PH Pen/meter

[ineke]

It's been a little while since Ph pens have been mentioned and I would like some thoughts on the pens currently available. I have been using a cheap PH pen which seems OK but it requires calibration each time it is used - I don't bother if it has only been a couple of days but I usually recalibrate when testing day comes around about  once a month. Since starting to use a pen I have learnt a bit more about them and would now like to get one that has automatic Temperature compensation and auto calibration. I know with auto calibration you still need to use the calibration solutions but the pen adjusts automatically instead of fiddling with the screw driver. So what are people using at the moment and do you store your pen dry or with the storage solution. At the moment I store dry which may be why I need to calibrate so often. 

[jayc]

1 hour ago, ineke said:

do you store your pen dry or with the storage solution. At the moment I store dry which may be why I need to calibrate so often. 

pH Pens MUST be stored wet with the storage solution. Never let it dry out.

That is why you are calibrating so often.

 

Here are some guidelines taken from Hanna Instruments:

Guidelines for pH Electrode Maintenance and Use 

 

The life of a pH electrode is not infinite. A number of factors can affect the life span of a pH electrode. The higher the temperature that the electrode is used at, the more extreme the pH, how often the bulb dries out and needs to be rehydrated, how roughly it is used; all these factors and more will shorten the life span of a pH electrode. An electrode that is well maintained and cared for can last many years, one that is not well maintained will last anywhere from just a couple of months to 1-2 years.

 

Following the guidelines outlines below will help to get the most from a pH electrode.

 

1. Hydration of the glass bulb

A very thin layer of water forms on the pH glass sensor. This layer is known as the hydrated layer.  A pH electrode is an electrochemical sensor that generates a voltage that is read by a voltmeter.

 

A probe that has a hydrated layer will generate a different voltage in a pH buffer than one that doesn’t. So calibrating a dry electrode will cause inaccurate readings when taking measurements as the hydration layer is formed.  A dry probe needs to be hydrated before calibration. The hydrated layer takes 3-4 hours to form when a probe is placed into storage solution.

 

2. Hydration of the junction

Every combination pH electrode has a junction. A junction is a barrier between the internal reference wire and the sample that is measured. The reference cell has a salt solution that leaks through the junction as part of the electrical circuit. If a probe is dry the salt will precipitate in the junction causing to be clogged. The pH readings will be erratic. A dry probe needs to have the junction hydrated to work properly.

 

3. Fill Solution Level for Refillable pH Electrodes

pH electrodes that are refillable need to have their electrolyte (salt) level checked.  The probe should be filled when the level falls below ½” from the fill hole opening. Having enough fill solution ensures that, for a single junction electrode, the reference wire is in solution.  A proper level also allows for adequate head pressure to force electrolyte through the junction.

 

4. Remove/Loosen Reference Fill Cap When in Use

In order to create head pressure the fill cap must be loosened or removed from the top of the electrode.  Removing/loosening the reference fill cap allows for air to enter into the reference cell. The solution within the probe will want to flow through the junction resulting in increased stability when taking a reading.  Note: Single junction and double junction electrodes use different fill solutions. An explanation of the difference can be found here (hyperlink to single junction versus double junction page).

 

5. Calibration Buffers

Fresh calibration solution should be used when calibrating a pH electrode.  All pH measurements are based on the pH calibration solution as a reference point. The pH calibration buffer is a water-based solution that will change over time. This is especially true with pH 10.01 buffer that will actually decrease in pH as atmospheric carbon dioxide enters the solution.  Bottles of buffer should be replaced after they have been opened for 6 months. Never pour calibration buffer back into the bottle.

 

6. Calibration of the pH Electrode

The pH electrode should be rinsed in purified water (RO, DI, or distilled) before placing in any pH calibration buffer. This will reduce the chance of contaminating the buffer.  A best practice would be to use two beakers/containers for each calibration buffer. The process would be to rinse the pH electrode with purified water then rinse the probe in one of the beakers with the buffer then place the probe in the second beaker with buffer. This procedure would be repeated for multiple calibration points. For two point calibration the pH buffers used should bracket the expected pH reading.  For example I expect a pH 5 reading then the pH electrode should be calibrated in pH 7 and pH 4 buffers.

 

7. Cleaning a pH Electrode

The pH electrode should be cleaned periodically.  A coating will form on the glass bulb that will cause errors in measurements and drifting/erratic readings. A typical cleaning procedure is to place the electrode in a cleaning solution for 15 minutes. After that the probe is rinsed with purified water and then placed in storage solution for at least 2-3 hours before using. It is important to note that there are many different types of cleaning solutions available based on the application. Solutions include ones for proteins, inorganics, food and wine stains to name just a few.

 

8. Storage of the pH electrode

The pH electrode bulb and junction needs to be kept hydrated. With benchtop meters a small amount of storage solution is placed in a beaker. With the probe lowered into the beaker the junction should be covered. For portable meters a small amount of storage solution is placed in the protective cap and the cap placed on the probe. 

 

NEVER STORE THE pH ELECTRODE IN PURIFIED WATER

Storing the pH electrode in purified water (RO, DI, or distilled) will shorten the life of your pH electrode. 

 

The reference cell has a high salt solution. Placing the probe in purified water will cause the salt to diffuse out and the water to go in. Storage solution is not only formulated to maintain the reference salt concentration but also has chemicals to keep bacteria and fungus from growing.  If storage solution is not available then use pH 4 buffer.

 

 

9. Tips

• Salt crystals around the fill cap of a refillable pH electrode or by the protective cap placed over the bulb are normal to see. Just rinse with water to remove the salt build up.
• Any air bubbles in the pH bulb or by the junction need to be removed. Gently shake the pH electrode like a spirit filled thermometer to displace the air bubble.
• Never wipe the pH electrode with a cloth or any other type of material. Any static formed will harm the electrode.
• Do not expose to high temperatures (>150 ^oF/65 ^oC) unless the probe is designed for it.
• Always rinse the probe with purified water to clean of contaminants before, during and after use.
• Calibrate frequently. Some guidelines to follow:

1. Periodic user (1-2 times per month) – calibrate the day of use
2. Weekly user (1-2 times per week) – calibrate the day of use
3. Daily user (1-2 times per day) – calibrate the day of use
4. Daily user (>2 times per day) – calibrate every 4-6 hours

[ineke]

Thanks JayC that's great information. Unfortunately the cheap Pens don't come with a cap that is suitable to use storage solution and their instructions don't mention it either. I have always used my normal liquid test kit as well as the pen but the test kits I have  don't go below 6 so I think a decent pen is the best solution. That's why I'm interested to see what people are using. The Hannah ph pens are not getting a good review although I do like their TDS pen . I'm looking at a mid range PH pen but can't afford much more than $100 - so I need to buy carefully. My tanks are all below 6 according to my low range ph but it doesn't say how low and when my pen is newly calibrated - supposing of course it is correct- they are around 5.5 . I'm not fussing too much as they are all the same, all stable and every tank has active breeding adults and high shrimplet survival rates so the shrimp are OK. However I would like an accurate  reading so I know exactly where they are sitting. 

[FMuscle]

The pen I bought doesn't have a storage solution, or means of calibration either visible or mentioned in the manual. And comparing the (dry stored) pen and test kit, they come up with the same results... 

[jayc]

I need more storage solution for my pH pen.

Does anyone know where we can get some? I don't mind sharing a bottle of this stuff.

[ineke]

@Jayc if you have a hydroponics store near you they seem to have the cheapest solutions ready made. The solution does have a 6 month expiry date from opening. Also there are recipes to make your own on google

[addicted2shrimp]

ineke is right i picked up some ph pens from my local hydroponic store for $50 each

ineke is right i picked up some ph pens from my local hydroponic store for $50 each

[jayc]

Thanks Ineke.

I'll check out some hydroponic stores.

[jayc]

Found a recipe to make your own pH pen Storage Solution.

Add 1gm of Potassium Chloride (KCl) to 100ml of pH 4.0 buffer solution.

 

I have a lot of pH 4.0 buffer, so I just needed to get KCl.

I now have heaps of KCl, since you can't just buy it in 1gm amounts.

 

Edited February 2, 2016 by jayc

[ineke]

I had that recipe bit didn't know where to get the KCl - where can I get it Jayc? I'm going to buy a new pen that has a storage sponge in the lid so will need some! 

[jayc]

@ineke

I bought KCl from this ebay store in SA Port Augusta!

1oz or 28gm for $3.50 plus $1.50 shipping.

http://www.ebay.com.au/itm/1Oz-KCl-Food-Grade-Potassium-Chloride-E508-salt-substitute-Vegan-/151735953500?hash=item23542ae85c:g:AkUAAOSwjVVVmPlo

Edited February 2, 2016 by jayc

[ineke]

Thanks Jayc I will get some.