Questions Re: Ph And Demineralized Water

[Aphotic Phoenix]

Although any change in your pH will of course be determined by the mineral content of your home tap water, I can attest (after several months of using the half "pure" half tap approach) that my experience has been quite positive. As a lover of SA cichlids, my selection has been somewhat limited due to extremely hard and alkaline water with a high TDS content. My 50/50 cutting experiment did not lower my pH significantly (unfortunate in my case), going from a 8.3 to ~8.0-7.9, but has stopped the necessity of scraping away limescale buildup every two weeks. It has also significantly decreased the diatom colonies that plagued my tank (which were present for over a year and half).

 

[petertr]

I'd just like to add to the statement that RO water has a ph of 7.0. This is incorrect unless I'm getting sold water that isn't actually RO. The RO water that I have been purchasing tests to have a ph of 6.0, the lowest that my API liquid test kit tests for. Also, several different different distilled waters test at 6.0 as well. I will have RO here in my home in a week or so and can't post results of that as well.

Matt

Exactly what Dave said. The pH of 7.0 for pure water has been confirmed time and time and time again. Unless you want to be completely technical about it in which I think that 7 is only accurate to 2 decimal places. I think at 25 degrees C, the pH of pure water is actually 6.998 +- 0.001. Though, for all but the most sensitive of applications, saying 7 is going to be accurate enough.

When the water is pure, it is very, very quick to take up anything that will dissolve in it, like CO2. Or any kind of residue in the container. Or almost anything else. That's why it is often very hard to actually measure the pH of a pure substance.

A touch of A Level Chemistry for you here. pH 7 is DEFINED as pure water at standard temperature and pressure. pH is a totally arbitrary scale - they picked water to be 7 on purpose.

 

[jnms]

Hello everyone,

I am new to this forum but not a newbie to fishkeeping.

I have a happy tankful of African Cichlids, and our tap water is naturally hard and as such the ph is always quite alkaline. Obviously this is great for the cichlids. I would like to start adding demineralized water to the tank as well as tap water when doing water changes. How will adding the demin. water affect the ph levels if at all? I am hoping that even though the demin water is exceedingly soft that smaller amounts will not disrupt the natural balance. What do you think?

Cheers,
Lorraine

When you say your water is very hard, what are its Gh and Kh readings?

 

[andywg]

A touch of A Level Chemistry for you here. pH 7 is DEFINED as pure water at standard temperature and pressure. pH is a totally arbitrary scale - they picked water to be 7 on purpose.

I am a little confused, are you claiming that pure water has a pH of exactly 7 rather than 6.998 as described by Bignose above?

 

[Bignose]

I'd just like to add to the statement that RO water has a ph of 7.0. This is incorrect unless I'm getting sold water that isn't actually RO. The RO water that I have been purchasing tests to have a ph of 6.0, the lowest that my API liquid test kit tests for. Also, several different different distilled waters test at 6.0 as well. I will have RO here in my home in a week or so and can't post results of that as well.

Matt

Exactly what Dave said. The pH of 7.0 for pure water has been confirmed time and time and time again. Unless you want to be completely technical about it in which I think that 7 is only accurate to 2 decimal places. I think at 25 degrees C, the pH of pure water is actually 6.998 +- 0.001. Though, for all but the most sensitive of applications, saying 7 is going to be accurate enough.

When the water is pure, it is very, very quick to take up anything that will dissolve in it, like CO2. Or any kind of residue in the container. Or almost anything else. That's why it is often very hard to actually measure the pH of a pure substance.

A touch of A Level Chemistry for you here. pH 7 is DEFINED as pure water at standard temperature and pressure. pH is a totally arbitrary scale - they picked water to be 7 on purpose.

No, please look this up. The pH of pure water at 25 degrees C is actually 6.998 +- 0.001, it is not 7.000. The value of 7.000 is a calculated value -- a value that fits exceptionally well with experiment until the experimental technique becomes very, very accurate. When the experiment can be that accurate, we find a small disagreement with theory. The theory isn't perfect (they rarely are).

Look at: http/www.millipore.com/bibliography/tech1/6s8ftj as a good example discussing this.

There are many others. You may have to look beyond just A level chemistry and look into some university or even graduate school level texts...

 

[petertr]

A touch of A Level Chemistry for you here. pH 7 is DEFINED as pure water at standard temperature and pressure. pH is a totally arbitrary scale - they picked water to be 7 on purpose.

I am a little confused, are you claiming that pure water has a pH of exactly 7 rather than 6.998 as described by Bignose above?

Its not exactly 6.998 either! There is an associated error built in!

I'd just like to add to the statement that RO water has a ph of 7.0. This is incorrect unless I'm getting sold water that isn't actually RO. The RO water that I have been purchasing tests to have a ph of 6.0, the lowest that my API liquid test kit tests for. Also, several different different distilled waters test at 6.0 as well. I will have RO here in my home in a week or so and can't post results of that as well.

Matt

Exactly what Dave said. The pH of 7.0 for pure water has been confirmed time and time and time again. Unless you want to be completely technical about it in which I think that 7 is only accurate to 2 decimal places. I think at 25 degrees C, the pH of pure water is actually 6.998 +- 0.001. Though, for all but the most sensitive of applications, saying 7 is going to be accurate enough.

When the water is pure, it is very, very quick to take up anything that will dissolve in it, like CO2. Or any kind of residue in the container. Or almost anything else. That's why it is often very hard to actually measure the pH of a pure substance.

A touch of A Level Chemistry for you here. pH 7 is DEFINED as pure water at standard temperature and pressure. pH is a totally arbitrary scale - they picked water to be 7 on purpose.

No, please look this up. The pH of pure water at 25 degrees C is actually 6.998 +- 0.001, it is not 7.000. The value of 7.000 is a calculated value -- a value that fits exceptionally well with experiment until the experimental technique becomes very, very accurate. When the experiment can be that accurate, we find a small disagreement with theory. The theory isn't perfect (they rarely are).

Look at: [URL="http/www.millipore.com/bibliography/tech1/6s8ftj"]http/www.millipore.com/bibliography/tech1/6s8ftj[/URL] as a good example discussing this.

There are many others. You may have to look beyond just A level chemistry and look into some university or even graduate school level texts...

Spot on.

 

[andywg]

A touch of A Level Chemistry for you here. pH 7 is DEFINED as pure water at standard temperature and pressure. pH is a totally arbitrary scale - they picked water to be 7 on purpose.

I am a little confused, are you claiming that pure water has a pH of exactly 7 rather than 6.998 as described by Bignose above?

Its not exactly 6.998 either! There is an associated error built in!

I don't see the word exactly in my post

Spot on.

So you quote Bignose stating it is 6.998 (+- 0.001) and say that the pH of pure water is described as 7. Bignose reitereates what he said about 7 only being correct to 3 significant figures and you then say he is spot on. What actually was the point of your first post? I am still somewhat confused about what you were trying to state.

 

[mike os]

For all practical purposes RO does not have a ph (OK outside very specialised environments it does not have a a significant Ph)

If the water is truly demineralised or nearly so you need some very sophisticated and expensive equipment to measure it, and for practical purposes it is taken as ph7, a principally arbitrary value that happens to be the middle of the ph scale. In reality it is an approximation... just as pi=3.142 is an approximation.

As there is so little buffering capacity (in fact there is none ie gh&kh=0 or nearly so) as RO absorbs atmospheric gases (typically CO2 in our case), and potentially some of the container it is in, the Ph will alter very quickly because there is nothing to "hold" the Ph at any given value (ie it has no buffering capacity) and can appear extremely acid or alkaline and swing with only a little agitation.

So for practical reasons testing Ro before reconstituting is about as useful as a chocolate fireguard in the sahara.

 

[petertr]

So you quote Bignose stating it is 6.998 (+- 0.001) and say that the pH of pure water is described as 7. Bignose reitereates what he said about 7 only being correct to 3 significant figures and you then say he is spot on. What actually was the point of your first post? I am still somewhat confused about what you were trying to state.

I was saying that I was wrong, and that the second quoted post was correct (I did some further reading). No need to get tetchy about it!

 

[andywg]

I was saying that I was wrong, and that the second quoted post was correct (I did some further reading). No need to get tetchy about it!

No tetchiness here, just confusion as the brevity of your post made it somewhat ambiguous. All clear now.

 

[Dave Spencer]

If the water is truly demineralised or nearly so you need some very sophisticated and expensive equipment to measure it....

Measuring the purity of water is considerably easier than trying to make it pure in the first place.

As there is so little buffering capacity (in fact there is none ie gh&kh=0 or nearly so) as RO absorbs atmospheric gases (typically CO2 in our case), and potentially some of the container it is in, the Ph will alter very quickly because there is nothing to "hold" the Ph at any given value (ie it has no buffering capacity) and can appear extremely acid or alkaline and swing with only a little agitation.

It is possible to produce demineralised water and measure the pH before it reacts with anything. At work we are only concerned about the pH to one decimal place, but there may be other industries needing a purer water and more accurate pH reading.

Dave.