Choosing the right pH meter?

[chee ping Tan]

Hi all, this is my first time joining this forum, nice to meet you all.

So I'm currently looking to buy a good pH meter but there are just simply too many option out there and I'm confuse and not sure which to choose.

I need a pH meter for measuring pH of seawater (salinity 25 ~ 40 ppt?) tank. From what I found so far, most pH meter don't come with a conductivity sensor for salinity compensation, and if they do, they either only measure up to 10 ppt only, or it is simply too expensive and meant for research uses (~ $ 1000).

My questions are:
1. Can a common pH meter without salinity probe work as accurate as those high-end pH meter that has automatic salinity compensation for seawater?

2. Do you all have any recommendation of a durable and accurate pH meter that work well for seawater and costs only around $ 150 ? (That's the max I can pay)

I'm considering the Apera pH 60, have anyone here used this brand before? How good is it?

That's all. Thank you guys so much for reading this.

Tan.

 

[Alfrareef]

Following.

 

[Randy Holmes-Farley Verified Community Expert]

You won't need or use salinity compensation for pH.

You definitely want 2 or more point pH calibration,a nd temperature compensation can be desirable (manual or automatic).

Here's what I suggest to look for in a probe:

Aquarium Chemistry: Measuring pH with a Meter ? Advanced Aquarist | Aquarist Magazine and Blog
http://www.advancedaquarist.com/issues/feb2004/chem.htm

from it:

Selection of Combination pH Electrodes
There are hundreds of different types and styles of pH electrodes. These vary in size, composition (glass or plastic sides, typically), shape, the nature of the glass membrane (even non-glass materials can be used), how that membrane is separated from the internal filling solution, (that is, single vs. double junction), whether the internal fluids are sealed or refillable, and a variety of other issues.

For most reef aquarists, the exact nature of the pH electrode doesn't much matter. Nearly all can give suitable pH readings. Here is my opinion about what is most important to look for:

1. Selecting an epoxy body electrode instead of a glass body electrode will make it less likely that you will break it by accident. The glass bulb on the tip is still very fragile (I've broken many), but the bulb can be protected by a plastic shield to reduce the chance of breakage.
   
   
2. If you intend on measuring pH directly in the aquarium or sump (rather than in a cup of water removed from the aquarium), then expect to occasionally get the whole electrode wet (by accident). If it is a refillable type which has a hole for adding more filling solution (usually KCl), then you might get aquarium water into the electrode. That can severely degrade performance by messing up the reference electrode. So selecting a sealed electrode may be a better choice.
   
   
3. Some folks argue that a double junction electrode is preferable, and that is likely true. Without going into the design details, a double junction electrode makes it much less likely that there will be any exchange of ions between the aquarium and the filling solution through the glass bulb. This option may be especially important if you are using an internal reference electrode other than Ag/AgCl. For example, if it is a calomel reference electrode, it contains mercury. It is desirable to keep mercury salts out of the aquarium due to its toxicity, so a double junction is likely preferable in that case. Likewise, it is desirable to keep sodium and other seawater ions from entering the reference electrode, and a double junction prevents that to a much greater extent than a single junction. That aspect is more important if you leave the electrode in the aquarium water than if you put in into a storage solution after every use. In general, it is likely better to have a double junction than a single junction, but it may not result in appreciably better measurements.
   
   
4. There are other attributes of certain electrodes that may make them more useful in laboratories, and hence justify prices that easily range to more than $350 for the electrode alone. There is not likely any significant advantage in using such electrodes for reef aquaria, however. Do not be worried about claims that you need a special electrode for high pH or high sodium environments. That is true for the most accurate measurements at high pH and high salinity, but aquarium water will be OK with a standard electrode, and even brief measurements at high pH (like limewater) will be fine.
   

An electrode that I have been using for the past few years is sold by Cole Parmer. It is catalog number U-59001-70. It has an epoxy body, is sealed, and has a double junction. I'm not certain what type of reference electrode it contains, but I believe it to be Ag/AgCl. Its list price is $60.

 

[chee ping Tan]

Dear Dr. Holmes-Farley, thank you so much for the info.

I'm so relief to know that salinity doesn't matter for the accuracy for the pH meter. I will take your advice into consideration when buying my pH meter.
However, I'm still fuzzy over the refillable/non-refillable pH probe, what is that actually?

Since I still need to do daily measurement of salinity, I suppose I will opt for a refractometer instead, what do you think?

P/S: Just checked the pH probe you recommended, yes, it is a Ag/AgCl combination reference electrodes, however, you may wish to update the price of that probe as it is now being sold at $ 116 USD each.

 

[Randy Holmes-Farley Verified Community Expert]

Yes, the article is quite old.

Refillable just means the electrode has a hole in to to add more fluid. Not needed for us.

I prefer conductivity probes for salinity, but many reefers use refractometers.