Conflicting descriptions of Alka-Seltzer reaction?

[david lewine]

Alka seltzer contains 1916 mg of NaHCO3 and 1000 mg of citric acid.

According to some descriptions of the reaction between alka-seltzer and water and then HCl (in stomach), there is an excess of NaHCO3 in the alka-seltzer in relation to the citric acid and the benefits from alka-seltzer come from the excess NaHCO3 reacting with the HCl in the stomach: NaHCO3 + HCl produces NaCl + H2O + CO2. According to these descriptions, the citric acid in the alka-seltzer is there just to create the effervescence as a marketing gimmick.

Other descriptions describe the reaction as 3NaHCO3 + citric acid produces 3H2O + 3CO2 + sodium citrate, and then sodium citrate + 3HCl produces citric acid and 3NaCl.

The potential benefit to the second description is that the CO2 is produced in the glass of water, not in the stomach (so less burping!) without reducing the amount of HCl in the stomach that can be neutralized.

I guess the idea is that citric acid is a weak acid so it won't dissociate as much as the HCl and will therefore reduce the acid effects in the stomach.

So I'm wondering which description is more correct? Is the benefit of alka-seltzer mainly due to the excess NaHCO3 reacting directly with HCl, or does the sodium citrate neutralize HCl as effectively as sodium bicarbonate?

Also, I see the formula for citrate sometimes written as C6H5O7 and sometimes as C3H5O7. why the difference?

Thanks,

David

 

[Kanesan]

The formula seems to be C6H8O7 for citric acid and C6H5O7 for tri-anionic citrate. These are accurate (I am chemistry PhD).

Based on the quantities you have given, it comes to 22.8 : 5.2 molar ratio (molar mass of sodium bicarbonate is 84 and that of citric acid is 192). So, there is more than three equivalents of bicarbonate and some bicarbonate will exist as such (at least 600 mg) when reaching the stomach. But, may be not all citric acid will be converted to tri-aninoic salt and leave even more bicarbonate.

In addition, the CO2 may have more solubility in this sodium bicarbonate, sodium citrate solution. So, this dissolved CO2 and the excess bicarbonate will bubble up when acidified more in the stomach.

 

[david lewine]

Interesting note about the dissolved CO2 ending up in the stomach.

Given that the purpose of alka-seltzer is to neutralize the acid (HCl mostly I think) in the stomach, to what extent is the sodium citrate produced in the reaction between the sodium bicarbonate and the citric acid able to neutralize the stomach acid?

 

[Borek]

Given that the purpose of alka-seltzer is to neutralize the acid (HCl mostly I think) in the stomach, to what extent is the sodium citrate produced in the reaction between the sodium bicarbonate and the citric acid able to neutralize the stomach acid?

pH in the stomach is around 2, which means citrate will be almost fully protonated (behaves as a threeprotic base).

 

[Kanesan]

Great point David. Never considered that. In fact, most sodium/potassium carboxylates are basic, not neutral.

In a similar note, I want to add that even trisodium phosphate was a base earlier used often in cleaning products!

 

[david lewine]

pH in the stomach is around 2, which means citrate will be almost fully protonated (behaves as a threeprotic base).

This might solve my puzzle. Doing a titration of a dissolved Alka-Seltzer solution I found that the solution neutralized more HCl than can be accounted for by just the NaHCO3 left over after the fizzing reaction, but less than would be neutralized by the whole amount of NaHCO3 in the tablet to begin with. However I was using Methyl Red as an indicator and stopping when the pH of the sample reached around 4. If a significant amount of the citrate remained unprotonated at this pH but would become fully protonated at the stomach's normal pH or below, then that might account for my results. I'll try with an indicator like Thymol Blue.

 

[Borek]

stopping when the pH of the sample reached around 4. If a significant amount of the citrate remained unprotonated at this pH but would become fully protonated at the stomach's normal pH or below, then that might account for my results.

Definitely. To get some intuition about numbers involved - at pH=pKa exactly half of the acid is protonated, move one pH unit and the ratio changes tenfold from 1:1 to 10:1. With pKa1 = 3.13 if you end the titration at 4 even less than half H₂Citrate^- is in the protonated form, yet it will get fully protonated in the stomach.