Water pH

[tropterrarium]

OP: triple *filtered* water is having more or less the same amount of dissolved molecules/ions as tap water. So your TOC value is not going to be lowered, and pH is more or less the same.

There is triple *distilled* water, also known as a form of ultra pure water for biochemistry applications. This is close to nothing but H2O, no minerals, no ions, no nothing. Using DI or distilled water can have strong effects on pH, and adding fertilizer can further complicate things. That's why three are "pure water" fertilizer versions.

When people in orchid growing circle talk about pure water, most refer to Reverse Osmosis (occasionally DeIonization columns). This is closer to distilled/rain water than tap/filtered water.

Just was wondering whether there was some confusion re triple type waters. Ray has explained that in various posts in case you are interested.

[ChipMolly]

tropterrarium makes a good point. All filtered water are essentially 'commercial' products because they remove different types of solubles from tap water. All I was interested in knowing is if there is widespread practice of adjusting the pH of irrigation water, and if so, with what.

RO/distilled water is difficult to use for irrigation purposes because it removes all solutes, and so reconstitution is generally required for plant culture, which has its own issues with pH drift after the fact.

[Orchid Whisperer]

Originally Posted by ChipMolly

Buffering capacity is to do with weak acids, which do not completely dissociate to produce equimolar amounts of H+ ions. Weak acids (which are almost always organic acids) have a weak dissociation constant, such that an equiilibrium exists. This is how buffering capacity works.

And pH >7 is alkaline. The measure of pH is the logarithmic concentration of [H+], not [OH-].

I think that you are confusing alkalinity (buffering capacity) with pH. Yes, you will commonly see pH above 7 referred to as alkaline, which confuses matters. It is really better to think of pH above 7 as basic. Alkalinity and pH are related in that the higher the alkalinity of the water, the more the water will resist changes in pH, due to buffering capacity. Have a look at http://www.uri.edu/ce/wq/ww/Publicat...alkalinity.pdf The Wikipedia article on alkalinity also explains alkalinity pretty well, and on a more complex level.

Think of it this way. You can have two water samples that both have a pH of 7. Sample 1 has an alkalinity near zero. Sample 2 has an alkalinity of 50. Add a little weak acid to Sample 1, the pH will plummet. Add the same acid to Sample 2, the pH may change a little, or may not change at all. This is due to the the difference in buffering capacity (alkalinity) between samples.

But this may be over-thinking things a bit. You are just wanting to change the pH of your water. Use the drop-by-drop method I suggested in an earlier post. You will get water at the pH that you want, easy-peasy.

[tropterrarium]

Re "filtering", that can be done at a variety of levels, from coffee filters, GFCs, to millipore and molecular sieves. RO water is in a sense also "filtered" just an extremely fine filter, usually then called a "membrane", or even semipermeable membrane. Still, bottom line, a filter.

Whether filtration affects pH depends on how fine you filter, whether compounds that affect pH are removed. Most household/gardening filters do not affect pH (charcoal, bacterial filters, etc.). They may *slightly* influence buffer capacity, but even that is a stretch.

Re changing pH, the first question is, is it necessary. It depends on the plants you grow: some are hardy, some are fickle. If you have fickle plants, I would think that pH adjusted filtered water is not helping much. I would then invest in an RO system and adjust the pH of that water.

Re chemicals used to adjust pH, for to alkaline water, vinegar is certainly great. If too acidic, you can use Sodium Hydroxide (NaOH) or Potassium Hydroxide (KOH). Those chemicals should be handled with great care as they can dissolve skin, including the cornea of the eye.

DI water has a tendency to be very acidic (pH <4). Once you add fertilizer, pH can be vastly different, so make sure you check with and without fertilizer. The purer the water, the stronger the potential pH swing is once you add fertilizer, because pure water has no buffer capacity.

[Ray]

It is very difficult to measure, but pure water, whether it be distilled, DI, or RO has a pH of 7. All of them will absorb carbon dioxide from the air and come to an equilibrium pH. With a global average of about 400 ppm CO2 in the air, that's in the neighborhood of 5.54.

I guess the bottom line is that most folks feel that a solution pH between pH 5 and 7 is adequate, and going outside of that a small amount is not going to be a huge issue (I still shoot for 5-6). However, the real thing to do is to measure the pour-though, not the incoming solution, as it's the pH in the root zone that's important.

Another thing to consider is that for orchids, a low electrical conductivity (EC) is also very important. I'd bet that a low EC solution at a less-than-ideal pH is better for the plants than the ideal pH in a high-solids-content solution.