pH of rainwater - alkaline?!

[D-novice]

After reading Bill Argo's articles on water / alkalinity / media / fertilizer, I got an alkalinity test kit and a pH/EC/TDS meter (Milwaukee Instruments).

I tested my tapwater, filtered tapwater (household drinking water filter, doesn't change things much according to these tests) and the rainwater I collect.

I am in a reasonably pristine area of Northern Calfornia, 25 miles from San Francisco, suburbs but nearly out in the country (and by country I mean about 50:50 agriculturearkland).

So, my rainwater pH, measured with my fancy new meter ranges from 7.6 when the probe is first inserted to 7.0 when left in a couple of minutes.

Isn't rainwater supposed to be acidic? What is the best explanation for this? Chemicals from degrading roof shingles? Chemicals leaching from plastic storage vats? (They used to be pickled pepper and olive containers, but have been rinsed, washed, and rinsed again, and this is their 2nd year collecting rainwater so they should be clean).

The alkalinity, EC, and TDS of my rainwater is all very low, so there's that. I just thought rainwater was acidic.........
I have started using MSU rain formula, and have been using dolomitic lime as a source of add'l Ca and Mg (and carbonate) - so I'm concerned my media pH is too high.

I haven't yet done the media test where you rinse through a pot, etc. I guess that's the most important one.

(Yes I calibrated the meter with 7.0 solution. And it's not the cheapest meter, was $150 on eBay and is professional grade)

Your thoughts.......

 

[John M]

Do you actually mean 7.01, not "7.0 solution"? I've never seen 7.0. I use 7.01.

Anyway, my advice is to recalibrate the meter with 7.01 solution and 4.01 solution. Never calibrate using just one solution. Also, the solutions have a shelf life. The bottle should have an expiry date on it. Do not rely on it after the expiry date. If there is no expiry date, then it could be that someone bought the solution in bulk and rebottled it. If that's the case, you have no idea when the expiry date is and you should NOT use it. I will NEVER accept calibration solutions that are "made up bottles" done in the back room of a horticultural supply place. I will ONLY use solutions that come in factory sealed bottles that have not reached their expiry date. If I am not the person who breaks the seal on the bottle, I won't trust that it's good to use.

Also, NEVER contaminate your calibration solutions by putting the pH probe directly into the bottle. Get a brand new, large, florist's water vial, or a glass test tube and pour a small amount of solution into that. Then, insert the probe to get a reading and recalibrate if necessary. Then, discard the solution that is in the vial/test tube. Never reuse it. After you open a bottle of calibration solution, the only thing that should pass through the bottle opening is small amounts of solution that you are pouring OUT into a test tube. Again, NEVER put the probe into the bottle and NEVER return used solution to the bottle.

Do the 4.01 calibration first and after making the adjustments to the meter, rinse it THOROUGHLY in clear rain water to remove ALL traces of the 4.01 calibration solution. Then, calibrate it in the 7.01 solution. Rinse again, of course. Then, go back and do the 4.01 again.

Now, you're ready to use the meter to get accurate readings.

I use the Hanna Gro-check pH meter. It's good and the calibration does not wander. I've used the pH pens and found them to be crap. The readings are all over the place and the calibration wanders wildly, making the pH pens completely unreliable. In my experience, the pH pens are inexpensive and useless. You do need a good quality meter if you're going to be watching your pH.

You say you got your meter from e-Bay. Hmmm...second hand? Personally, I'd never buy a second hand meter. The risk of damaging your collection is too great. It could be for sale because the first owner found it to be garbage and unreliable. Or, the fluid inside the bulb has been used up and the probe needs replacing. Sorry to say it; but, better to buy a brand new, high quality meter if you really want accurate results.

One thing I know: If you are adjusting your pH and you do not take the accuracy of your readings seriously, you'll soon severely damage or kill all your plants. 'Been there, done that and learned a VERY valuable (and expensive), lesson! You MUST do this right, or not bother. Do not go cheap on the meter; or get lazy with calibration and rely on numbers that may not be correct. If you do, you'll be so sorry.

 

[D-novice]

Thank you.

This is the meter, which as I said in my long post, wasn't an inexpensive pen.

The meter was brand new - lots of dealers sell on eBay nowadays, at least in U.S.

You are correct, the calibration sol'n was 7.01, not 7.00. The meter, despite having two decimal places, doesn't appear to give readings in the hundredths.
The calibration solution I just opened, it expires 11/20.

So, your post might be more useful to someone asking about whether to buy a cheap used meter, but doesn't apply to me or my question.

Yes it would be ideal to calibrate the meter to 4.01 as well. I don't have that solution on hand at the moment.

So, ignoring all of that, let's just say that my brand new, $150 meter is reasonably accurate, and my rainwater pH is 7.00. Any ideas relevant to that (my original) question?

 

[naoki]

I don't completely understand this, but most pH probes are not stable for near pure water. I think ChrisFL had an explanation in one of the threads. I have multiple lab-grade (well for molecular biology) pH meters, but they behave in the similar way (fluctuate widely and doesn't work well). It isn't too meaningful to measure the pH of pure water any way. For plants, it is probably more interesting and useful to measure the pour-through.

 

[D-novice]

Thanks, will do that next.

Argo says pH of water isn't all that meaningful, actually, relative to alkalinity of water, which helps determine the appropriate fertilizer to use.

 

[PaphMadMan]

A couple points. If you are only testing a solution that is close to neutral, say 6-8, then a one point calibration at 7.0 is perfectly reasonable. If you want to be accurate over a range then a 2 or 3 point calibration is better, but your meter doesn't support that anyway.

The fact that the reading was 7.6 initially is meaningless. It takes most pH electrodes at least a couple minutes to reach equilibrium with the solution, especially a low ionic strength unbuffered solution like rainwater.

7.0 or 7.01 is a meaningless distinction in terms of realistic pH measurement, and actually variable depending on temperature of the solution. Unless you have a moderately sophisticated meter and electrode that does temperature compensation, and is capable of that kind of accuracy and precision, it just doesn't matter.

 

[gonewild]

You can safely assume your rainwater is not alkaline. You said your pH meter first reads 7.6 and then drops to 7.0(neutral). The fact that it drops from an alkaline reading down to neutral pretty much proves that.

I think you can consider rainwater as being neutral regardless of what your pH meter says. It is "neutral" because when it comes into contact with minerals it will quickly shift pH up or down and reflect the pH of the newly dissolves salts.

 

[D-novice]

OK, so my next question would be, why isn't it acid? Rainwater is supposed to 5.5 or so.

I didn't mention, I did test some vinegar, just to confirm that my meter was capable of registering acid, and it came out at about 3.3 (I don't remember exactly, I was just doing it to confirm the meter would work on that).

Presumbably, if it were acid, my meter would register that, since it is correct for calibrating solution and for vinegar. I'm going to assume that my meter was correct (7.0) for my rainwater.

Could it be that there is an assumption that most rainwater has picked up sulfuric acid and the like from coal-burning plants, as would be the case in much of the world, but that mine, coming from storms blowing straight off the Pacific Ocean, isn't carrying the same acid load?

 

[PaphMadMan]

The mild acidity of rainwater in most places is due to dissolved carbon dioxide, typically putting it between 7.0 and 5.5 or so. But as nearly pure water without any buffering capacity that acidity is nearly meaningless. As soon as you add something like fertilizer to that water the pH is determined by the fertilizer regardless of the original dissolved carbon dioxide. Or when you put rainwater into organic media like bark or moss it is the media itself and soluble minerals and organic compounds in it that make the final pH, not the weak acidity from carbon dioxide.

If your meter had been calibrated at 7 and the reading eventually equilibrated to 7, it was probably about 7 and just wasn't anywhere near carbon dioxide saturation.

Acid rain caused by sulfuric acid emissions is usually a more localized effect directly downwind, and for an air mass coming off the Pacific that would not be an issue.

 

[Ray]

The equilibrium pH for pure water dissolving carbon dioxide from the air is 5.3. However, that is not all it can dissolve. Even some of the alkaline components of glassware can affect it, if they haven't been extracted by acid "curing" and thorough rinsing before use.