Reverse Osmosis?
- Thread starter matty2k
- Start date
elisew
I don`t count sheep to get to sleep, I count fish
elisew
I don`t count sheep to get to sleep, I count fish
I`ve changed my setup from Tropical to Tanganyikan now so don`t need the RO but the RO water is stored in a 200L water butt in the garden, I was using a mix of 3 parts RO to 1 part tap water, this brought my ph down from 7.6 to 6.6-6.8.
If you`re going to use RO water then you really need to start off mixing 1 part RO with 3 parts tap and then increase the RO ratio over about 4-6 water changes so the fish have time to acclimatise to the difference.
If you`re going to use RO water then you really need to start off mixing 1 part RO with 3 parts tap and then increase the RO ratio over about 4-6 water changes so the fish have time to acclimatise to the difference.
RO water, by definition, has a pH of 7.0. RO water alone cannot bring pH below 7.0. You'll need something acidic to go below the neutral value of 7.0.
elisew
I don`t count sheep to get to sleep, I count fish
My ph also went below 7.0 
Matty, if your ph with just RO is 5 then obviously you will need to find the correct ratio you want by adding some tap water and keep testing until you've got it right. As already said please make sure you slowly up the amount of RO over a few water changes and don't just whack a whole load in at one go, otherwise you'll likely see some of your fish struggling and dying
Matty, if your ph with just RO is 5 then obviously you will need to find the correct ratio you want by adding some tap water and keep testing until you've got it right. As already said please make sure you slowly up the amount of RO over a few water changes and don't just whack a whole load in at one go, otherwise you'll likely see some of your fish struggling and dying
RO water is so pure, that pH is almost a meaningless statistic about it. Any tiny addition to the water will cause the pH to swing by a large amount. This is because there is no buffering in RO water.
I suspect that your readings below 7.0 is because the carbon dioxide in the air mixed with the very pure water and formed carbonic acid. Again, with the water with no buffering, a little bit goes a long way. And, when the water is so pure, very sophisticated test equipment is needed to accurately measure pH -- test strips and drip tests are not accurate enough to be meaningful at all.
Carbonic acid is very weak and temporary. This is why CO2 injectors have to continuously add CO2 to maintain the low pH. It is temporary because almost anything else added to the water will be stronger and the determining factor in what the pH will be -- the carbonic acid is negligible at atmospheric pressure.
Again, RO water, as pure water, by definition, has a pH of 7.0. RO water cannot be mixed with any quantity alkaline water to bring the pH below 7.0.
I suspect that your readings below 7.0 is because the carbon dioxide in the air mixed with the very pure water and formed carbonic acid. Again, with the water with no buffering, a little bit goes a long way. And, when the water is so pure, very sophisticated test equipment is needed to accurately measure pH -- test strips and drip tests are not accurate enough to be meaningful at all.
Carbonic acid is very weak and temporary. This is why CO2 injectors have to continuously add CO2 to maintain the low pH. It is temporary because almost anything else added to the water will be stronger and the determining factor in what the pH will be -- the carbonic acid is negligible at atmospheric pressure.
Again, RO water, as pure water, by definition, has a pH of 7.0. RO water cannot be mixed with any quantity alkaline water to bring the pH below 7.0.
my question would be, WHY? what use id RO in a tropical tank. especially, as bignose says, it often cant do the job people use it for.
my opinion, too many people using it, to few people understanding it.
i would, however, like to know why you feel you need it. or, more probably, what you have been told you need it for?
I take it you will be 'cutting' (mixing) the RO with tap water?l
I want to write a little be here about what pH actually is, because I still get the feeling that RO and distilled water is some sort of "magic pH remover" when that just isn't the case.
First, let me define what pH is. p is mathematical shorthand for the negative base 10 logarithm of the concentration of what follows the p. H in this case stands for [H+] where the square brackets is normal notation to mean the concentration of. That is, [H+] represents the concentration of H+. Finally, the + is important, because H never exists on it elemental form on its own -- a single H atom will be very strongly incline to bind with something else, and this inclination to bond is denoted to be positive. There are also negatives out there, and they are denoted as such because when balanced quantities of negative and positive material comes together, they form substances without charge. Pure hydrogen will be found in a form H2 -- 2 hydrogen atoms bonded together.
Let me show you an example of definition of pH at work. Lets say I have a water solution with a concentration of H+ being 0.000 002 3 moles per liter. (A mole is a number of molecules of a substance -- it is very useful in chemistry.) The pH of this solution is - log_10 (0.000 002 3). The log base 10 of 0.000 002 3 is -5.638. The negative of that number is 5.638. Hence the pH of this example solution is 5.638.
Some more nomenclature: A pH greater than 7.0 is called basic or alkaline. A pH less than 7.0 is called acidic. A pH of exactly 7.0 is considered neutral. I will get to the motivation of these terms below.
Now, let me talk about how H+ gets into the water in the first place. In most circumstances, this is because of the addition of something to the water. For example, let's say I add a quantity of hydrochloric acid to water. Hydrochloric acid is HCl and in water, the hydrogen and the chlorine atoms will disassociate. HCl <--> H+ + Cl-.
I wrote the disassociation reaction with arrows pointing both ways, because this is a reaction that is reversible. That means that HCl is constantly breaking apart into H+ and Cl- and H+ and Cl- are constantly coming back together and forming HCl. Chemists have a measure of what percentage of HCl is broken into H+ and Cl- at any time, known as the disassociation constant, usually denoted K.
This K has a formula of: K = [H+][Cl-]/[HCl]. (Remember that the []'s denote the concentration of whatever is inside them.) Despite normally being called a "constant", the value of K is normally sensitive to temperature changes, and the above formula as written is only valid when HCl is the only substance in the water -- other things in the water affect the disassociation.
Nevertheless, the above equation gives us what we need to calculate pH. If a known amount of hydrocloric acid is added to a known amount of water at a known temperature, the above equation allows calculation of [H+], which we can then take the negative of the base 10 logarithm of and report the pH.
In general, an acid is of the form HR. Where R denotes any of the bases -- Cl is the base above. R = SO4 for sulfuric acid. R= PO3 for phosphoric acid. And so on.
For the ones of interest to fishkeeping, R = NO2 (what we know as nitrite) is the base of nitrous acid and R = NO3 (nitrate) is the base of nitric acid.
All of these have their own disassociation constants, and in fact how much disassociation occurs is one way in which people talk about how strong or weak an acid is. Strong acids are acids which high disassociation constants.
Bases work the same way. If I add a base like sodium hydroxide (NaOH) or baking soda (sodium bicardonate NaHCO3) it too will disassociate in water:
NaOH <--> Na+ + OH-. And, in fact, one can even talk about a pOH to measure the concentration of OH ions in the water. And just like the acids, Ks as also defined for these disassociation.
The last piece of the puzzle is that water itself will disassociate.
H2O <--> H+ + OH-. Water is constantly breaking itself into hydrogen and hydroxide ions and constantly reforming. And, just like the acids and bases, we can define a disassociation for water: K = [H+][OH-]/[H2O].
For pure water at 25 degrees C, the value of K is known to be 10^-14 (= 0.000 000 000 000 01).
So, 10^-14 = [H+][OH-]/[H2O]. Now, two more things. Since it is pure water, we take [H2O] to be equal to 1. And, we note that in the disassociation reaction H2O <--> H+ + OH-, for every H+ made there is also exactly 1 OH- made. Therefore, the concentration of H+ and OH- has to be same [H+] = [OH-]. We put the two facts into the equation:
10^-14 = ([H+]^2)/1. Solving [H+]=10^-7. Then, turning [H+]=10^-7 = 0.000 000 1 into a pH we get pH=7.0.
This is why pure water has a pH of 7.0. Because water itself disassociates.
Now, look at what happens when we add an acid or a base. An acid, especially a strong acid, will release lots of extra H+ in the water. All those extra H+ around means that whenever water disassociates, the OH- that forms is very, very quickly scooped up by one of the many extra H+s around. In the same way, when there is a strong base, all the extra OH- around scoop up any H+ when they are formed
In effect, the disassociation of water is insignificant whenever there is a strong acid or a strong base around.
This is why when RO or distillation removes all contaminants from the water, those acids and bases are removed, and the pH changes dramatically.
As I wrote above, the carbon dioxide in the air is enough to form carbonic acid. RO water that is needed to be kept pure is stored in a container with an atmosphere of nitrogen gas, not air, so that the pH remains 7. Carbonic acid is a weak acid, however, and stronger stuff will overwhelm what little influence carbon dioxide has on pH, however.
-------------
So, after all that, I hope that it explains a little better what pH is, why RO isn't a magic pH eliminator, and why RO water has a pH of exactly 7.0. The pH of 7.0 is a property of pure water, and isn't something that can be changed without adding something to the water.
First, let me define what pH is. p is mathematical shorthand for the negative base 10 logarithm of the concentration of what follows the p. H in this case stands for [H+] where the square brackets is normal notation to mean the concentration of. That is, [H+] represents the concentration of H+. Finally, the + is important, because H never exists on it elemental form on its own -- a single H atom will be very strongly incline to bind with something else, and this inclination to bond is denoted to be positive. There are also negatives out there, and they are denoted as such because when balanced quantities of negative and positive material comes together, they form substances without charge. Pure hydrogen will be found in a form H2 -- 2 hydrogen atoms bonded together.
Let me show you an example of definition of pH at work. Lets say I have a water solution with a concentration of H+ being 0.000 002 3 moles per liter. (A mole is a number of molecules of a substance -- it is very useful in chemistry.) The pH of this solution is - log_10 (0.000 002 3). The log base 10 of 0.000 002 3 is -5.638. The negative of that number is 5.638. Hence the pH of this example solution is 5.638.
Some more nomenclature: A pH greater than 7.0 is called basic or alkaline. A pH less than 7.0 is called acidic. A pH of exactly 7.0 is considered neutral. I will get to the motivation of these terms below.
Now, let me talk about how H+ gets into the water in the first place. In most circumstances, this is because of the addition of something to the water. For example, let's say I add a quantity of hydrochloric acid to water. Hydrochloric acid is HCl and in water, the hydrogen and the chlorine atoms will disassociate. HCl <--> H+ + Cl-.
I wrote the disassociation reaction with arrows pointing both ways, because this is a reaction that is reversible. That means that HCl is constantly breaking apart into H+ and Cl- and H+ and Cl- are constantly coming back together and forming HCl. Chemists have a measure of what percentage of HCl is broken into H+ and Cl- at any time, known as the disassociation constant, usually denoted K.
This K has a formula of: K = [H+][Cl-]/[HCl]. (Remember that the []'s denote the concentration of whatever is inside them.) Despite normally being called a "constant", the value of K is normally sensitive to temperature changes, and the above formula as written is only valid when HCl is the only substance in the water -- other things in the water affect the disassociation.
Nevertheless, the above equation gives us what we need to calculate pH. If a known amount of hydrocloric acid is added to a known amount of water at a known temperature, the above equation allows calculation of [H+], which we can then take the negative of the base 10 logarithm of and report the pH.
In general, an acid is of the form HR. Where R denotes any of the bases -- Cl is the base above. R = SO4 for sulfuric acid. R= PO3 for phosphoric acid. And so on.
For the ones of interest to fishkeeping, R = NO2 (what we know as nitrite) is the base of nitrous acid and R = NO3 (nitrate) is the base of nitric acid.
All of these have their own disassociation constants, and in fact how much disassociation occurs is one way in which people talk about how strong or weak an acid is. Strong acids are acids which high disassociation constants.
Bases work the same way. If I add a base like sodium hydroxide (NaOH) or baking soda (sodium bicardonate NaHCO3) it too will disassociate in water:
NaOH <--> Na+ + OH-. And, in fact, one can even talk about a pOH to measure the concentration of OH ions in the water. And just like the acids, Ks as also defined for these disassociation.
The last piece of the puzzle is that water itself will disassociate.
H2O <--> H+ + OH-. Water is constantly breaking itself into hydrogen and hydroxide ions and constantly reforming. And, just like the acids and bases, we can define a disassociation for water: K = [H+][OH-]/[H2O].
For pure water at 25 degrees C, the value of K is known to be 10^-14 (= 0.000 000 000 000 01).
So, 10^-14 = [H+][OH-]/[H2O]. Now, two more things. Since it is pure water, we take [H2O] to be equal to 1. And, we note that in the disassociation reaction H2O <--> H+ + OH-, for every H+ made there is also exactly 1 OH- made. Therefore, the concentration of H+ and OH- has to be same [H+] = [OH-]. We put the two facts into the equation:
10^-14 = ([H+]^2)/1. Solving [H+]=10^-7. Then, turning [H+]=10^-7 = 0.000 000 1 into a pH we get pH=7.0.
This is why pure water has a pH of 7.0. Because water itself disassociates.
Now, look at what happens when we add an acid or a base. An acid, especially a strong acid, will release lots of extra H+ in the water. All those extra H+ around means that whenever water disassociates, the OH- that forms is very, very quickly scooped up by one of the many extra H+s around. In the same way, when there is a strong base, all the extra OH- around scoop up any H+ when they are formed
In effect, the disassociation of water is insignificant whenever there is a strong acid or a strong base around.
This is why when RO or distillation removes all contaminants from the water, those acids and bases are removed, and the pH changes dramatically.
As I wrote above, the carbon dioxide in the air is enough to form carbonic acid. RO water that is needed to be kept pure is stored in a container with an atmosphere of nitrogen gas, not air, so that the pH remains 7. Carbonic acid is a weak acid, however, and stronger stuff will overwhelm what little influence carbon dioxide has on pH, however.
-------------
So, after all that, I hope that it explains a little better what pH is, why RO isn't a magic pH eliminator, and why RO water has a pH of exactly 7.0. The pH of 7.0 is a property of pure water, and isn't something that can be changed without adding something to the water.
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