Ph Buffer Bicarbonate Of Soda

[Essjay]

When I was an undergraduate, the lab class technician used to like telling us gory tales. One was of a female student with long nails who used these nails rather than a spatula to handle sodium. She got a sliver of sodium caught under one nail. When she went to wash off the oil on her hands from the sodium, the resulting reaction took her nail off.

For non-chemists, metallic sodium is so reactive it will react with the moisture in the air, so it is stored in oil which must be washed off with petroleum ether before the sodium metal can be used. Messy students will get oil all over themselves.

 

[TwoTankAmin]

Thanks for clearing things up. But this still leaves me with one more question. Since the sodium ion is "unstable" unless it combines with anions to form a stable compound, I wonder what compound it forms in a tank?

 

[Essjay]

All ions exist in combination. Positively charged ions are called cations, and negatively charged ones are anions. They exist together in proportions that will allow the number of negatives to cancel out the positives. An ion with a double negative or positive charge can be balanced by two single-charged ions of the opposite type.

Sodium bicarbonate as a powder in the tub of baking soda has sodium ions combined with bicarbonate ions. Sodium bicarbonate (or sodium hydrogen carbonate as I suppose I should call it now) is a salt. When salts dissolve in water, the ions split up, they don't stay together in the solution. But the solution still has the same number of positive and negative charges.

The water in a tank is a bit of a soup. It's not just pure water, with sodium ions and bicarbonate ions if you choose to add it. The water itself exists partly as hydrogen ions (H+) and hydroxide ions (OH-). There will be nitrate ions (NO3- ;yes, nitrate is an anion), traces of nitrite ions, and some ammonium ions (freshly made by the fish and just needing to be sucked into the filter). Calcium ions and carbonate ions (these are what cause the water to be hard, and there will be some even in soft water). And hundreds if not thousands of other ions too. Because these ions are not bound together in water but can travel freely of each other, the sodium ions don't form a compound. If you evaporate all the water off, the various ions will come together and crystallise out (think of sea salt). The exact compounds made will depend on how the crystals form, it'll be a mixture of all the ions just jumbled up together. In sea salt you can't separate the sodium chloride from the magnesium sulphate etc, they are all mixed together in the same crystal. Some of the sodium will be sodium sulphate, some of the magnesium will be magnesium chloride. And there's a lot more than just these four in sea salt.

So as a solid, sodium ions will always be in combination with an anion, trapped in a particular arrangement. This arrangement will vary with the anion as these compounds have their own particular crystal lattice. When the sodium ions are dissolved in water, the anions are still there with the sodium but not in a fixed arrangement. And if there are other ions there too, like in a fish tank, you can't even say that the sodium ions are in combination with any particular anion.




Sorry, I'm not very good at explaining things.



Edit: must learn to hit the right keys when typing

 

[SamLovesCorys]

Try Caesium, much more fun.

Or, if you can get some, a little Francium would do wonders!

 

[StandbySetting]

Good luck finding some!

 

[katdixon58]

My GH is exceptionally high and my KH exceptionally low. My ph has been falling gradually as these extremes are moving further apart. What would I fix the imbalance with?

 

[coolie]

A much better buffer, and something found in the natural environment is Dolomite.

 

[Gvilleguy]

And what do the chemists (and arm-chair chemists) on this thread say about crushed coral in the filter flow? I have used it for two years to help buffer my 6.0 pH, zero KH/GH water supply. Seems to work quite well. But I still use Arm & Hammer Baking Soda during fishless cycles!

 

[Essjay]

Crushed coral - calcium carbonate - will dissolve very slowly putting both calcium and carbonate into the water. I suppose strictly speaking I should say the acid in the water will dissolve the calcium carbonate, and if your pH is low, it's acidic. That's the straight forward bit. If you dissolve calcium, that increases GH. If you dissolve carbonate that increases KH. Very little cruched coral dissolves so the effect isn't very great.

When they dissolve in water, all salts like calium carbonate dissociate into ions, so rather than calcium carbonate molecules you get calcium ions and carbonate ions in the water. You can look at it another way and say you have the salt of calcium hydroxide and carbonic acid in the water. Bases such as calcium hyrdoxide always dissociate into ions, but acids can be strong or weak. This doesn't mean concentrated or dilute, it means whether all the acid dissociates into ions (strong) or only some of it (weak). Carbonic acid is a weak acid. So the carbonate ions will tend to attach to the hydrogen ions in the water to make carbonic acid molecules rather than ions.
pH is a measure of how many hydrogen ions are in the water, but it's an inverse measure; the more hydrogen ions there are the lower the pH and the fewer hydrogen ions, the higher the pH. The carbonate binds hydrogen ions, reducing the amount of free hydrogen ions so the pH goes up.


Edit to add:
Sodium bicarbonate (bicarbonate of soda, baking soda) does exactly the same as far as increasing KH and pH goes. Carbonic acid is H2CO3; it can lose both its hydrogens to make carbonate or just one to make bicarbonate. The problem with bicarbonate of soda when there are fish in the tank, as explained earlier in this thread, is that it adds sodium ions rather than calcium ions, and few freshwater fish have evolved to cope with high levels of sodium ions in the water. I ignore Rift Lake cichlids here as their water is just downright peculiar when compared to other freshwater sources!

 

[coolie]

Essjay, nice post, I also have been trying to reduce usage of Sodium Thiosulphate because of not wanting to put Sodium ions into the water. What I was doing was leaving a bucket of water 24 hours first to remove chlorine and then only adding half the amount for the chloramine, but I just got a report from the water authority and it turns out it's nearly all chloramine, and I was kinda wasting my time trying this. that is unless you've got a few weeks to let the chloramine escape.

 

[Essjay]

It's question of scale. When using sodium bicarbonate during fishless cycling, the starting point is 1 tablespoon (15ml spoon) per 50 litres/13 galls tank water, increasing to as much as 3 tablespoon per 50 litres if necessary. This is a huge amount compared to the amount of thiosulphate used to dechlorinate water. And it's even more than you'd think. Because the thiosulphate part of the molecule weighs more than the bicarbonate part, the amount of sodium in each gram is less with sodium thiosulphate than with sodium bicarbonate.