pH Meter Calibration

[schematix]

And then I saw the rest.

Yep so i am firmly in alignment now that the calibration routine on this meter is trash, and there's no sense attempting to use it any further. With that in mind i guess my only option is to manually correct the readings after doing my own calibrations...

1. Delete calibration from meter - it will never say 'CAL' on the front.
2. On brew day i will remove the cover from the probe, rinse it with tap water to get the salt off, and then give it a final rinse in DI water .
3. Pour a pH7 buffer and soak/swirl meter for 10 minutes. Record reported value.
4. Perform cleaning procedure (rinse/shake/dry)
5. Pour a pH4 buffer and soak/swirl meter for 10 minutes. Record reported value.

I'll then calculate a calibration curve and apply it to any of my measurements.

All of this still assumes my probe is good.

You can see in the last 2 pictures the calibration had been deleted again (it doesn't say CAL on the display) and the probe was reading nearly 4.1 when it should have been reading 4.00.

I'll perform the drift/stabilization test as you describe.

 

[ajdelange]

Yep so i am firmly in alignment now that the calibration routine on this meter is trash, and there's no sense attempting to use it any further.

If your electrode passes the stability test you can work around this flaw. Let it do its cal. After it has calibrated itself then take reading of the two buffers waiting until the readings are stable before recording them. Get the actual buffer pH's from a table or the formulas in the sticky and get e4 and e7, the errors at, respectively, pH 4 and pH 7. When you get a sample reading of pH
correct by subtracting e4 - (pH -4)*(e4 - e7)/3 (check my math) from the meter's reading. The reason I suggest this rather than what you are thinking of is that I am uncertain as to what the meter does with the temperature when cal if disabled. Perhaps our correspondent from Hanna can answer that. What I am suggesting may be simpler. Depending on what you get for e4 and e7 and noting that mash pH is half way between 4 and 7 you may find that the correction you calculated by this formula is pretty constant over the range say 5.0 to 6.0 and that you don't need to consult a curve but rather just subtract the correction.

All of this still assumes my probe is good.

I'll perform the drift/stabilization test as you describe.

Yes.

 

[mbobhat]

Not trying to be the last word but based on AJ's mention of the acetate buffer it may be that a pH 5 buffer may suit you if you wanna keep using your meter since it is very close to mash pH. Then you could just use a correction from one known point. AJ you should market this for homebrewers!

I imagine the reason you don't see a pH 5 buffer is just that the buffers are not as easily made by just adding powder, you'd need to get the liquid or make it yourself with caustic and sodium acetate/acetic acid:

Here's one for sale

http://www.carolina.com/ph-buffers/buffer-solution-ph-5-500-ml/849663.pr

 

[day_trippr]

To our @Hanna_Brewing correspondent: Is it safe to say that an air bubble inside the electrode is non-optimal, and that the larger the bubble the more non-optimal?

Further, should a brand new probe not have any air bubble inside?

Cheers!

 

[ajdelange]

To make the acetate buffer you mix sodium acetate with acetic acid in the right proportion. But what's the easiest way to get sodium acetate? Adding NaOH to acetic acid so that's how I'd do it. Acetic acid is a liquid above 16°C so no, you can't buy powdered acetic acid for inclusion in little capsules.

The way to prepare the buffer is to mix acetic acid and sodium acetate in the correct proportions or acetic acid and NaOH in the correct proportions, make up to volume and trim to the target pH with either more acetic acid or more NaOH as required. Obviously this requires monitoring pH with a working pH meter. So while you can prepare a buffer based on measurements of weights or volumes alone hitting the target pH exactly is unlikely because of measurement error, uncertainy in the purity of the chemicals. Thus the suggestion here is really that one can prepare something that is quite stable in pH for the purpose of checking electrode stability closer to the pH at which we work than the 4 or 7 buffers.

One can do the same thing with citric acid which is a powder and is readily available. It has a pK within 0.01 of vinegar's. It's buffering won't be as sharp as acetic acids because the 4.76 pK we'll be using with citric is the second pK and the first is only a little more than 1 unit lower thus broadening the buffering peak at 4.76. Citric acid and lye could be packaged together to make a 5.4 buffer.

No, a bubble in the pH sense half cell is not a problem for a pH meter unless it is somehow wedged in the bulb in a way that the electrode (the wire) is inside it. If the electrode isn't in contact with the acid in the bulb no potential will be generated and the meter won't work. So if you have a bubble in the bulb try to get it back up the stem by jiggling, shaking, tapping etc. Don't get too carried away, obviously.

Should there be some gas in the pH cell? Absolutely! If there weren't and you exposed the bulb to higher temperature the pressure would build up and the bulb would burst. As it is exceeding thin it wouldn't take much of a temperature rise to do it.

 

[day_trippr]

My thought wrt the bubble was if the reduction in contact area of the electrode wire to the electrolyte would cause a corresponding flattening of the rate-of-change slope. If true it could inform why the probe has slowed so significantly that it's effectively non-calibratable.

fwiw, I did try pulling the cloth strip out about 1 millimeter as suggested by the Hanna rep, but there was no obvious improvement in the response rate.

So I'm going to purchase a replacement electrode - through Hanna's retail outlet in the hope of avoiding old stock - and keep my fingers crossed...

Cheers!

 

[ajdelange]

My thought wrt the bubble was if the reduction in contact area of the electrode wire to the electrolyte would cause a corresponding flattening of the rate-of-change slope. If true it could inform why the probe has slowed so significantly that it's effectively non-calibratable.

If the business part of the wire (at the end and usually a different color i.e. darker than the rest) is sitting in gas it isn't sitting where it should be and is going to cause a problem. Slowing of response is, assuming the electrode tip is properly bathed, a normal phenomenon as an electrode ages.

 

[day_trippr]

fwiw, I finally had to concede my ~20 month old pHep 98128 electrode had reached the end of its useful life and needed to be put down.
Replacement arrived yesterday and after a short soak in storage solution and a DI rinse it calibrated like new - waaay faster response.
And a quick check of the 4 and 7 buffers just now shows it held the calibration to .01 for each.

So, great, I'm back in bidness.
But I reckon I'm in for ~$60 every couple of years

I almost decided to switch to the Hach PP+, and downloaded what passes for the user manual. Regarding electrode storage it said "check the website"; the web site said "see the manual". hahaha!

Contacted Hach support and the answer was to keep a few drops of tap water (not DI) in the sample cap at all times.
Interesting - from all appearances the electrode is very similar to that used on the Hanna - but the latter recommends saturated KCL for storage.

Anyway...decided not to up the ante by another $30 and just stick with the Hanna...

Cheers!

 

[mbobhat]

fwiw,

Contacted Hach support and the answer was to keep a few drops of tap water (not DI) in the sample cap at all times.
Interesting - from all appearances the electrode is very similar to that used on the Hanna - but the latter recommends saturated KCL for storage.

Anyway...decided not to up the ante by another $30 and just stick with the Hanna...

Cheers!

I have one of those probes, but I never liked the cap design really, when you put the probe on its side the fluid looks like it could leak leaving the probe dry, that's probably why they say tap water, because if KCL leaked out you would get salt drying on the glass. if you wanted to you could make up a KCL solution (I think they sell potassium chloride at aquarium stores) and just make it and put in a tall screwtop plastic container, cut a hole in the lid so that your probe fits in suspended in the container,and keep your probe upright soaking in KCL without the cap. KCL storage solution will last forever, especially if you cap the container. If it starts to dry just add some tap water to keep the level up.

 

[ajdelange]

Interesting - from all appearances the electrode is very similar to that used on the Hanna -

Take a closer look. They are not at all the same. The pH electrode itself is visibly different but the reference junction is glaringly so be a rag junction on the Hanna and some kind of porous frit (unlike any I've ever seen before) on the Hach.
[/QUOTE]

 

[ajdelange]

I have one of those probes, but I never liked the cap design really, when you put the probe on its side the fluid looks like it could leak leaving the probe dry, that's probably why they say tap water, because if KCL leaked out you would get salt drying on the glass.

Certainly KCl deposited on the glass is not a problem for a pH bulb as most manufacturers bathe their bulbs in a saturated solution of it in storage.

if you wanted to you could make up a KCL solution (I think they sell potassium chloride at aquarium stores) and just make it and put in a tall screwtop plastic container, cut a hole in the lid so that your probe fits in suspended in the container,and keep your probe upright soaking in KCL without the cap. KCL storage solution will last forever, especially if you cap the container. If it starts to dry just add some tap water to keep the level up.

KCl is subject to creep. If there is any way out of a container a KCl solution will find it, dry out and deposit crysals. Everyone who has owned a pH meter has squirted dried KCl off various parts of the electrode.

One can, of course, easily prepare a saturated KCl solution for use as a storage solution using KCl bought at the supermarket (Co-salt) but one would be foolish to do so as opposed to using one prepared by the manufacturer of the probe one is storing. The reference electrolyte is very probably KCl but is not necessarily so. Don't use KCl for a Hach electrode, for example.

The people at Hach can't seem to make up their minds as to whether to store their electrode dry or not. It just makes sense to me that as the bulb clearly does not need to be hydrated in storage but does in use one could keep it partially hydrated by putting a couple of drops of DI water in the cap this keeping the RH in there pretty high. I'm guessing that this would allow quicker hydration when putting the electrode to use and people, including Hach, seem to have adopted this suggestion (or perhaps they thought of it themselves).

If you store a Hach meter in this way keep it vertical. It doesn't matter wrt the bulb but I don't think you want the water on the reference.

 

[schematix]

View attachment ImageUploadedByHome Brew1487556162.084940.jpg

 

[ajdelange]

That's it. Aqueous solutions of KCl rand liquid helium at near 0 K are apparently the only fluids that exhibit this phenomenon.

 

[schematix]

After several uses I deem this meter only fit for a dumpster... waste of money. Steer clear.

Others have mentioned the issues and i can confirm they are 100% accurate.

If anyone else would like to experience the frustration of dealing with this thing i'll send it to you for the cost of shipping, otherwise it's going in the trash.