[pcm81]

I am trying to make and titrate a very dilute solution of HCl.
My process is to use KHP as primary standard, calibrate an NaOH solution against KHP (approximately 0.03M NaOH) then  use the NaOH to titrate HCl, which is about 0.004M.
The problem that i am running into is that my pH curve does not look like a strong acid / strong base curve should look. I get a very slow and drawn out linear fall. I am also seeing pH value drift if i stop for a while. This tells me that the reaction rate between such dilute solutions is very slow, hence my pH readings are wrong. I tried magnetic stirrer and it makes pH fall, but if i turn stirring off pH rises as much as 0.3pH points within the few minutes i observed it for. Is there a trick / proper technique in titration of such dilute solutions? Or is leaving solution to settle for 30 minutes at every point is my only choice?

[Borek]

It is less about kinetics, more likely about mixing, it can take quite long to homogenize the solution. At least the theoretical curve for your concentrations looks nice.

[pcm81]

It is less about kinetics, more likely about mixing, it can take quite long to homogenize the solution. At least the theoretical curve for your concentrations looks nice.

Thanks

[pcm81]

Hmmm, This is going to take a while...
Just did a test of pH change vs time with stirring Takes over an hour to stabilize. Stirring was stopped when ph readings were taken and pH was allowed to stabilize to no more than 0.01 change in 10s.
When stirring was stopped to take pH reading the pH value would rise with time. Progressively less so as overall time went on.


Minutes   pH
0   9.86
5   9.66
10   9.5
15   9.32
25   9
35   8.36
45   7.85
55   7.72
65   7.68

[pcm81]

Hmmm, This is going to take a while...
Just did a test of pH change vs time with stirring Takes over an hour to stabilize. Stirring was stopped when ph readings were taken and pH was allowed to stabilize to no more than 0.01 change in 10s.
When stirring was stopped to take pH reading the pH value would rise with time. Progressively less so as overall time went on.


Minutes   pH
0   9.86
5   9.66
10   9.5
15   9.32
25   9
35   8.36
45   7.85
55   7.72
65   7.68

6 hours and 55 minutes after initial addition pH is down to 7.56. So 7.68 was not the end point for volume added. Makes me wonder how many other titrations have the same time dependence issue.

[Borek]

Have you isolated the solution from the atmosphere, or was it absorbing the carbon dioxide all the time?

[pcm81]

Have you isolated the solution from the atmosphere, or was it absorbing the carbon dioxide all the time?

Absorbing CO2. I thought that at such dilute concentration should not matter. Atleast not THAT MUCH of pH drop. I'll try again with sealed off from atmosphere.  This morning, 12hr after stable point was reached 6pm last night, pH is still the same.

Another reason why I ignored CO2 absorption is that if I stir pH goes down, I stop stirring it goes up a bit. I figured that if pH was decreasing due to NaOH reduction by CO2, stirring would not have a short term effect on pH.