Reply With Quote
distilled water= no
Hi presently beside using buffer water sold in LFS to calibrate, can we use mineral or distile water to calibrate the setting back?
distilled water= no
Jinxed fishy photographer
[Specialist]
pure water doesn't necessary have a pH of 7.0
you need the calibration solutions.
ck
The main purpose of using calibration fluid is becos the pH of tat fluid is already known so tat u can adjust the pen's reading to this value. I dun see any reason y mineral water or distilled water can't be used for calibration purpose if e pH of these liquid is also known beforehand. This is easily achieved by taking e pH reading of these liquid with a calibrated pen while u still hv the original calibration fluid. If e pH of mineral water or distilled water is 6.9 or 7.1, when u calibrate the pen, just adjust the reading to 6.9 or 7.1 respectively instead of 7.0 by using the calibration fluid. The most important thing to watch out for is the pH of the mineral water or distilled water must not vary over a period of time. I'm not sure whether e pH of mineral water will vary over timer but I'm sure the pH of distilled water will not.
Any 1 who says mineral water or distilled water can't be used to calibrate pH pen pls state yr reasons. Dun jus say NO.
NO...[]
Pure water expose to ambient air will quickly dissolve CO2. The water can dissolve up to 0.53ppm of CO2. This CO2 amount is sufficient to drop the pH from 7.0 to 5.65.
But the problem is we will never know exactly how much CO2 is dissolved. Therefore, any "pure" water samples can have pH from 5.65 to 7.0.
Convinced?
BC
OTOH, buffered solutions are used for calibration.
Buffered solutions of certain make-up will always maintain certain pH. This buffered solutions will buffer against pH fluctuation from small amount acid(CO2 included) or alkaline, thus give you pretty good pH reference.
BC
Jinxed fishy photographer
[Specialist]
Wow.
As far as I understand, pH meters usually have fixed calibration points of 4, 7 and 10. If you a calibration solution that is off (say 6.9), it will be taken that that is 7.0. So your reading is off because your calibration is off. It doesn't adjust itself to understand that the solution is exactly 6.9, which is why you are calibrating it to fixed pH solutions in the first place. It is like you use a solution of pH 8.12 and expect the meter to know that you are using a pH 8.12 as a calibration point. 4, 7 and 10 are factory settings, cannot change one.
The calibrating solutions are buffered. This means that the pH stays almost constant. Mineral water and distilled water is not. Any slightest acid/base (even Co2, and you ensure how much Co2 stays in the water?) will cause a change in pH. Since it will probably not be at pH 4, 7 or 10, it cannot be used as a calibrating solution.
Also, anyone using an uncalibrated pH meter and claim that it is accurate, even though it is new from the box is fooling himself. pH readings drift (even though though it is sitting nicely in the box), always calibrate before you test.
Sheesh, this is probably my longest answer to a one-line question. []
ck
No. I'm not convinced. Are u telling me if I place a bottle of pure water over time, the pH will fluctuate btw 5.65 n 7.0 ? U must be kidding ! If that's the case, LFS dun hv to sell pH down soln liao.----------------
On 3/7/2003 10:08:16 AM
NO...[
Pure water expose to ambient air will quickly dissolve CO2. The water can dissolve up to 0.53ppm of CO2. This CO2 amount is sufficient to drop the pH from 7.0 to 5.65.
But the problem is we will never know exactly how much CO2 is dissolved. Therefore, any "pure" water samples can have pH from 5.65 to 7.0.
Convinced?
BC
----------------
Moreover, distilled water comes in bottle with cap. How much CO2 can u find inside the bottle to lower the pH to significant amount ?
If wat u said abt the fixed calibration pts at 4,7 n 10 is true, then I fully agree with u, a pH 7.0 fluid is a must. Then again I'm puzzled on 1 thing. If u only use pH 7.0 to calibrate, y are there calibration pts at 4 n 10 ?----------------
On 3/7/2003 10:27:02 AM
Wow.
As far as I understand, pH meters usually have fixed calibration points of 4, 7 and 10. If you a calibration solution that is off (say 6.9), it will be taken that that is 7.0. So your reading is off because your calibration is off. It doesn't adjust itself to understand that the solution is exactly 6.9, which is why you are calibrating it to fixed pH solutions in the first place. It is like you use a solution of pH 8.12 and expect the meter to know that you are using a pH 8.12 as a calibration point. 4, 7 and 10 are factory settings, cannot change one.
The calibrating solutions are buffered. This means that the pH stays almost constant. Mineral water and distilled water is not. Any slightest acid/base (even Co2, and you ensure how much Co2 stays in the water?) will cause a change in pH. Since it will probably not be at pH 4, 7 or 10, it cannot be used as a calibrating solution.
Also, anyone using an uncalibrated pH meter and claim that it is accurate, even though it is new from the box is fooling himself. pH readings drift (even though though it is sitting nicely in the box), always calibrate before you test.
Sheesh, this is probably my longest answer to a one-line question. [
ck
----------------
Any 1 actually measured the pH of distilled water over time ? Does it vary or stay const ? Facts r better than hypothesis here.
The aim here is to sourse for cheap replacement for pH calibration fluid. So lets explore all possibilities n back up with facts.
P/S : I dun use pH pen so I dun understand its details.
I find this debate rather meaningless. You could simply do some experiments over time and write up an article on your findings. Just bare in mind that not everything is agreeable among all, just like those algaes findings and research.----------------
No. I'm not convinced. Are u telling me if I place a bottle of pure water over time, the pH will fluctuate btw 5.65 n 7.0 ? U must be kidding ! If that's the case, LFS dun hv to sell pH down soln liao.
Moreover, distilled water comes in bottle with cap. How much CO2 can u find inside the bottle to lower the pH to significant amount ?
----------------
You need not calibrate your PH meter very often. A PH buffer sells like $7.50 and I think the most you could use it is like 4 times in a year? It assures you of the PH value as stated on the bottle. If you are very sure of the value of the distilled water will not change over time, over each production, use it. For many others like me, we will rather pay more for assurance.
Do you want me to show you the calculations and equilibrium constants?
When we talk about "pure" water, we do not have any form of buffering.
Here it go:
CO2 is about 0.0355% of the atmosphere.
Partial pressure of CO2 is 0.0355% x 1atm = 0.000355atm.
Equilibrium constant of CO2 dissolving in water is 3.38x10^-2 mole/L.atm.
Therefore, [CO2(aq)] = 3.37x10^-2 x 0.00035 = 1.2x10^-5 mole/L (0.53ppm).
CO2 (aq) + H2O <---> H+ (aq) + HCO3- (aq)
Ka = [H+][HCO3-]/[CO2(aq)] = 4.45x10^-7
In this case,
[H+] = [HCO3-]
Therefore,
Ka = [H+]^2/[CO2(aq)] = 4.45x10^-7
Solving for the above equation
[H+] = 2.3 x 10^-6
pH = -log[H+] = 5.6
So you just need a tiny winy amount of 0.035% of CO2 to drop the pH of water by so much.
Satisfied?
BC
feel free to test, get bottle of distilled water, while you
have your ph meter in, blow across the surface.
next: try this:
calibrate it against a ph 7.0 solution
dip the ph pen into the distilled water, check ph. should be 7.0
now dip ph pen into eg: a ph 8.6 solution (ie: 1/4 small teaspoon of sodium bicarbonate in a glass of water)
next dip the ph pen into the distilled water.
wash ph pen with tap water, calibrate with ph 7.0 again to be sure.
dip it into your distilled water solution (the one with a residual amount of sodium bicarbonate.
--
what you will realise is distilled water is very sensitive to slight amounts of acid/alkaline contaminants.
--
what exactly is the ph 7.0 buffer solution? KCL?
pH down solutions are just diluted acids. If added to distill water, you will probably get pH of about 3 or 4.
In most cases, the water is buffered. These pH solutions will react with the KH bringing both KH and pH down.
BC
Jinxed fishy photographer
[Specialist]
pH meter have two types. 2 point calibration and 3 point calibration. Can you imagine in your mind a line graph? 3 points on a graph is more accurate than 2 rite? That's what the 4, 7 and 10 are for; they are calibration points, not suka suka anywhere you want it to be.----------------
On 3/7/2003 10:50:43 AM
If wat u said abt the fixed calibration pts at 4,7 n 10 is true, then I fully agree with u, a pH 7.0 fluid is a must. Then again I'm puzzled on 1 thing. If u only use pH 7.0 to calibrate, y are there calibration pts at 4 n 10 ?
----------------
Gary, it is a very very well established scientific fact. Go look it up. There is really no need to reinvent the wheel.----------------
Any 1 actually measured the pH of distilled water over time ? Does it vary or stay const ? Facts r better than hypothesis here.
The aim here is to sourse for cheap replacement for pH calibration fluid. So lets explore all possibilities n back up with facts.
----------------
The point with the pH varying in distilled water is not that it stays contant or not. The point is that it can flutuate very easily, even CO2. Which is why in plant tanks, people buffer (does this mean anything to you?) their water with HCO3 so that the addition of CO2 will not crash the pH. Calibration solution's pH does not change easily, distilled water's pH can!
You win loh!----------------
P/S : I dun use pH pen so I dun understand its details.
----------------
ck
Aiyah, CK, it is good for Gary to probe for alternative answers/solutions to things. That's how knowledge expand.
I, myself, like to challenge views from others. Many times I find myself wrong, but I learn much more this way.
BC
Guys,
Dun get so work up ! The purpose of this discussion is to establish some facts on y mineral water or distilled water can't be used to calibrate pH pen. Some ppl jus said no w/o giving explanation. To me, I want to find out the real reasons. Is there anything wrong with tat ? If any of u find this discussion is meaningless, by all means dun read n dun reply. Simple as tat.
Thks for the understanding, bro.----------------
On 3/7/2003 11:20:28 AM
Aiyah, CK, it is good for Gary to probe for alternative answers/solutions to things. That's how knowledge expand.
I, myself, like to challenge views from others. Many times I find myself wrong, but I learn much more this way.
BC
----------------
Chris, I do not think pH 7 buffer is KCl because, by itself, it does not have any buffering capacity.
One example of a pH 7 solution is KH2PO4 + NaOH
H2PO4- <---> H+ + HPO4-- <---> 2H+ + PO4---
You gotta make some calculations to find out how much of each component to mix to achieve the disired pH.
BC
U didn't get wat I meant. u said e pH pen has 3 calibration pts : 4,7 n 10. But u only calibrate it using pH 7.0 fluid. So wats the purpose of the pH 4 n 10 calibration pts if u dun calibrate it at these 2 pts using pH 4 n 10 fluid ? BTW pH = -log [H+], its not a straight line.----------------
On 3/7/2003 11:14:12 AM
pH meter have two types. 2 point calibration and 3 point calibration. Can you imagine in your mind a line graph? 3 points on a graph is more accurate than 2 rite? That's what the 4, 7 and 10 are for; they are calibration points, not suka suka anywhere you want it to be.
ck
----------------
Gary, just imagine a chart...
if you have only one point of reference, you can draw a line of any gradient through it.
You need at least 2 points to determine how a straight line run on the chart. ie, at which point it will intersect pH 7 and at what gradient.
Hope this help you to understand.
BC
Posting Permissions
Bookmarks