# Distilled Water and pH



## Hoppy (Dec 24, 2005)

Lately there have been a few questions about the pH of distilled or RO/DI water. I couldn't answer the questions, so that bothered me enough to do some studying of the subject. Here is some of what I learned, and I hope others with better chemistry backgrounds will help fill in the blank areas or correct the mistakes.

All of the water parameters we measure are about concentrations in parts per million of substances in the water, except for temperature and pH. Temperature is an energy measurement. But, pH is the logarithm of a concentration of H+ ions (if the concentration is expressed as 10 to the x power, x is the logarithm of the concentration). That makes dealing with pH much different from dealing with hardness, concentration of various nutrients, etc. You can mix equal parts of water having 10 dKH and 0 dKH and get water with 5 dKH. But, if you mix equal parts of pH 6 water with pH 8 water, you don't get pH 7 water - you get just slightly below pH 8 water.

Distilled or RO/DI water has virtually nothing dissolved in it. It has a KH that is virtually zero, for example. Since it has virtually no H+ ions in it, it is neutral on the pH scale, or pH 7. However, as we planted tank people should instinctively know, water in the presence of carbon dioxide absorbs carbon dioxide. So, a test tube of distilled water will immediately begin absorbing CO2 from the atmosphere. But, distilled water has virtually zero KH, and as we all know, it is KH that stabilizes the pH of water when CO2 is dissolved in the water. So, the distilled water in the test tube will have a slowly dropping pH, as it absorbs CO2 from the air. The actual value of the pH will end up as something below 7.0, possibly as low as 5.5, due to that absorbed CO2.

So, we pour distilled water (pH 7) into a bucket, where the water absorbs CO2, dropping its pH to 6.0, for example. Now, we want to add that water to our tank water, which we feel has a too high pH, pH 8.0 for example. Remember pH is the logarithm of the concentration of H+ ions in the water, not the actual concentration. Our pH 6 distilled water actually contains about 1% of the concentration of H+ ions as the tank water at pH 8. Clearly, adding a gallon of that pH 6 water to a gallon of pH 8 tank water will not change the tank water pH at all, at the sensitivity our pH test kit can detect.

If we were to add just a tiny pinch of sodium bicarbonate to that gallon of distilled water, the water will still absorb the same amount of CO2 from the air, but the pH will barely change, if it changes at all, and it will remain pH 7 water. Now, if we add the water to a gallon of our tank water (pH 8.0), we get what? We still get 2 gallons of pH 8.0 water!

As I said, I hope the chemistry majors in our group will correct any errors in this or further explain it.


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## KevinC (May 24, 2004)

Hoppy:

It's all pretty complicated. I think you are oversimplifying in your first example (mixing pH=6 with pH=8 water). What was used to lower and raise the pH values initially? If it was a strong acid and a strong base then they will indeed reach pH=7 on mixing. If either has a buffering ability then it could end up either side of 7. Also this assumes no CO2.

Kevin


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## Hoppy (Dec 24, 2005)

I know it is an oversimplification. It does explain why distilled or RO/DI water doesn't necessarily have a pH of 7.0 as it seems logical that it should have. And, using RO/DI water for water changes, hoping to lower the tank water pH, isn't likely to work well.

Is there a more accurate, but equally simple way to explain this?


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## Captivate05 (Feb 23, 2010)

I think there is no simple way to really explain pH accurately. It's a pretty advanced subject.

Perhaps breaking down pH would help. It's not an easy thing to understand, but teaching others about the properties of pH in water may help some people learn about it better.


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## Hoppy (Dec 24, 2005)

Captivate05 said:


> I think there is no simple way to really explain pH accurately. It's a pretty advanced subject.
> 
> Perhaps breaking down pH would help. It's not an easy thing to understand, but teaching others about the properties of pH in water may help some people learn about it better.


Can you contribute an explanation of some of those properties, where pH is involved?


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## Darkblade48 (Jan 4, 2008)

Hoppy said:


> Our pH 6 distilled water actually contains about 1% of the concentration of H+ ions as the tank water at pH 8.


The above sentence is wrong. A pH 6 solution would contain 100x more hydronium ions than one of pH 8.


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## Hoppy (Dec 24, 2005)

Darkblade48 said:


> The above sentence is wrong. A pH 6 solution would contain 100x more hydronium ions than one of pH 8.


Yes, and a hydronium ion is H3O+, correct? So, for simplicity it is a surplus of H+ compared to OH-, but the H+ is attached to a water molecule, H20. ( I couldn't remember the name, hydronium, so I just used hydrogen. ) Also, isn't it more accurate to say that pH 6 solution has a 100 times bigger surplus of hydronium ions over hydroxide ions than a ph 8 solution, because the pH 6 solution has a 10 to the 8 minus 6 power bigger surplus?

There must be some way to make pH a concept that can be easily understood by those of us not chemically well educated, and a way to intuitively grasp what happens when you mix solutions of different pH. Maybe not, though.


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## Darkblade48 (Jan 4, 2008)

Hoppy said:


> Yes, and a hydronium ion is H3O+, correct?


Yes.



Hoppy said:


> So, for simplicity it is a surplus of H+ compared to OH-, but the H+ is attached to a water molecule, H20. ( I couldn't remember the name, hydronium, so I just used hydrogen. )


Not exactly correct, but OK. It is not as simple as a proton attached to a water molecule.



Hoppy said:


> Also, isn't it more accurate to say that pH 6 solution has a 100 times bigger surplus of hydronium ions over hydroxide ions than a ph 8 solution, because the pH 6 solution has a 10 to the 8 minus 6 power bigger surplus?


This is the same as saying what I said, in a more roundabout way. However, I think you are confusing pH with pOH here. 



Hoppy said:


> There must be some way to make pH a concept that can be easily understood by those of us not chemically well educated, and a way to intuitively grasp what happens when you mix solutions of different pH. Maybe not, though.


Unfortunately, even if you have a high school chemistry background, it is likely not enough to understand pH fully. With high school chemistry (at least senior chemistry, i.e. grade 12, I think is the highest?) knowledge, you should be able to grasp the fundamentals, however.


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## zdnet (Aug 13, 2010)

Darkblade48 said:


> A pH 6 solution would contain 100x more hydronium ions than one of pH 8.


Glad that you pointed it out for I too had made the same mistake by flipping the direction of magnitude progression.

Mixing equal parts of pH 6 and 8 water should dilute the concentration by half (assuming there is no buffer). Thus, going from 1/1,000,000 (pH 6) down to 1/2,000,000 which gives a pH of 6.3.

Replacing 10% of a volume of pH 8 water with pH 6 water should yield a concentration of approximately 1/8,000,000 which gives a pH of 6.9.

Please correct me if that is wrong.




Hoppy said:


> There must be some way to make pH a concept that can be easily understood by those of us not chemically well educated, and a way to intuitively grasp what happens when you mix solutions of different pH.


What got me was being used to having a measuring scale whose numbers are monotonically related to magnitude, i.e. the larger is a number the larger is the magnitude. Thus, going from 6 to 8 represents an increase in magnitude. But the pH scale is a reverse - going from 6 to 8 is a decrease, NOT an increase, of the magnitude (of concentration).


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## Captivate05 (Feb 23, 2010)

Hoppy said:


> Can you contribute an explanation of some of those properties, where pH is involved?


Heh, I can ask my dad to write out the properties of pH with water, but I guarantee it'll be a book that few people here (me included) could understand. I've come to him often about issues with getting correct pH in my aquatic environments, and every time he gives me an hour long advanced chemistry lecture about _why_ the pH is where it is and I can never take in any of it. So then he tells me how to correct it, and _why_ it will correct it, but by that point I just care about it being corrected, not necessarily why it will be that way. In the end, we both decided that pH just doesn't seem to impact the aquatic environment all that much, and that the minerals that impact hardness is far more important. I know several others have come to this conclusion as well.

Some aspects of chemical interactions is fascinating to me; pH (which seems so basic) is over my head. It's dynamic, complex. Whenever I have issues, I run to Daddy, who fixes them. :hihi: Guess that's the benefit of having a dad who's a PhD Chemist... That, and he also pretty much did my chemistry homework in high school. I guess it was too simple for him to explain... :biggrin:


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## Hoppy (Dec 24, 2005)

Captivate05 said:


> Heh, I can ask my dad to write out the properties of pH with water, but I guarantee it'll be a book that few people here (me included) could understand. I've come to him often about issues with getting correct pH in my aquatic environments, and every time he gives me an hour long advanced chemistry lecture about _why_ the pH is where it is and I can never take in any of it. So then he tells me how to correct it, and _why_ it will correct it, but by that point I just care about it being corrected, not necessarily why it will be that way. In the end, we both decided that pH just doesn't seem to impact the aquatic environment all that much, and that the minerals that impact hardness is far more important. I know several others have come to this conclusion as well.
> 
> Some aspects of chemical interactions is fascinating to me; pH (which seems so basic) is over my head. It's dynamic, complex. Whenever I have issues, I run to Daddy, who fixes them. :hihi: Guess that's the benefit of having a dad who's a PhD Chemist... That, and he also pretty much did my chemistry homework in high school. I guess it was too simple for him to explain... :biggrin:


Well, if I need to get a doctorate in chemistry, I guess I need to get started. :biggrin:


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## Captivate05 (Feb 23, 2010)

Hoppy said:


> Well, if I need to get a doctorate in chemistry, I guess I need to get started. :biggrin:


LOL, those things take a while, come to find out. Dad spent 12 years at MSU.


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## sepehr (Oct 6, 2010)

I use 3/4 RO and 1/4 tap water for my 35% weekly water changes. The RO has a PH of 6.6 and the KH is almost non existent and on the other hand my tab water has PH of 7.8 and a KH of 21. The final mixture gives me a PH of 7.2 and a KH of 4-5. Again I use 3/4 RO & 1/4 tap. At first I used 1/2 RO & 1/2 tap expecting to get a PH of 7.2 but it stayed at 7.8.

By the way here's an easy formula to follow for mixing RO & tab water in order to come up with the desired kh:

tab water KH - desired KH = gives you parts of RO to be used
RO KH (usually 1) - desired KH = gives you parts of tab water to be used

For example my tab water has a KH of 21 and the RO has a KH of 1 and I want to shoot for KH of 5 so I made the following calculation according to the formula:

21 - 5 = 16
1 - 5 = 4 (ignore the negatives)

So thats 4/16 same reduced to 1/4... That tells me I need to use one part tab and 3/4 RO


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## Satirica (Oct 3, 2005)

Hoppy said:


> I know it is an oversimplification. It does explain why distilled or RO/DI water doesn't necessarily have a pH of 7.0 as it seems logical that it should have. And, using RO/DI water for water changes, hoping to lower the tank water pH, isn't likely to work well.
> 
> Is there a more accurate, but equally simple way to explain this?


RO and DI water is not distilled and is not pure water. Some RO units produce water that is acidic and it is quite effective in lowering pH. All distilled water will lower the pH of bases and raise the pH of acids, including buffered acids and bases. It is simply a question of degree.

Water is one of the most complicated substances known to man. Luckily, this hobby does not demand that we do everything with scientific certainty.


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## guppygolucky (Nov 9, 2009)

pH goes from 1 (acidic, there are a few that are a negative value) to 7 (neutral) and then 14 (basic). We will only concern ourselves with the H+ or H3O+ concentration also written out [H+] or [H3O+]
pH of 7 = ([H+]*[OH-])/[H2O] = 1 because 1*1/1 = 1 (the 1's are the superscript to the concentrations)
Since pH is a logarithmic scale; 7 = 10^0 (10^0=1). if your pH goes lower (more acidic) your [H+/H30+] goes up exponentially. so a pH of 6 = 10^1 (or 10 times stronger or has 10 times as much H+/H30+ compared to a liquid with a pH of 7). a pH of 8 = 10^-1 (.10 times as strong or has .10 times less H+/H3O+ than a pH of 7)
this is true for pure water when adding acids or bases, this will differ when buffers are added. Buffers in solution resist a change in pH by forming either the conjugate base/acid to mitigate out the drastic effects of a pH swing.


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