[frozenheart2511]

Hello all,

I have some sample of paper and I want to determine the exact amount of acid. I used 100 mL of water to soak about 1,000 g of sample and titrate 10 mL of that solution with sodium hydroxide 0.1N and phenolphtalein and the equivalent point was reach with only one or two drops of sodium hydroxide. And I dont have much original sample too.

With such low amount of acid, how can I determine exactly?

Please help...

[Hunter2]

Two ways, increase the sample amount or decrease the molarity of the sodium hydroxide. Or do back titration that you treat you are sample with a known amount of sodiumhydroxide and titrate back with hydrochloric and calculate the difference.

[khanhduy0388]

yes, have you tried to reduce the concentration of NaOH from 0.1N to 0.05N or lower?

[Hunter2]

The question is what is your consumption. A good titration should require 5 -10 ml of hydroxide

[frozenheart2511]

Hi guys,

Thanks for reply,

I've tried 2,000 g of sample and sodium hydroxide 0.01 N and it only gave about few mL at equivalent point. I also tried back titration when treated 2,000 g of sample with 15 mL of sodium hydroxide 0.01N and titrate with hydrochloric acid 0.01N (methyl orange as indicator) and i found it very hard to figure out the equivalent point as my solution is very dilute (as i supposed).

I wonder
1. if i can use a pH meter for potentiometric titration in this case?
2. i used a pH meter for titration of sodium hydroxide 0.01 N with hydrochloric acid 0.01 N and found out the equivalent point is not clear, delta(pH)/delta(V) graph gave me many peak near the equivalent point, what's the problem?

Thank you in advance,

[Arkcon]

How much free acid do you expect is in paper.  I understand a certain level leads to deterioration over decades, but I don't know what amount is too much.  If you got all of it out of 2 kilos of paper, and its still barely detectable, maybe titrations aren't ever going to work.  Like Hunter2: said, a proper titration will require more than a few drops of titrant.

If you're having trouble with the methyl orange endpoint, which isn't the easiest one to see, I'll grant you, you can use phenolpthalein, to a clear endpoint that doesn't return for 15 sec.  But still, how much free acid do you expect in a kilo of paper, and if you prepare a solution of HCl to that level, can you execute the titration or not?  If you can't, you may have to analyze by a different method.