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Rain water and a case of its curious pH

ScareCrow

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Hi All,

I'm in the process of setting up a new tank and thought I'd check my water providers test results as I knew the water is hard but I also wanted to check other parameters, so I had an idea for fert. dosing. The tank will be low tech with a beech/oak leaf litter substrate mixed in equal parts with cat litter and capped with cat litter. As suspected the water is hard 293ppm TDS @20°C (according to water provider. My TDS pen reads 311 at a room temp of 18°C). So I thought I'd check my rain water, 58ppm TDS (using my own TDS pen at a room temp of 18°C). This all seemed great as I could soften my hard water with rain water and everything would be great. However, I thought I'd check the pH. Tap water from provider results = 7.5pH. My pH pen tap water = 7.8. My pH pen rain water = 8.4. I thought that the high pH reading of the rainwater was due to the inability of the pH pen to measure accurately in such soft water. So I setup a few tests combining different combinations of rain water and tap water and measured them across 3 days. Below are the results (I don't have a Gh or dKH test kit, so can't provide that info but I've included what is available from the water company). The reason I question the results is that I'd expect adding rain water to my hard tap water would either maintain or slightly lower the pH but instead it's pushing the pH higher. I collected the rainwater in a bucket to remove the possibility of contamination from anything on my roof and I didn't collect rain from the first rain shower, to avoid air pollutants as much as possible. I'd really like to get you opinions on this as I've not managed to find anything or come up with a reason, other than:
1/ The pH pen is reliable (I get a fairly consistent reading +/- 0.3) but inaccurate and it was a coincidence that the water company and my own reading are fairly similar.
2/ There is something in the rainwater that really has an impact on pH.

Test Description
Water TestedHydrogen ion (pH)Conductivity (ppm)°dHAlkalinity (mg/l CaCO3)Alkalinity (mg/l HCO3)Comments
Tap Water Only7.8311Results are the mean of 3 days of testing at room temperature (approximately 18°C water would be colder).
Rain Water Only8.458Results are the mean of 3 days of testing at room temperature (approximately 18°C water would be colder).
50:50 Tap : Rain 7.9195Results are the mean of 3 days of testing at room temperature (approximately 18°C water would be colder).
25:75 Tap : Rain8147Results are the mean of 3 days of testing at room temperature (approximately 18°C water would be colder).
Water Company Results7.51293161631990.5 conversion factor applied to get conductivity in ppm from µS/cm@20°C

Thanks for you time.
 

dw1305

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Hi all,
So I thought I'd check my rain water, 58ppm TDS (using my own TDS pen at a room temp of 18°C). This all seemed great as I could soften my hard water with rain water and everything would be great
I'm a rainwater user, the ppm TDS value is the important bit, and it does mean that you can soften your tap water with it. I don't worry too much about pH, I just use conductivity (and the state of <"the snail shells">) as a proxy for water hardness (both dGH/dKH).

Apologies for the rest of the post, it gets a bit "chemical" (via the linked threads), but there isn't really any way around that. I think that pH and buffering are probably the most <"problematic water parameters to understand"> and the most misunderstood on forums etc.

<"Conductivity"> (TDS meters actually <"measure electrical conductivity in microS">). Conductivity is a linear scale, so 50:50 rain and tap water should have a TDS value of about (311 + 58)/2 ~ 185 ppm (near enough what you got). It is also temperature dependent, and conductivity values rise with increasing temperature
However, I thought I'd check the pH. Tap water from provider results = 7.5pH. My pH pen tap water = 7.8. My pH pen rain water = 8.4. I thought that the high pH reading of the rainwater was due to the inability of the pH pen to measure accurately in such soft water.
That is part of the answer, pH meters are modified conductivity meters and don't perform <"well in low ionic strength solutions">.
The reason I question the results is that I'd expect adding rain water to my hard tap water would either maintain or slightly lower the pH but instead it's pushing the pH higher
The issue here is actually two fold, it is:
Calcium carbonate is insoluble in water, but in water with carbonates present the small amount of CO2 (that goes into solution as H2CO3) is in equilibrium with the HCO3- to give a stable value of ~pH8 at atmospheric CO2 levels (400ppm CO2) and standard barometric pressure (1013mb).
All the time you have some <"carbonate buffering"> the pH will return to ~pH8, but you now (in your 50:50 mix) have fewer <"HCO3- and/or CO3-- ions"> to accept a proton (H+ ion). (Bi)carbonate is a base (a <"proton acceptor">) and CO2 is an acid (a "proton donor").

Your rainwater has many fewer proton acceptors than your hard tap water, so you need a smaller addition of acids to neutralise that buffering and for the pH to eventually start to fall.

Even in carbonate buffered water pH can be <"directly affected by photosynthesis"> and the changes in the ratios of dissolved oxygen and CO2 it produces.

cheers Darrel
 
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ScareCrow

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Thanks for your reply Darrel,

"Your rainwater has many fewer proton acceptors than your hard tap water, so you need a smaller addition of acids to neutralise that buffering and for the pH to eventually start to fall". This was what I wanted to try next. I was going to add some alder cones to the rainwater before adding it to the tap water to see if I could get a pH reduction overall. Unfortunately, the only Alder I knew of near me has been cut down, so I'll need to search further a field.

I'm not really concerned with chasing any pH value, I was just interested why the pH seemed to increase with the addition of more rain water. So thank you for your reply.

I intend to keep Killis so wanted to try and get it a bit softer for them and they did fine in rainwater last time.
 
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dw1305

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Hi all,
Thanks for your reply Darrel,
You are good, I'm sorry it is such a complicated reply.
Unfortunately, the only Alder I knew of near me has been cut down, so I'll need to search further a field.
I'm a great <"Alder cone fan">. There are a few sellers <"on Ebay"> (I've bought food from "A World of Fish" (including some "decapsulated brineshrimp eggs" last week) and would recommend him as a seller). Otherwise look by rivers and ponds, and another good source is Supermarket/Industrial Estate car parks, where <"Italian Alder (Alnus cordata)"> is often planted as a "landscape tree".

I have <"structural leaf litter"> in all the tanks now.
I intend to keep Killis so wanted to try and get it a bit softer for them and they did fine in rainwater last time
I kept <"Epiplatys annulatus in 100% rainwater"> for a couple of years, and I may get them again. My only other <"relatively recent"> foray in Killis was with <"Poropanchax normani">, so I don't have a lot of experience to offer.

We have some proper <"Killifish keepers on the forum"> @jolt100 , so they maybe able to advice you for other species.

cheers Darrel
 
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ScareCrow

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Thanks for the tip about the use of Alnus cordata as a landscape tree. I would have thought with Alders preference for water it wouldn't have been used in that way.

I've been considering Aphyosemion Striatum (I think that they prefer slightly cooler temperatures for breeding), Epiplatys annulatus (would be perfect if I could find some and also get the pH down), Aphyosemion australe (are probably the most suitable option) or Epiplatys dageti. I've kept F.gardneri in the past but found them to be too aggressive.
 

ScareCrow

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Thanks Darrel,

I have used oak leaves in the past and the humic acids are one of the reasons I'm going to use composted leaf litter as a substrate. I find that alder cones are more concentrated than leaves but I'll add some leaves anyway as I like the aesthetic.
 

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