and control the display.
If the voltage&curent specs from the sensor are within the ADC chip's imput range (I dought) then u can dich the amplifier.

 

[mira]

hi all I am mongolia and I happy on forum/than i beginer of english language/

 

[ayien]

Thank you

dear tarsil
thank you very much..thats what i'm gonna to do straight away after i got the electrode..maybe for the next 1 month i can't post anything for this project since i have to sit for my semester exam and busy writing the progress report for this project.. wish me luck!!