Test pH value in the low TDS water

[zhihuai1982]

I have read some articles about wild caught of apistogramma of TomC. I find that the pH value of the water in the amazon is quite low, always 5 or even lower, with TDS near zero. So I fell very strange that compare with the RO water, both of them has low value of TDS, but when I test the pH of the RO water with pH meter, the reading is 6.8. Why? In my mind, TDS near zero means nothing in the water, what makes the water so acid in the amazon?

BTW: Dose any body know what method did they use to test the pH value when they were in the wild?

 

[Simon Morgan]

RO water is fairly pure. In many ways it ought to be neutral but it's likely that some CO2 from the air has made it a little acidic. The Amazon on the other hand is full of decaying leaves and wood and the water is full of Tannic and Humic acids, hence the low pH.
I believe it is unwise to use a pH meter in purified water as it can corrode the probe, try colorimetric tests instead.

 

[dw1305]

Hi all,

So I fell very strange that compare with the RO water, both of them has low value of TDS, but when I test the pH of the RO water with pH meter, the reading is 6.8. Why? In my mind, TDS near zero means nothing in the water, what makes the water so acid in the amazon?

My suspicion would be that TDS should really read "electrical conductivity". To actually get a value for TDS you need evaporate a known weight of filtered "water" (actually a "dilute solution", rather than pure H2O) to dryness, and weigh the residue. If you start with pure H2O you don't have any residue or dissolved solids.

In water where the TDS are from "salts" electrical conductivity and TDS are directly comparable (100ppm TDS is about 150 microS.) but in the Amazon if you actually measured the TDS, the colloidal humic and tannic compounds that contribute to the acidity and black tint to the water would also contribute to the TDS. They aren't electrical conductors, so they don't contribute to the EC.

The other problem is to do with the nature of the pH scale. pH is a ratio of the H+ ion donors (acids) and H+ ion acceptors (bases), because it is a ratio it doesn't tell us anything about amounts. In water with no salts we by definition have no carbonate buffering and the addition of any small amounts of acids will cause the pH to fall, in this case we only need a very small amount of humic acid, or even dissolved CO2, to produce low pH.

Have a look at this thread for a more complete discussion: <http://www.apistogramma.com/forum/index.php?threads/ok.12048/>.

cheers Darrel

 

[gerald]

One small clarification on Darrel's post: What he means (I think) is that pH doesn't tell us anything about amounts of any other ions besides H+ and OH-. A pH measurement does in fact tell you the concentrations of those two ions: For example, pH 5.0 means the H+ concentration is 10-5 and the the OH- concentration is 10-9 . The sum of the two exponents (-5 and -9 in this case) is always -14 (roughly, in water at room temp). Sorry - don't know how to use superscript for the exponents).

pH is a ratio of the H+ ion donors (acids) and H+ ion acceptors (bases), because it is a ratio it doesn't tell us anything about amounts.

 

[dw1305]

Hi all,

What he means (I think) is that pH doesn't tell us anything about amounts of any other ions besides H+ and OH-. A pH measurement does in fact tell you the concentrations of those two ions: For example, pH 5.0 means the H+ concentration is 10-5 and the the OH- concentration is 10-9 . The sum of the two exponents (-5 and -9 in this case) is always -14 (roughly, in water at room temp). Sorry - don't know how to use superscript for the exponents).

Yes Gerald is right and I do, pH is the measure of H+ ions as the negative log10 value, the initial 1 x 10-14 ions coming from the self ionization of pure water (H2O). Pure water is an ~ electrical insulator, because we only have 10-14 ions. When we add an acid we add an H+ ion donor, the pH will fall, and the electrical conductivity rises.

If that still doesn't make sense, have a look at Regani's posts in this thread <http://www.apistogramma.com/forum/index.php?threads/700-liter-amazone.13197/#post-71427>.

cheers Darrel

 

[gerald]

Pure water has 1 x 10-7 H+ ions and 1 x 10-7 OH- ions, so the total ion concentration is 2 x 10-7 Moles per Liter.
Their product (H+ concentration X OH- concentration) is 1 x 10-14, but their sum is 2 x 10-7. The conductivity of totally pure water is supposedly about 0.06 uS. The lowest conductivity measured in natural streams (e.g. Rio Negro) is around 8 uS.

 

[dw1305]

Hi all,

Their product (H+ concentration X OH- concentration) is 1 x 10-14, but their sum is 2 x 10-7. The conductivity of totally pure water is supposedly about 0.06 uS. The lowest conductivity measured in natural streams (e.g. Rio Negro) is around 8 uS.

Thank you, cruelly snubbed again I'll get my coat.

I will eventually manage to answer these water chemistry questions correctly, it may just take a few more attempts.

cheers Darrel

 

[gerald]

No cruelty intended; watching out for each other's gaffes we usually get it right eventually, or at least we come up with something together that sounds believable and nobody questions it further |;>)

 

[zhihuai1982]

thanks dw1305 and gerald, but I still wanna know what method did they use to test the pH value when they were in the amazon? cause in my mind, the ph meter is not so accurate when the tds is low.

 

[regani]

What we generally talk about as 'low TDS' (or better 'low conductivity') is actually not that low. Generally in the context of pH measurements and the difficulty of measuring pH in high purity water the conductivity values are well below 1 micro Siemens (uS), so still quite a bit lower than anything measured in naturally occurring water sources.
Ultra-pure water with a conductivity of 0.06 uS is for example used in the manufacture of microchips. The theoretical value for pure water (based on the self-dissociation of water molecules into H3O+ and OH-) is about 0.055 uS.
If e.g. ultra-pure water is exposed to air just the equilibration with atmospheric CO2 will lead to a conductivity of approx. 1 uS and a (measurable) pH of 5.6. Any other organic or inorganic material present will then affect conductivity and pH on top of that.
So compared to really, really pure water all natural water sources have a much higher conductivity and and a good quality pH meter will be able to measure it.

for more details about measurements in low conductivity water see e.g. here or here

 

[dw1305]

Hi all,
I'll have another go.

but I still wanna know what method did they use to test the pH value when they were in the amazon? cause in my mind, the ph meter is not so accurate when the tds is low.

That is right, but in some ways the actual pH value is a bit of a red herring, and it doesn't matter what method they measured the pH with.

I think that Regani has covered the really important bit, basically that pure H2O is very effective solvent, which means that it can acquire solutes from a myriad of sources.

Geology and climate
In the case of the Amazon basin geology and climate are important. Away from the Andes piedmont the rocks are ancient shield rocks, over lain by silica based sediments, and this is a region of heavy rainfall. The net effect of this is that any soluble compounds will have already been washed from the soil (the process of laterization) and there aren't any carbonate rich rock sources to replenish calcium and HCO3-. In fact there are virtually no water soluble compounds in either sediment or rock.

Sources of acids and bases
The outcome of this is that any small amount of acid compounds that go into solution (such as humic acids from leaf litter and including the small proportion of CO2 that dissolves as H2CO3), will lower pH. In the case of CO2 there is no "conjugated base" HCO3- ions (dKH) to balance the CO2 ~ HCO3- equilibrium and maintain a neutral pH. We know there aren't many potential solutes because the conductivity is so low, although some ions may be organically bound by the DOC.

pH measurement
If you look at the linked "OK" thread Ted says

that you can stabilize pH with NaCl, and that this doesn't effect any other parameter, and the fish themselves are as happy at 300microS as they were at 30microS.

and this on pH meters, from Gerald's post at <http://www.apistogramma.com/forum/index.php?threads/natural-ph-alterations.14868/#post-78348>

Even the good meters have a hard time measuring pH in very low conductivity water. When there's so few ions present, a tiny shift in real numbers of ions creates an apparent BIG shift in pH. The meter itself leaches out KCl and affects the reading as you're measuring it. (That's why you keep stirring). The "ISFET" meters are supposed to work a little better than typical glass electrodes in very low conductivity water, but all electric meters have limitations.

cheers Darrel

 

[Mike Wise]

The 2 times I went with Tom we used simple Hannah pH/cond/temperature hand probes. The values given are adequate for keeping the fish at home. There is no reason to keep water values precisely at given values. There is always a range of values fish tolerate. This isn't technical water chemistry testing like I did for geologic studies, where water was run through an emission spectrometer and contents need to be stabilized.