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What pH does TO filtration give?

Discussion in 'Water Chemistry' started by Siggi, May 9, 2018.

  1. Siggi

    Siggi Member

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    Hi, all.
    I'm getting ready to set up a large aquarium, mainly to house Apistos and tetras.
    I have moved away from Lisbon, capital of Portugal, to a mountainous region in the central part of the country. The mountains are granitic and local spring water has the following characteristics:

    15258612094011948859095.jpg
    (Note: "resíduo seco" means "dry residue" and is an idea of suspended mineral load)

    Tap water is close to pH 7 and leaves some stain on glasses, windows, cutlery, etc. I guess they add some carbonates to harden and take some acidity off the water.
    My plan is to buy a reverse-osmosis filter and use that water to fill the aquarium.
    My question is this: Will the RO filter produce pH 7 "pure" water or will it give me back the acidity the water had before the additives, around pH 6?
    Thank you for you time and responses.
    Cheers!
  2. Mike Wise

    Mike Wise Moderator Staff Member 5 Year Member

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    R/O water straight from the unit should give you about pH 7, but once it mixes in air CO2 in it will drop the pH into the pH 5.5 - 6.0 range. My reservoirs typically produce a pH ~6.5 because they aren't exactly clean and sterile.
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  3. Siggi

    Siggi Member

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    Thank you, Mike.
    I'll keep that in mind. Maybe add peat extract, oak leaves, or the likes. As the hardscape will be made with local rocks and gravel (granite), that will also keep the pH on the acidic side - not much, but consistently.
  4. dw1305

    dw1305 Well-Known Member 5 Year Member

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    Hi all,
    Brilliant, I really liked the Lima valley when I visited Portugal.
    Do you have a conductivity meter?

    That would give you an idea of any added (carbonate) salts. If the reading was less than 100 microS out of the tap, I would just use the tap water and some added tannins.

    If you have access to trees like Quercus spp. (Oak) andArbutus unedo (Medronho or Strawberry tree) it would supply wood and leaves that should be suitable tannin sources.

    cheers Darrel
  5. regani

    regani Active Member 5 Year Member

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    "resíduo seco" is the same as TDS, that is residue left after evaporating the water - conductivity is just a shortcut way of measuring TDS.
    So the water in that bottle would have a TDS of 19. The tap water will likely be different because it generally would be treated before it goes into the water supply. But if that is what they start from the TDS of the tap water is probably not going to be that high. A TDS meter or test kits for GH and KH will give you the answer.
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  6. dw1305

    dw1305 Well-Known Member 5 Year Member

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    Hi all,
    @regani, thank-you it would have made much more sense if I'd put that bit in my post in the first place.

    Because TDS would have been measured by evaporation (of a known volume of water to dryness), the maximum conductivity in the spring water would be 19/0.64 ~ 30 microS (1 microS conductivity is ~ 0.64 ppm TDS). This is fairly close to RO water straight from the spring.

    Thirty microS would be a maximum value, because any non-ionic compounds (humic compounds etc) would contribute to the TDS (measured by evaporation), but not to the conductivity.

    Most TDS values are estimates, based on measuring electrical conductivity and then using a conversion factor (usually 0.64) to get a ppm TDs value

    cheers Darrel
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  7. gerald

    gerald Well-Known Member 5 Year Member

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    Also, the low pH of the spring water might be due to elevated CO2 from the groundwater, if the analysis on the bottle was done without aerating the sample. If so, then the pH of spring water left in an open container will rise as it equilibrates with the lower CO2 concentration in the air.
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  8. TCMontium

    TCMontium Member

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    My RO systems (I had several in last 8-9 years) always gave a pH of 5.0 to 6.0. I have a Dennerle Osmose Professional 190 right now and it produces water with approx. 15-20 microSiemens conductivity and 5.5 pH. Even if I let the water wait for a day or a week in a bucket the pH does not go up. All my aquariums also have pH of around 5.5 (liquid test measures only 0.5 difference of pH) with only use of RO water and approx. 1 catappa leaf per 10 liters of added water at every other water change.
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  9. Siggi

    Siggi Member

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    Hi again.
    Thank you all for your valuable inputs.

    I just visited the municipal water service and the officer in charge of water quality let me check the samples.
    The most recent values are from last October (end of dry season after which there was an unusual drought all through February...).
    Values are listed as follows:
    mg/l NH4 <0,01
    mg/l NO2 <0,04
    mg/l Ca 4,7
    mg/l CaCO3 (listed as total hardness !?) 18
    uS/cm 51
    mg/l Cl2 0,8
    pH 7,1
    mg/l Fe <40
    There are other values about benzo... and bacteria, but they are all zero or way below permitted minima.

    It's been raining all through March and April so portuguese soils are about saturated with water and dams are gradually getting full also. At present there is less pressure on acquifers by human needs and e.g. chlorine will be equal or less than last October...

    Does this help you?
    Thx
    Last edited: May 10, 2018
  10. Siggi

    Siggi Member

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    This whole region is a granitic mountain with some areas of slate - all of Hercynian (/Appalachian) age. Normally granite is considered an 'acidic rock', as opposed to basalt or limestone. I do not know if the bottled water is measured after aeration, but my guess is it is measured at the bottling factory, which is here in Manteigas, where I live.
    The sping is at ~1350m (4400ft) in the valley south of here and the water is brought down in a pipe under the road. The factory is in the southern end of the town, at 760m (2500ft), some 8km (5miles) away from the spring. I don't know3 if the piping is in a siphon or if the water aerates on the way - probably not...
    Anyway - granite springs in Portugal are normally all acidic, with pH between 5,5 and 6,5... only one exception: Monchique spring in Algarve is at pH 9,2.
  11. gerald

    gerald Well-Known Member 5 Year Member

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    In very soft water with low mineral concentrations, a very small change in dissolved ions (or CO2) can cause an apparent BIG shift in pH.
  12. TCMontium

    TCMontium Member

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    Doesn't that happen if there is a considerable amount of buffer solution like kH (CaCO3) in the water? If you use "nearly pure" water with only CO2, O2 and other dissolved gases inside, will it still experience sudden pH drops and spikes at particular points like buffer solutions do when the buffer region is being crossed?
    I sucked (and still suck) at acids, bases and buffers at chemistry...
  13. Siggi

    Siggi Member

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    Hi, all.
    @TCMontium I'd say it's the other way around: if there is buffer present, that avoids sudden changes in acidity. Without a buffer substance in pure H2O, any small amount of OH- releasing substance will spike the acidity.
    But like you, I'm no ace in chemistry. Hopefully someone more knowledgeable will tap in and straighten this out...

    Cheers!
  14. TCMontium

    TCMontium Member

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    Yes, buffers make it so that in their buffer range pH will change extremely slowly, it will pretty much stay the same even if you add lots of acids or bases. But the sudden huge spikes/drops of 1.0-4.0 points of pH happen if you exceed the buffer range. That is why african cichlid keepers have problems with dying fish caused by sudden big pH drops after water changes, if kH (the buffer) level is not high enough to deal with the added "acidic/less alkaline" water. You can see examples of buffer effect if you search for "buffer titration curve" on images.

    I suppose carbon dioxide alone becomes an acid (HCO3) in water and has a corresponding base (which is ?), which would create a buffering solution, but I do not think it has a "strong" enough buffer range to cause huge drops/spikes at it's "limits". So RO water would become more acidic or alkaline/basic at a "normal" rate rather than keeping the pH stabilized for a while and than having big drops/spikes, right?
  15. dw1305

    dw1305 Well-Known Member 5 Year Member

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    Hi all,
    It depends a little bit on the carbonate hardness of the source water. If none of the residual solutes (that give you the 15- 20 microS) are bases, the pH will be dependent upon the dissolved CO2, and will be ~pH5.5. The working <"are here"> for 355ppm CO2, but the level of CO2 has now risen (to 400ppm atmospheric CO2), slightly lowering the equilibrium pH.
    [​IMG]
    If you have hard water flowing into the RO unit you are likely to have some residual HCO3- ions (in the RO output) and these will add enough carbonate buffering to raise the pH to ~pH8 (the pH value for carbonate buffered water).
    That is the one, as you get close to pure H2O, pH is not really a very useful measurement, because small changes in water chemistry cause large changes in pH. In buffered water the opposite is true, you need large changes in water chemistry to change the pH. Soft vegetated water will show large diurnal pH swings as the relative proportions of CO2 and O2 change during photosynthesis.
    Perfect, you don't need the RO unit.

    The 18 mg/l (ppm) CaCO3 is equivalent to 1dKH, and the conductivity is only 50 microS. Any addition of humic or fulvic acids from Oak leaf litter etc should drop the pH below pH7.
    Back to needing @regani, but I think that is right, if a substance accepts a H+ ion it is a "base", so you have OH- (a base), and when you add an acid (a proton (or H+ ion) donor) the OH- ion accepts an H+ ion to form neutral (pH7) H2O.

    Carbon dioxide (CO2) is an acid, because a very small proportion of the gas dissolves as carbonic acid, (H2CO3) or H+ and HCO3- (or CO3-- at higher pH ). You've liberated a H+ ion and the pH falls.

    You always have the same amount of total inorganic carbon (TIC), and the equilibrium points are related to the atmospheric CO2 level.

    [​IMG]

    The exception is if you add CO2 (aquascapers etc.), you then lower the pH (you've added more H+ ions), but you haven't changed the alkalinity, it is just you have new equilibrium pH values.

    cheers Darrel
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  16. regani

    regani Active Member 5 Year Member

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    Darrel's got it.
    Anything that gives away a proton (H+) is an acid, anything that captures a proton is a base.
    There is an important difference between strong and weak acids. Strong acids, like hydrochloric acid (HCl) will completely 'fall apart' in water, that is they give away all their protons. Weak acids will only give away some protons while holding onto others.
    That means if we have the same number of molecules of hydrochloric acid and carbonic acid (H2CO3 - the acid generated by dissolving CO2 in water) in in two separate volumes of water the HCl solution will have a much lower pH compared to the carbonic acid solution (even though the carbonic acid can in theory release two protons per molecule and the hydrochloric only one!).
    There is some good info on the Wikipedia page for carbonic acid. Just ignore all the equations in there and read the general explanations. The table and the comments on there illustrate the issue quite well, I think.
    As mentioned multiple times in various threads, in low TDS water the pH is not that important as changes in pH occur quite readily because of the lack of buffering capacity. This can be caused e.g. by dissolved CO2, humic acid etc. If the pH changes from 7 to 5.5 in low TDS water, this doesn't necessarily mean there is a dramatic change in the general water chemistry that the fish can't handle.
  17. dw1305

    dw1305 Well-Known Member 5 Year Member

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    Hi all,
    Good, I'm always worried that I've misinformed people, particularly as a lot of it is isn't very intuitive for me.

    The concept of an acid as a proton donor, and a base as a proton acceptor has really helped me with water chemistry, and I think I now have a better idea about buffering and pH.

    This is partially why I suggest using conductivity as my "weapon of choice", rather than the intuitively more useful pH measurement.

    Once you know that @Siggi's tap water reads 50 microS, you don't really need to know anything else, the water is a fairly blank canvas and you can easily adjust the pH up and down with weak bases and weak acids respectively.

    cheers Darrel
    Last edited: May 13, 2018
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