How to analyze the acid level (HCl) in ferric chloride Solution

A. Titrate a 5 ml sample diluted to 100 ml with 1.0N NaOH [1N NaOH on Amazon] to the yellow methyl orange [on eBay or Amazon] endpoint. mls NaOH used x 7.3=g/l HCl mls NaOH used x 19.67=ml/l 32% HCl mls NaOH used x 1.97=% v/v 32% HCl

A. If you use 1.0N NaOH [1N NaOH on Amazon] , you will also precipitate a fair amount of the iron giving you poorly repeatable results that will be high. Use 0.1N and divide the fudge factors by 10 to reflect the 10 times weaker titrant. Use lots of agitation and a relatively slow addition of the titrant. Play around with the amount of indicator that you use. Too little or too much make it a very hard endpoint to find. The right amount makes it a piece of cake. You can also use a pH meter. Just remember that it takes 10 to 20 seconds for it to give you a true reading.

A. There are alternatives to the base titration. The NaOH titer by indicator will include a portion of any ferric ion [Fe(III)] as HCl. The amount of base titer consumed and counted as HCl will vary directly with the level of ferric ion in the solution. To avoid poor endpoints and pH plots, you may measure both the total chloride by ISE and Fe(III) by EDTA, using a glycine buffer and sulfo-salicylic acid as indicator - see Complexometric Titrations by Schwarzenbach and Flaschka. To estimate the free HCl molarity subtract four times the Fe(III) molarity from the total chloride molarity. Calculate the difference as HCl. Use this technique if extreme accuracy isn't needed and total acid strength is not important.

A. You seem to have all the answers that you need, but one other suggestion is to use a small sample and the dilute NaOH and add approx. 1g of potassium fluoride [affil links] (KF) to hold up the iron.

A. Dear Mr. Tsang,

The first thing you have to do is "tie-up" the iron ion, thereby achieving the most accurate results. The procedure is as follows:

1. Pipette [pipettes on eBay or Amazon] a 5 mL aliquot of the working solution into a 250 mL Erlenmeyer flask.
2. Add 100 mL DI water and 20 mL potassium fluoride solution, 10 w/v%.
3. Add 10 drops phenolphthalein [on eBay & Amazon] solution, 1.0 w/v %.
4. Titrate sample against 1.0 Normal sodium hydroxide solution. Color change will be turbid white to constant pink.
5. Record volume of titrant consumed.

Calculation: v/v % 22Be HCl = mL 1.0N NaOH X 1.82

A. What you suggest will work IF you add the NaOH very slowly. If you add it too fast, the pH in a localized area will drop out some iron giving you a false reading. The iron hydroxide is very slow to redissolve.
I would prefer to use a 1 ml sample and titrate with 0.1N NaOH.

I would use a class A pipet or a calibrated cheap one for the one ml sample. I would also let the main sample set for a few min before taking the 1 ml sample.

Another way is to do an accurate dilution of the sample and then take a 5 ml sample of that.

Adjust your fudge factor accordingly.