Thanks for that - so I'm guessing it's also unimportant to know why the rainwater (once collected) is also higher in pH than we might expect? I thought maybe whatever was causing this would have effects on my plants, but sounds like that's not the case; whatever is causing above 7 pH is trivial. Still seems odd though. Thanks again.

Rain water is supposed to be healthy, maybe they way are testing the water is incorrect? It should have a neutral ph. The tank you collect the water in could be metal or something like that?

In theory it should have a neutral pH. In practice it almost never does. As Tox mentioned, rain water has no buffering capacity as so pH can change with very little input. Even when using pure distilled or deionized water you can stick a meter in the sample, stir it with the probe and watch the pH drop quickly as CO2 from the air is absorbed. Some alkaline dust on the roof could easily push it the other way. But there will be such little buffering capacity it won't matter. I think the fact that the OP can get accurate readings from the calibration solution is evidence enough that he's testing properly.

"Normal" rain water has a pH of 5.6 because as that water travels through the atmosphere it picks up stuff that might be considered pollutants.

Here is a link that might be useful: pH of rain water

Agreed, although I would not consider CO2 in the atmosphere a pollutant. I too am curious about the slightly alkaline pH of the OP's rainwater. Roofs and metal tanks are not likely to change it significantly. I suspect soil dust floating around and being absorbed into the rain as it falls, or, settling on the roof in dry weather and being washed down. 7.8 is approaching the 'carbonate buffer' pH of 8.2 or so which you would see in a dry area with a lot of limestone (carbonate) rock. What type of rocks do you have around you and what is your natural soil pH?

Thanks everyone for your thoughts. The error is mine. As I have now learned, and as suggested above, rainwater is so devoid of everything but water, the pH meter has a difficult time reading the pH (salts help). I left the meter in the solution for an hour and it kept dropping and dropping, going to 6.4 before I stopped. Temperature of the water did not matter. I use the meter usually for making beer, so all this was a surprise to me - I never had to wait before. Thanks again

You might have been observing the absorption of CO2 into the water over time, which would bring it down to a slightly acidic end point once the alkalinity already there was neutralized. So, both readings (before and after) may have been correct, but you were observing the process take place.

toxcrusadr - thanks. It's really more a matter of a lack of chemistry for the pH meter to measure. If it was CO2 this would not have happened as it was rainwater collected in a jar the night before. so the only thing changing in the time of measurement is contact of the sensor with the solution; thanks

Did you check total dissolved solids with a TDS meter? That might give you a clue if something else is involved from the roof. Also, if you have a metal roof - could cause chemical reaction to affect the ph. There is also the acid rain situation for some folks depending on where they live. I’m in VA USA, my rain water ph,is about 6.8 and the TDS is a low of 4 ppm to a high of 12 ppm depending on the debris in my gutters and how much rain we have had to keep the gutters flushed out. I try to collect after it has rained for about 10 minutes or I just make it easy, and throw a old sheet over the can to keep fine debris out. If you use an actual rain barrel many use screens but still let the fine stuff through. Now to ph..it is important - too high or too low and the nutrients you feed the plants can’t use. This is very important for potted plants as they are in soil-less mixes i.e. peat, coir, perlite, etc. - all inert and must be fed everything NPK and all trace minerals and elements. You can lower your ph (take from ph 7 to 6) by using a few grains of citric acid per gallon of water or raise the ph (ph 5 to 7) with the tiniest bit of baking soda per gallon. You must use a good ph meter and creep up to what level you want. These work well but are not stable to sit around so use right away. If using reservoirs, or storing fertilizer solutions, use commercial products like PH Up or PH Down. If your TDS readings are above 50 ppm, you might need to find a better source of water i.e.rain, distilled, RO if this is due to chemical issues etc. If your rainwater is high once or twice and you know the gutters have tons of debris (trees growing in the gutters, leaves, twigs etc) then don’t worry about it. If you are using tap - have it tested by a lab versed in plants like JR Peters. I believe a typical sample is $40 but it is well worth it if you grow plants for sale, have expensive plants or very sensitive ones like orchids- why waste nutrients and not have healthy plants over $40. They also test soil, potting mixes, and plant tissue. They can tell from the tissue if your plants are deficient in particular nutrients. Also on the web you can find many charts (or from hydroponics shop) that show optimum ph ranges for all the main nutrients to be taken up by the plant. If a particular nutrient like say calcium or magnesium is available to the plant at ph 6.5 -7.0 and your irrigation water or your potting mix has broken down to a very acidic ph 5 - the plant can’t absorb any calcium or magnesium you give it. Your plants might be deficient and you dump dolomite limestone or calcium magnesium supplement - and the plant won’t be helped. Now - if you use a product like GH CaliMagic which is chelated with iron and it allows it to work over a wider range but you still want to be as close as a normal range. Remember uptake and mobility in the plants with all NPK nutrients and minerals is dependent on ph but also type of plant, light, temperature, humidity, etc. Read articles from First Rays, St. Augustine Orchid Society, Maximum Yield, and search for university reports and growing house articles that specify on the plants you are growing for the best information. For example tropical hibiscus is sensitive to fertilizers with a lot of phosphorus (keep it very low like below 5);but they almost can’t get enough potassium and there aren’t many commercial fertilizer mixes that have enough so you have to supplement. If you grow these - check out articles from Hidden Valley Hibiscus. I hope this helps.