alk titration

[eroan]

Randy,
Please describe the titration technique you use in your article for this magazine for Feb. 2002 for your tank water. What was the sample size and could you please show the math for the bottom value of acid added (figure 1). When do you record the ph value for the vertical value, when the ph swings down and bottoms out after the addition of the titrant. THX

 

[randy holmes-farley]

I believe that I was titrating 500 mL.

I also believe that I was using 0.1 M HCl (0.1 eq/L= 0.1 meq/mL).

(I've since tossed the notes that I took)

So the addition of 1 mL of acid corresponds to 0.1 meq of acid. If you add that 1 mL to the 500 mL of water, the acid added is equal to 0.1 meq/500 mL = 0.2 meq/L.

So I would add some acid, record the acid added, and the pH, and repeat over and over. I waited for the pH to stop it's rapid change after each addition, but not long enough to allow any substantial CO2 equilibration. I probably waited about 30 sec between additions. It is pretty obvious when you get it mixed in by the fact that the pH stops changing.

I actually added the acid in a way that minimizes additive errors: I took a 10 mL graduated cylinder and filled it to 10 mL. I then withdrew amounts from it, and watched the amount left. That way, small errors on each volume addition don't add together into one big error. Instead, the error for an entire 10 mL is no greater than the error of the original measurement. This is all beside the point, but if you try to add 0.1 ml over and over, and you really are adding 0.11 mL, you'll be off by 10% at the end of the experiment.