I have an RO system, so according to RPavlis method, I need to add 100mg of potassium bicarbonate per 1 liter of water.

As you can see in this bag, it says the serving size is 1300mg, but 490mg of Potassium per serving (as potassium bicarbonate)

So the question is,

Do I need to extrapolate the values and add 265.3mg of this stuff to 1 liter water?

Or

Is it saying I'll be consuming 490mg of potassium in the 1300mg of potassium bicarbonate?
Which would just mean i follow the original formula of 100mg per 1 liter of water.

Any help would be appreciated!

Yes, it is saying there are 490 mg of potassium in the 1300 mg of potassium bicarb.

So I would just use 100mg of this stuff per 1 Liter of water, correct?

Since RPavlis calls for 100mg of potassium bicarb, not 100mg of potassium.

My non chemist response is, Yes, you are correct.

Also, I would look at a home brew supply distributor for your potassium bicarb if/when you run out. It will be much cheaper than something labled for dietary use, and just as good if not better quality.

In case there's any misunderstanding: the bicarb is important, not the potassium. You can also use sodium bicarb (baking soda) in the Pavlis recipe:
potassium bicarb (KHCO3) 100mg/l ~= sodium bicarb (NaHCO3) 84mg/l

For convenience, make up a 500X concentrate (5g/100ml KHCO3 or 4.2g/100ml NaHCO3) and store it in the refrigerator. Get a pipette and use 2ml concentrate per liter (or 1ml if you prefer half strength) for a liter of Pavlis water.

Agree with everyone here. (Also not a chemist, but have studied and worked enough in chemistry to handle simple questions like this.)

I like John's suggestion to use a concentrate. With one of those 750 ml swingtop lemonade bottles you could use the 5 gram potassium bicarb or 4.2 gram sodium bicarb added to the bottle of purified, then you could use a tablespoon rather than a pipette - - 1 tablespoon per liter for full strength or half tablespoon per liter for half strength rpavlis. This recipe is very forgiving - does not demand high precision. At 1/2 tablespoon you would have ~ 25 mg/L alkalinity, enough to buffer your pH up to a healthy level for corrosion protection with this water, which is devoid of corrosion issues from chlorides or sulfates. And even if you went up to 2 tablespoons your alkalinity would be only 100 mg/L, which is still not so high that you 'd likely notice dulling of acidity in an espresso extraction. (I think that If you are very particular about the perceived acidity in your coffee, then you might notice some dulling of that in a pour-over brew of some coffees when using the higher alkalinity recipes. )

P.S.
(TLDR - just showing the geeky calculations)
recipe math:
1 tablespoon = 15 ml
5000 mg / 750 ml = 6.67 mg/ml
6.67 mg/ml * 15 ml = 100 mg

a little basic chemistry:
KHCO3 molar mass = 100 g/mole
CaCO3 molar mass = 100 g/mole (by happy coincidence)
two moles of KHCO3 are equivalent to one mole of CaCO3
(because HCO3- neutralizes one H+ ion to form H2O + CO2, whereas CO3-- neutralizes two H+ ions to form H2O + CO2)
100 mg/L KHCO3 = 50 mg/L CaCO3 equivalent

I'll explore this route so I can use a tablespoon for convinience.
Just to confirm, I will add 5g of potassium bicarbonate in a 750ml of my RO water to create the concentrate,
Then add 15ml (1 tablespoon) of this concentrate per every 1 liter of water, correct?

JPMech wrote:Just to confirm, I will add 5g of potassium bicarbonate in a 750ml of my RO water to create the concentrate,
Then add 15ml (1 tablespoon) of this concentrate per every 1 liter of water, correct?

correct