How do I describe 4 dKH reference solution to a chemist?

[rusticitas]

I have a friend who is a Chemistry professor where I work (Lehigh University). I usually go to her to ask questions when I absolutely get stuck or confused.

I am having trouble translating from aquarium-speak to chemist-speak what a 4 dKH reference solution for a drop checker is. Our language of "degrees of hardness" does not mean anything to her.

How do I describe a 4 dKH reference solution for use in a drop checker to a chemist, so that she can help me mix up my own accurately? (And teach me a little chemistry in the process.)

 

[VaughnH]

4 dKH = 71.4 ppm of carbonate. A 4 dKH reference solution contains 71.4 ppm of carbonate. Maybe that will be easier for a non-aquarium chemist to handle.

 

[Tom Barr]

Then convert to molar concentrations, let her do that, it's not hard for her.

Regards,
Tom Barr

 

[rusticitas]

And for "carbonate" I should use sodium bicarbonate (aka baking soda) like what is found at the grocery store?

 

[Tom Barr]

sodium carbonate is likely best.

Regards,
Tom Barr

 

[rusticitas]

Slightly confused...

(I'll ask my chemistry professor friend this, but for the time being...)

You keep referring to 'carbonate' (ex. sodium carbonate). Is that the same as, or different from sodium bicarbonate?

I had in my head that a carbonate was related to GH and bicarbonate was related to KH/Alkalinity...

 

[VaughnH]

Bicarbonate is HCO3- and carbonate is CO3-- (The first has one excess electron and the second 2 excess electrons.) The first is slightly heavier per mole, although most sodium carbonate/bicarbonate also has water molecules incorporated into the crystal structure, so the problem involved in trying to calculate how many grams of either one to use to make X dKH water is not knowing how much water is in the powder you have. (You might think I know chemistry. You would be wrong!)

 

[rusticitas]

Ah, so if I recall my high school chemistry from 1988 (old!) that Na is +1 and HCO3 is -1, hence NaHCO3 which is sodium bicarbonate.

Now, since that's the only source of (bi)carbonate I know of, what else is there that can be used with relation to a drop checker reference solution?

-Jason

 

[VaughnH]

Sodium carbonate, Na2CO3, is used when laboratory grade certified standards are made. The reason isn't clear to me.

 

[rusticitas]

Calcium carbonate?

I was just looking over aquariumfertilizer.com and see calcium carbonate (CaCO3) listed, but not sodium carbonate (Na2CO3). Is CaCO3 "interchangeable" with Na2CO3 or NaHCO3 as a basis of making a 4dKH reference solution?

-Jason

 

[Tom Barr]

Na2CO3 is used because it has less water and is more stable when heated.
Thus you can get a more accurate measure of it's weight.

Fert cals do not measure Na2CO3, but they can be adapted.
The issue here is the amount of water in the salt and how heat might affect that.
So to get around error, you can use a lot of NaCO3, as well as a lot of water to make the initial solution, then subdivide the solutions.

You can also do a density for solid Na2CO3 reference as well.

Regards,
Tom Barr



Regards,
Tom Barr

 

[swylie]

Fortunately you can make sodium carbonate from sodium bicarbonate very easily. Just cook the stuff in a frying pan on your stove until it stops offgassing CO2. This process also cooks off any water that might be affecting the purity of your sample.

If you need a more rigorous method, bake the sample in an oven for a certain time and temperature. I forget the appropriate time/temp, but I can look it up if anyone is interested. If you cook too hot or too long, you risk decomposing the sodium carbonate, though I seem to remember carbonate requiring very high heats to decompose. If you cook too cool or too short, of course, you won't convert all the bicarb into carbonate, and then you're worse off then when you started.

I guess one solution would be to dry sodium bicarbonate (at low temperature, so as not to decompose it into carbonate) to constant mass, or to cook sodium bicarbonate into sodium carbonate until it reaches a constant mass. 'Course if you have a chemist friend then she'll already know all of this and more.

 

[rusticitas]

Ah, ok. That was the reference in another post on here about keeping the temperature below some threshold to prevent NaHCO3 from changing, hence about keeping it in above boiling for awhile to boil off/dry the powder out before weighing and mixing.

Okay. Perhaps I've made the entire question more complicated than necessary. After much re-re-re-*-thinking, what was bothering me was that I had no way I could think of to verify that what I mixed was actually 4 dKH, as the test kit indicated, but which I have no correlation for.

I think that's it. If I have a TDS meter, pH meter and EC meter (combo of all 3 actually), can that be used to independently verify that it is 4 dKH? If so, and considering that all that's being used is dried/desiccated NaHCO3 and distilled water, what could/should I use on that digital meter to check against the KH test kit?

-Jason