Looking for an analytical method for citric acid

A. Analysis of citric acid is a simple titration with sodium hydroxide. I don't remember the exact sample size, sodium hydroxide normality, and indicator, but phenolphthalein [on eBay & Amazon] should do the trick. Make a standard from a known amount of powder in water if you aren't sure of the calculation factor. I'm sure someone reading this has the information handy.

A. citric acid :
10 ml sample
phenolphthalein [on eBay & Amazon]
titrate with 1.0N NaOH [1N NaOH on Amazon]
% by wt = (mls of NaOH X 0.7)/Specific Gravity of solution

Hope this helps.

A. The formula to calculate citric acid titrimetric method is:
W (weight en grams)=V of NaOH(ml) x Molarity x Molar Mass x 10^-3.
You have to Know the mass of the sample and the Specific Gravity D=m/v) just in case you want to calculate %(v/v).

%(w/v)= W(analite)/w(sample in g)x100

A. % CA = Normality x volume of NaOH x Equivalent weight of CA
Weight of sample (g) x 10

Equivalent weight of citric acid = Molecular weight of CA
Basicity of CA

= 192 / 8 = 24
NOTE: Basicity of CA = Number of Hydrogen ion present in CA, Which is 8.

A. pH is a logarithmic scale, so there is no way to get a 0.1% accuracy.
Density will reflect both the acid and any soluble material in it, giving you a false high reading that gets progressively worse with use.
Titrations can be done wrong also. Many people do not use proper mixing and get false answers. It takes the correct indicator or a very good pH probe and meter to get good results. Free iron will react with the hydroxide titrant, so use a weak one with spin bar agitation. There is a possibility that the citrate will form complex ions which can skew the results. Get a good procedure from a good reference or one of the companies in the business.

A. Your vendor can't help?

Then, something like this ought to work:

Take a suitable sample (10 ml?) and dilute to about 100 ml with DI water. Adjust the pH to about 9 with sodium hydroxide, then add about 1 g sodium sulfide (best done in a fume hood) to precipitate Fe as FeS. Filter, and wash filter cone well with DI water.

Then, titrate under suitable conditions (that you'll have to figure out) with 0.1 N KMnO4, to a purple endpoint.

You may find more details in the AOAC methods manual. This has methods for foodstuffs, likely several for citric acid .

A. Two things:

First, I forgot to mention that you need to bubble air through the filtrate after the iron precipitation step, to oxidize any excess sulfide ion. Otherwise, you will get a positive error in the titration.

Secondly, another way to go might be to add an excess of 0.1 N ceric ammonium sulphate, some sulfuric acid, and then boil the mixture briefly. After allowing it to cool, you can then back titrate the excess oxidant with standard ferrous ammonium sulphate, using ferroin as indicator. You may then calculate the citric content by simple arithmetic.

A. Brad,
Naturally, all the easy monitoring methods are not highly accurate, though they are generally good enough to keep a bath at the proper concentration for passivation.

pH monitoring is traditional for a standard citric acid bath, titration works as well. Some citric baths are buffered to a higher pH though, which takes away the equivalence between citric concentration and both titration and pH readings.

In this case the general fallback is specific gravity/density, which does assume that the bath is kept well filtered so that the citric acid concentration is the major source of your reading. Other methods that should be viable include refractive index and conductivity.

More complex analytical methods such as those Dave suggest should be fine as long as you have a staff chemist and the necessary equipment on hand.

A. I was in the library yesterday, and I looked up 'citric acid ' in the ASTM stds index. I found ASTM D4608 and ASTM D3598 - methods for citric acid in detergents.

One (I forget which) involved titrating a sample with a standard copper solution, using a copper ion selective electrode as the endpoint indicator. I imagine you could also use an chemical indicator, like PAN.

A. Parash,
This may be a language barrier issue.

Titrating an acidic solution is good for telling you how much acid is in there. It won't even really tell you what kind of acid or acids.

Purity means how much of stuff that isn't citric acid is mixed in with your citric acid . Titration won't tell you that.

But let's suppose for the moment that you are asking to find the concentration of a citric acid solution via titration. The basic idea of titration is to add an indicator like phenolphthalein [on eBay & Amazon] that will give you a visual indication of reaching the acid-base neutral point, then slowly add a hydroxide solution of known concentration until that neutral point is arrived at. Then it's just a matter of running the stoichiometry math.