# Titration method to determine HCl concentration in a mixture

1. Jan 8, 2008

### Bluetheice

I have a solution which contains unknown amount of dissolved ferric chloride (FeCl3) and hydrochloric acid.

Can I use the titration method with NaOH to determine the concentration of HCl?
I was concerned since dissolved FeCl3 is also acidic, and the fact that NaOH will try to neutralize it as well. Thus the titration result may not be accurate.

Further more the solution is a brown to begin with, what indicator should I use to clearly tell the end point?

2. Jan 8, 2008

### symbolipoint

Some of this response is a little vague, so you would need to do an information search:

The Fe(III) can either be titrated with EDTA or some chelator; you might not need to adjust the pH too much; or you might try(?) an ion specific electrode for measurement. The acidity to pH 6 or 7 should be possible, but I'm not sure how much interference would occur from the Ferric chloride. You could either try a titration for total chloride (Mohr or Fajans method), or use a chloride specific ion electrode to measure chloride. Check a good quantitative book for finding how to do some of these. (Maybe there is a precipitation method for ferric?)

3. Jan 8, 2008

### mrjeffy321

Shouldn’t the excess HCl be neutralized first by the NaOH, before the FeCl3?

If the solution is still acidic and you add a small amount of NaOH solution, one of two possible reactions could occur. The OH- ions could react with the excess H+ ions and simply form water, or the OH- ions could react with the Fe+3 ion and form an Iron Hydroxide precipitate. But I would think that if Iron Hydroxide is formed and the solution is still acidic, then the excess H+ ions should react with the Fe(OH)3 and take it back to FeCl3 and water.

4. Jan 9, 2008

### Bluetheice

Hi, first of all thank you both for the reply.

Perhaps I should have describe the problem in more detail, the mixture is used for etching steel. We know that the initial solution is 40%w/w FeCl3 and 1.7% w/w HCl.

During etching Fe3+ is consumed, the reaction is
2Fe3+ + Fe -> 3Fe2+

and Fe2+ react with hydrochloric acid and oxygen in the air to regenerate Fe3+
The reaction is
3FeCl2 + 3HCl + 3/4 O2 -> 3FeCl3 + 3/2 H2O

So basically we want to know the HCl concentration in order to ensure the regeneration takes place.

To: symbolipoint
I think the ferric chloride will have significant influence, because we know the content of FeCl3 is so much higher than HCl. As you can see from the reaction above, the chloride is not involved in the reaction and thus is does not represent the HCl content.

To: mrjeffy321
I am not sure which will be neutralize first. Lets' assume that there are no HCl present in the solution, then how could I determine the end point? Wouldn't I end up neutralizing just Fecl3?

5. Jan 9, 2008

### chemisttree

This reaction produces additional FeCl3 (and you know where the extra chloride comes from...). You need to replace the chloride (as HCl) that is consumed in the regeneration step. Analyze the concentration of Fe+3 and F3+2 before and after the process and account for the amount of chloride before and after. The difference will indicate the amount of HCl consumed.

The total reactions:

$$Fe^0 + 2FeCl_3 \xrightarrow~ 3FeCl_2$$

$$2FeCl_2 + 2HCl + \frac{1}{2}O_2 \xrightarrow~ 2FeCl_3 + H_2O$$

You can monitor the H+ concentration with a pH electrode.

Last edited: Jan 9, 2008
6. Jan 9, 2008

### Bluetheice

To: chemisttree

Yes, the problem is figuring out how much HCl to add. To do that we first need to know how much is in the mixture.

Finding difference of Fe3+ and Fe2+ should work, but I was concerned that it may only work IF HCl is added AFTER the etching process. Our mixture contains 1.7% w/w of HCl before etching, this means HCl is consumed in 2 ways:

1: Reaction with steel during the etching
Fe+ 2HCl -> Fe2Cl

2: The regeneration after etching
3FeCl2 + 3HCl + 3/4 O2 -> 3FeCl3 + 3/2 H2O

This means some HCl is already consumed before the regeneration, so that the difference is not really the HCl consumed. I don't know if FeCl3 dissolve steel more rapidly than HCl. If it does then only a small amount of HCl would be consumed during etch and the difference should give fairly accurate estimate of HCl consumed. Could you give me more details, what do I need to find the concentration of Fe3+ and Fe2+?

I have tried to measure pH with a pH meter, however the change is very small. Perhaps the content of HCl is too low, and/or the change is too small to make a difference?

7. Jan 9, 2008

### symbolipoint

Here is another idea which might still be too crude for what you want. It assumes (maybe wrongly) that the ferric chloride is like weakly acidic. You can experimentally determine the acidity of ferric chloride, and you already know the acidity of HCl. Choose two different endpoints for an acidity titration. Maybe choose pH 4 and pH 7. When you then know the acidity of HCl and the acidity of ferric chloride, you can use this information when you titrate your sample to both endpoints. You can choose whatever acidity units that you like; best suggestion is either as HCl, or as milliequivalents or equivalents per unit volume.

8. Jan 11, 2008

### chemisttree

You can think of this reaction (it should be FeCl2 rather than Fe2Cl... a minor typo) as the operative one if you wish but hydrogen is produced in this reaction as well. I don't believe that much hydrogen is produced from your process but it is possible I suppose. Regardless, the additional iron salts come from the zero valent steel and are associated with the chloride that ultimately comes from consumption of HCl if all of the Fe+2 salts are converted to Fe+3 in the following reaction:
To do this properly, you will need to determine the total iron content (both Fe+2 and Fe+3 separately) and calculate the total chloride associated with that. Determination of the amount of chloride (from some previous determination/speciation) will tell you how much HCl was consumed since ALL additional chloride will come from the HCl.