Carbonic acid effect on pH in soda water

[zwiller]

Can someone point me in the right direction? Looking to estimate the effects of carbonic acid in soda water. I would like to first be able to create a soda with a slightly alkaline pH say around 7.5 and am curious how much alkalinity would be required to do so (assume using distilled). Then I would would like to make a soda fountain style soda that is acidic near a pH of 5. From memory, carbonic acid is weak so maybe the effect is minor and I can stop overthinking it and approach it straight forward. Let's assume carbonation of soda at 4vols or 6g/L.

Thanks in advance.

 

[ajdelange]

Can someone point me in the right direction? Looking to estimate the effects of carbonic acid in soda water. I would like to first be able to create a soda with a slightly alkaline pH say around 7.5 and am curious how much alkalinity would be required to do so (assume using distilled).

That's sort of like asking how to create an honest politician. It's a contradiction in terms. The species carbonic acid, bicarbonate ion and carbonate ion are in fixed ratios dependent on the pH. At pH 7.5 the carbonic acid is 7.07%, the bicarbonate 92.81% and the carbonate 0.12%. Thus for a given level of carbonic acid (CO2) at that pH there needs to be 92.8/7.07 = 13.1 times as much bicarbonate. To make a solution with 6 grams CO2 dissolved per liter at pH 7.5 one would dissolve 154.84 grams of sodium bicarbonate in a liter of water and then apply CO2 under pressure until 5.885 grams of CO2 dissolved. This would be carbonated to the extent of 6 g/L CO2 but would also contain 112.61 g/L bicarbonate ion.Thus the difference in probable existence of this solution and the honest politician is that this solution is, at least theoretically, possible but obviously completely impractical.

Then I would would like to make a soda fountain style soda that is acidic near a pH of 5. From memory, carbonic acid is weak so maybe the effect is minor and I can stop overthinking it and approach it straight forward. Let's assume carbonation of soda at 4vols or 6g/L.

At pH 5.5 the carbonic acid is 96%, the bicarbonate 4% and the carbonate 0%. Thus for a given level of carbonic acid (CO2) at that pH there needs to be only 4/96 = 1/24th as much bicarbonate as carbonic. One can make a soda with 6 grams/L CO2 content by dissolving 1.728 grams of sodium bicarbonate and dissolving 6.00 grams of CO2 per liter. This solution is also carbonated to about 3 vols (6 mg CO2/L) but contains onl 1.26 g/L bicarbonate.

I can have whatever pH and carbonation level I want by using bicarbonate (or anything with finite alkalinity). If I don't do that I lose control over the pH. For example, in pure water the pH of a CO2 solution of strength 6 g/L is 3.62. If I pressurize water with alkalinity 50 ppm as CaCO3 to 6 g/L the pH is 4.25. If the alkalinity of the water is 100 ppm then the final pH at 6 g/L is 4.54

Things work out this way because 5.5 < 6.38 < 7.5 with 6.38 being the first pH of carbonic acid which does indeed make it a weak acid. The first pK for sulfuric acid is -7.

 

[ajdelange]

This all might seem quite confusing. In a nutshell, adding CO2 (carbonic acid) to a soda is the same problem as adding any weak acid to any other beverage. What happens to the pH will depend on the alkalinity/acidity of the mixture before the CO2 is added.

Were you to make a soda by adding sugar, flavoring and phosphoric acid to water (a real, old fashioned tradtional 'phosphate') we could figure out what the pH would be pretty easily using the math at
https://www.homebrewtalk.com/showthread.php?t=473408

 

[zwiller]

Thank you AJ! I will continue to study this but my take on this so far is that RO/distilled water is probably not the best water to use for soda and folks will likely need some alkalinity to buffer theirs at home. This is somewhat baffling to me as I as convinced I would need acidity... I am very interested to see the effects of potassium bicarbonate used as alkalinity as it appears to be used in many sodas. Might need to use pickling lime to hit 7.5? Math isn't clicking right now, but the fundamentals are.

 

[ajdelange]

I expect they are using KHCO3 to keep pH from going too low given the use of citric acid, lime or lemon juice for flavor but it doesn't matter how you get to pH 7.5, At that pH you are going to have 13.1 moles of bicarbonate for each mole of CO2.