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Best way to measure pH of water?


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Initial post: Nov 14, 2012 10:06:27 AM PST
Last edited by the author on Nov 15, 2012 8:41:31 AM PST
Peregrinn says:
My daughter needs to measure the pH of water samples as part of her science project. There are many pH test strips and pH meters available here on Amazon, but some of them specify that they are best for measuring the pH of highly buffered solutions and may not give accurate readings for water. Does this make sense? If so, what is the best inexpensive way to measure the pH of water? I am considering this product and would love to hear from someone who can comment on its use for this purpose, or who can recommend an alternative. Thanks.

Banggood Digital pH Meter Tester+2 Pouches of Calibration

Edit: In case anyone pulls up this email in the future, I wanted to note that the item specifically listed above would have been slow to ship. Instead, I found what appears to be the same meter but fulfilled by Amazon for faster shipping:
CyberTech PHTester PH-107 Digital pH Meter Tester

In reply to an earlier post on Nov 14, 2012 1:16:25 PM PST
Last edited by the author on Nov 14, 2012 1:17:26 PM PST
Doctor Who says:
That looks much more complicated than what you need. I would use something simpler, like LuckyStore 6pcs 80 Strip Full Range pH Alkaline Acid 1-14 Test Paper Water Litmus Testing Kit
because it will be easer to use and it is cheaper, even after shipping.

In a chemistry lab, you actually use different reagents. Its complicated but basically the right tool for the right job.

In reply to an earlier post on Nov 14, 2012 1:46:41 PM PST
As a chemist, I disagree here. I think a meter that reads to 0.1 pH unit is better than the pH strips--especially for water samples which are going to measure generally between pH4 to pH9.

Posted on Nov 14, 2012 1:53:04 PM PST
Last edited by the author on Nov 14, 2012 2:13:15 PM PST
Rev. Otter says:
if it's for a school science FAIR project, i'd use actual litmus strips because those can be put directly into a display of results. otherwise, you'd only have lists of recorded numbers.

(or, if nothing else, pictures showing the litmus results ...)

if she's doing something less visual -- a paper or report -- then ignore the above and go digital for accuracy.

good luck to your daughter, let us know how it goes! :)

In reply to an earlier post on Nov 14, 2012 2:20:03 PM PST
The Weasel says:
For a middleschool project I'd go with the strips. Watching the strips actually turn color is kind of a revelation when you're young. I used to think the pool chlorine testers were magical when I was 5 or so.

In reply to an earlier post on Nov 14, 2012 2:47:12 PM PST
i bought one at home depot to measure soil ph

should work fine on water as that is what it measures with
you have to get the dirt into mud pies to measure it
which means you add a lot of water

so plain water should work too

In reply to an earlier post on Nov 14, 2012 2:51:09 PM PST
Re OP: Because "pure" water has nothing else in it but water, the pH is given directly by the dissociation constant of water: H2O <--> H+ + OH-. This constant is 1E-7, so the pH is 7.

Tap water has various minerals in it which will affect the pH by a point or two. (Larger deviations would make the water undrinkable.) Thus, the cheapest reasonable approach is to get a few test strips (or indicator dye solutions) which can show pH values in the range from 5 to 9. Meters can be had which will measure pH directly, but these aren't cheap and I can't recommend them for an experiment of the sort contemplated here.

If the water sources at issue are from non-potable sources, larger deviations in pH may be seen. If you get stuff which will measure from pH 4 to 10 or so, that should be adequate for these.

In reply to an earlier post on Nov 14, 2012 2:51:13 PM PST
Peregrinn--Most pH meters do not work very well at low ionic strength. One way to check this out is to add a neutral salt, such as sodium chloride (table salt will do fine) and see if it affects the reading. If not, you are good to go. If it does, use the reading with the added salt. I am not a big fan of pH test strips, which are highly subjective and affected by at least as many variables as meters.

Posted on Nov 14, 2012 6:17:22 PM PST
Last edited by the author on Nov 14, 2012 6:34:20 PM PST
Peregrinn says:
Thanks to all who have offered comments. I am leaning towards the pH meter I posted in my initial comment. Here is some more background.

Some of you may have heard of SODIS, or solar disinfection of water. Particularly useful in third world countries where provision of clean water is not reliable, SODIS is performed by putting water in a clear plastic bottle (like the Deer Park and other water bottles that are so common) and exposing it to sunlight for several hours, allowing the synergy of UV radiation and rising temperatures to disinfect the water, killing organisms that cause diarrhea.

SODIS only works with clear water, and many areas have clay or other substances clouding the locally available water. Recent research shows that these clays can be removed easily and without great expense by using table salt (sodium chloride) as a flocculating agent. My daughter hopes to build on this research by exploring whether the pH of the water affects the ability of salt to remove the clay particles. (There is some research showing that lime juice aids in the SODIS process, so she is theorizing that lower pH may work better - but neither she nor I know much about the process of flocculation.)

In any case, there will be dissolved salts in the water that she will be testing. Therefore, I hope the pH meter will provide valid measurements over the anticipated pH range, which is between 5 and 9 units.

In reply to an earlier post on Nov 14, 2012 6:19:25 PM PST
Last edited by the author on Nov 14, 2012 6:26:46 PM PST
Doctor Who says:
Do *NOT* try that with normal plastic bottles. The UV will cause the plastic to leach toxic chemicals into the water. You need a bottle made of some type of special plastic or glass.

In reply to an earlier post on Nov 14, 2012 6:33:27 PM PST
Last edited by the author on Nov 14, 2012 6:36:43 PM PST
Peregrinn says:
Wrong. The World Health Organization and others support the use of SODIS using plastic bottles (but not PVC). Given the risk of dysentery from untreated water, any unproven risks from leaching toxins are trivial in comparison. Here is one basic story on the process: http://greenliving.nationalgeographic.com/use-solar-power-purify-water-3062.html

In reply to an earlier post on Nov 14, 2012 6:53:41 PM PST
Last edited by the author on Nov 14, 2012 7:00:27 PM PST
Doctor Who says:
Yes I know they use bottles. I looked up the process before I posted last time. But that still leaves the fact that the bottles are not meant to be heated and the DO leach chemicals. You can try tasting the water if you don't believe me when I say that process will degrade plastic.

Yes, a little plastic is better than dysentery and you have to make do in the third world, but this is not the third world. You should really go with glass as it really is not ideal to use normal plastic. My background here is as a physicist. I have done my share of heating plastic water bottles and seeing what happens.

Look at it this way. If your daughter got a deep cut you would probably have a doctor clean it with an antibiotics and have the wound closed with stitches. You don't just tie a dirty garment around her arm. If you were in the third world that might be the best option that is available. It does not make it the best option or the safest.

In reply to an earlier post on Nov 14, 2012 7:10:25 PM PST
Last edited by the author on Nov 14, 2012 7:16:36 PM PST
Doctor Who says:
Look here:

http://en.allexperts.com/q/Food-Safety-Issues-767/2009/8/RE-Bottled-Water.htm

http://www.snopes.com/medical/toxins/plasticbottles.asp

I completely agree that given the alternatives its better than a life threatening illness, and I really do appreciate that you want to educate your daughter in science, however the plastic bottles really are not meant to be used that way. I strongly suspect that the bottles actually used are specially treated. Basically, you should not take a risk when you don't need to.

EDIT: WHO is not like the FDA. Their mandate is to do the best they can for as many people as they can with what they have. In this case, the plastic is the less of two evils.

In reply to an earlier post on Nov 14, 2012 8:10:28 PM PST
Peregrinn,

You might try a good ohmmeter with 2 MΩ, from eBay sold under $15.
The graph of pH-conductivity is found here:

http://lss.fnal.gov/archive/pbarnote/fermilab-pbar-note-674.pdf

The graph of pH-resistivity is found here:

http://www.che.ufl.edu/ren/course/4404l/ga/GA%20-%20Water%20Conductivity%20and%20pH%20Handout.pdf

The conductivity of pure water at a pH of 7 = 0.0548μS / cm
The resistivity of pure water at a pH of 7 = 18.5 MΩ / cm

Even though resistivity decreases as pH deviates from 7, you could still get a very good idea on the exact pH by simple math, if you know whether your solution is acidic or alkaline.

Very pure water will require you to bring the two tips of the ohmmeter as close as 1 mm which gives 1.825 MΩ.

If you are an electricity oriented person, you might lean towards more sophisticated setting for measuring resistivity over 2 MΩ with the ammeter/voltmeter method.

Mohamed F. El-Hewie

In reply to an earlier post on Nov 14, 2012 9:47:18 PM PST
Re Peregrinn, 11-14 6:17 PM: Adding salt to the water may help precipitate the crud, but it has its own problems: if you add too much, you get what is basically sea water -- which is undrinkable. Some sort of filtration process would be better, such as to run the raw water through a column of sand or suitable mineral.

Irradiation bottles of fused quartz would be preferable if available; they do a good job of passing UV radiation.

Posted on Nov 14, 2012 10:39:37 PM PST
the problem is to measure ph
buy a ph meter
stick it in the water
read the answer

if you want drinkable water
there are filters that take out everything down to cryptosporidium size
our boyscout store sells them as do most outdoor/hunt/fish/camp stores

you can also distil the water and get only pure water
sunlight can do that
any survival book shows how

Posted on Nov 15, 2012 3:40:29 AM PST
Peregrinn says:
My daughter did a project in the past that used a passive solar still to desalinate water, so she is well aware of that process and of the need to minimize the salt content of drinking water. The amount of salt needed to remove the clay particles is still well below the level that is problematic to human health. The focus in this research is to develop a low-cost feasible method of water purification for areas of the world where the lack of clean water causes high levels of disease. While fused quartz and filtration may produce higher water quality, they are not easily within reach for much of the population. http://www.sciencedaily.com/releases/2012/05/120501134315.htm

In reply to an earlier post on Nov 15, 2012 4:18:29 AM PST
Last edited by the author on Nov 15, 2012 4:21:02 AM PST
Bubba says:
The problem with glass bottles for this application is that the glass normally used to make bottles greatly attenuates UV-A and essentially blocks UV-B and the shorter wavelengths of UV light. This is why you can't get a suntan through a normal glass window.

In reply to an earlier post on Nov 15, 2012 8:30:15 AM PST
Doctor Who says:
They employ glass bottles, so I don't think that has a significant factor in the process. It was listed ahead of the plastic bottles on the home page. Ideally, you could use a solar oven to heat the water, which would be more efficient and less time consuming. That is also expensive. I suspect that is why they are willing to make do with plastic.

In reply to an earlier post on Nov 15, 2012 3:29:36 PM PST
Peregrinn,

I am afraid that you are mixing up between desalination and purification.

Removing salt from water is an energy-dependent process that requires rich communities that could afford the technology.

Purification is less expensive. I worked in remote parts of the Sahara desert in Africa, where underground water was the only source of water available. Near residential area, all agriculture land was deemed unsuitable to cultivation due to human activities, sewer, garbage, etc. But getting drinking water took less effort by digging deeper, using simple filters and boiling the water.

The major problem was the lack of education and money for common villagers, most of whom cannot even afford heating water or filtering it. Hence, kidney diseases and gastrointestinal and skin infections were common endemic diseases.

Mohamed F. El-Hewie

In reply to an earlier post on Nov 15, 2012 5:06:10 PM PST
Peregrinn--"In any case, there will be dissolved salts in the water that she will be testing."

>>JGC: The pH meter should work fine then. I am a little suspicious of it because it is so cheap. A laboratory pH meter typically costs several hundred dollars and the electrode(s) another 100 or so. But if it works, who cares?

I had a friend who bid on a contract. He did not get the contract, so he asked the client if he was underbid. The client said, "No, the winning bid was considerably higher, but we were suspicious of your bid because it was so low!" My friend said, "I'd be happy to charge you more!"
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Discussion in:  Science forum
Participants:  10
Total posts:  21
Initial post:  Nov 14, 2012
Latest post:  Nov 15, 2012

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