[Andy66]

I make vinegar.

The only NaOH I can buy locally says .2N (Not .2M, NOT .1N). the formulas I find online (and all the unhelpful high school chem video lessons) describe .1N NaOH...

I need to find a solid titration formula. I do NOT want to get a lesson or lecture or be put down in a condescending manner please. if you don't know, please DO NOT reply speculatively.

I have a good pH meter (but also use Phenolpthaline to see color fix).


I use a Burette,

Typically at ~70F sample temp.

I need to dilute the sample w water in order to visualize the color fixing. 5 or 10 parts sample to water is ideal.  I typically use un chlorinated distilled or bottled water for dillution, not de ionized that is about pH 6.8 -6.9. my tap water is very close to neutral as well, but I don't use in my lab much.

I can easily change every input variable except the .2 N NaOH.

I see several formulas listed online but none are specific as to amount of sample, pH target at color fix of Phenolpthalin, pH target for end. etc.

In one test variation I used 5ml sample, 95ml water and tested to pH 8.2 which requiring 26ml NaOH. 
Multiplied x .3 this equalled 7.8% acidity.  Is this at all valid as a (ballpark) Result?

is there a simple such forumla that gets me in the ballpark? Testing is repeated often so comparative values that are approximately accurate are fine. Say within 1/2 % max.

Thanks for any help.

A

[Borek]

0.1M or 0.1N is just a way of describing concentration - (and actually for NaOH 0.1M and 0.1N is exactly the same). You can use 0.2N NaON (which will be also 0.2M) for titration of the acetic acid, no problem.

To calculate results of the titration you have to follow the stoichiometry of the reaction, that's the only sure way. See this approach explained and described here:

http://www.titrations.info/titration-calculation

Note: while the NaOH you are buying is advertised as 0.2M it doesn't tell much about the real concentration (it can be 0.185 or 0.234 or something else, that will make the result of your calculations completely off). You need to check with the supplier what the "0.2N" really means.

Using tap water is generally not a good idea, as you don't know how much base it will consume on its own (can be checked with a blind trial). Just because the pH is close to neutral doesn't mean anything. OTOH it should not change the result by much, most likely just a few tenths of a mL.

Also see:

http://www.titrations.info/acid-base-titration-acetic-acid-in-vinegar

If titrating against pH meter end point is around pH 9.0 (exact value depends on the vinegar concentration).

[pcm81]

What Borek has stated is 100% correct. I realize that you are not asking for a lecture and i am not trying to give you one, however here is my personal experience which i gained trying to setup a titration method for a different acid than yours, but same idea applies.

1. Accuracy of 0.5% that you are asking for is difficult to get, but it is doable. Most burettes will have 50ml volume and 0.1ml divisions. So that means the error is 0.1ml over a less than 50ml volume which is 0.2% just from burette. You will have to add it to all other errors in you set-up. Also, most of the time your titrating volume may be less than full 50ml, hence error from burette reading grows. The 26ml of NaOH reading has 0.4% error on its own.

2. NaOH solution which you are buying will age and it will not be 0.2M or what ever it is called out for very long after you open the bottle. NaOH will interact with atmospheric carbon from CO2 to form sodium carbonate changing concentration of your "standard" solution. To overcome this you have to titrate NaOH vs another known standard to determine it's actual concentration. KHP (Potassium Hydrogen Philate) is such a standard. KHP is knows as a primary standard, because it is not greatly affected by atmosphere unlike NaOH.

3. I titrate for phosphoric acid, but process is the same as for vinegar in your case:
a. I use KHP as my primary standard. I weigh a small amount of it, measured on a very accurate scale, and titrate Sodium hydroxide solution vs this mount to determine concentration of my NaOH solution. The exact pH of the equivalence point is unknown, because it depends on concentrations, however as you log pH values vs volume of NaOH added you will see a steep rise in pH at some point. The point with steepest slope is your equivalence point and is when molar ratios of NaOH and KHP standard are equal. So, knowing mass of KHP and ml of NaOH solution needed to get to that point gives you exact concentration of NaOH solution. At this point i titrate phosphoric acid and you would be titrating the vinegar using now a well calibrated NaOH solution.

Normality vs Molarity:
in case of NaOH those 2 things are the same. Normality = Molarity * X where X is the number of protons acid has or number of OH- groups base has. Since NaOH has 1x OH- the molarity and normality are the same.
Read this: http://www.smc.edu/projects/28/chemistry_10_experiments/ch10_titration.pdf