phosphates can buffer water?

In the text it is said that a build up of phosphates will end up destroying the buffering capacity of the water, but my understanding has always been that phosphates are themselves buffering ions, so I believe it would be more accurate to state that an accumulation of phosphates will tend to buffer the water at a lower level. The difference is that a phosphate-buffered water in the wrong pH should be almost as hard to correct as a carbonate buffered one, in contrast to the idea that a water that lost its buffering capacity is easy to correct.

My most painful personal aquarium disaster occurred several years ago when I decided to use a phosphate remover in a 4 year old planted tank with some of the favorite fish I've every kept, just to help me with algae control. It turned out that over the years the water had been buffered at about 6.5 by the phosphate and when the resin removed it the pH swung to 8.5 or more overnight, killing nearly all of my 4 year old fish :-(

Finally, the Tap Water Purifier method certainly works very well as described, but I lived for 2 years in a city where the tap water was extremely hard and alkaline, and as a consequence the resin lasted very little and it became very troublesome and costly to replace it all the time. The solution I found at the time was to simply go to the supermarket next door and stock up gallons of distilled water for the water changes. At a cost of US$0.29 per gallon it was a much cheaper solution (at least for my small tank), and all I had to do was mix in a bit of tap water before using it in the tank.

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dw1305 said:

Hi all,
The water companies add ortho-phosphate ("for control of plumbosolvency") to water and these combine with any lead (Pb) in the tap water and precipitates the lead out of solution as insoluble, pH stable, lead phosphate compounds. If you want the technical term it is - PIMS -"phosphate induced metal stabilisation".

There are a lot of phosphate buffers - "potassium dihydrogen phosphate" KH2PO4 (buffers just below pH7) for example, so I suppose it is possible, but very much a chance occurence.

The volume question Mac mentions is also very relevant, this is why Marine and Lake Tang. cichlids are unable to cope with much change in temperature, pH, salinity etc. they've evolved in huge stable water bodies with very highly buffered water, it's also why big tanks are easier to maintain water quality in than small tanks.

pH Buffers (warning contains a bit of chemistry)
All a pH buffer is a compound that is soluble at one pH range, but insoluble in others. They consist of a mixture of a weak acid and its base. It has the property that the pH of the solution changes very little when a small amount of strong acid or base is added to it, so it "buffers" changes in pH.

This is also how the buffering of KH by calcium carbonate CaCO3 works (in this case the acid and conjugated base base are carbonic acid and bicarbonate).

CaCO3, or calcium carbonate is insoluble and is the main constituent of limestone. H2CO3, or carbonic acid is what you get when you dissolve CO2 in water by the following formula:

CO2 + H2O <--> H2CO3 but when you add some KH Ca(HCO3)2 <--> CaCO3 + H2CO3

Ca(HCO3)2, actually Ca++ and HCO3- ions, is soluble and is what you measure as the carbonate hardness (KH) of your aquarium water.

Both of these reactions are at equilibrium and reversible. If you add components to one side of the reaction, you drive it in the other direction. if you add CO2 to an aquarium that has calcium carbonate decorations (e.g. dolomite or coral), you will dissolve some of the CaCO3 to make Ca(HCO3)2 and thereby increase the hardness of water. On the other hand, if you have fairly hard water and the CO2 content is decreased, CaCO3 will precipitate out (limescale) and H2CO3 is released into the water until an equilibrium is eached. In this way, Ca(HCO3)2 acts as a buffer in aquarium water. This is also why hard water resists pH changes much better than soft water, it has a much larger reserve of buffer which we call its carbonate hardness KH.

cheers Darrel

Click to expand...

The water companies add ortho-phosphate ("for control of plumbosolvency") to water and these combine with any lead (Pb) in the tap water and precipitates the lead out of solution as insoluble, pH stable, lead phosphate compounds.

Click to expand...