Ferric chloride etch -- titration / analysis issues

A. Titrate a 5 ml sample diluted to 100 ml with 1.0N NaOH [1N NaOH on Amazon [affil link] to the yellow methyl orange ⇦ on eBay or Amazon [affil link] 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 [affil link] , 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.

"Cpmplexometric Titrations"
by Schwarzenbach & Flaschka

on Amazon or eBay
(affil link)

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 & 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 ⇦ on Amazon [affil link] (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 [affil link] 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 [affil link] 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

Dear Albert,

The only response I have to that question is as follows:

1. Can you say filtration? (sorry, trying to be funny) The carbon particulates you describe do not contribute to the titration reaction, and can easily be filtered out of solution using fluted, qualitative paper or microfiber filters.

2. It sounds like what you need to do is the same thing that I always do when I encounter an "unknown solution": Prepare standards.

3. Here's how I would prepare the standard which you describe:

a) Make up an ideal lab standard solution of the working bath. A standard which contains the optimum working agents.
b) Perform your accepted titration procedure upon this solution. Record your results.
c) Heat a known volume of the sample (in the lab) to the optimum working temperature.
d) Pass some "work" through the sample. This work should consist of brass and copper parts which you usually run. Keep a weighed, mass measurement of the parts you ran through.
e) Filter the sample, then run the same titration procedure again upon the second sample. Record results.
f) Establish a linear constant based upon the difference between the first standard and the second standard. These numbers will yield the math you need to your process control procedure.

Keep me in touch. I'm interested in your plight.

Truly,

I think now might be a good point to step back and ask yourself why am I doing this?

Why are you wanting to analyse the hydrochloric acid content?

If it is solution control a better answer might be to just go with the very simple answer of run a sample until it is not doing the process you want. Take samples every few loads (be sensible so you end with 5 to 10 samples for a bath life) Analyse these by a very simple titration NaOH and phenolphthalein ⇦ on eBay & Amazon [affil link] (I hope I can still spell) and see if there is a pattern - do the results rise? do they fall?

If you see a pattern is it such that you can draw a line and say once it crosses that line it is time to dump the solution.

I expect many of the people will have reached the point where the lab say the solution is fine, the shop floor say it is not and when you look you find it is some contamination issue that is not tested for so the lab are right when they say they can not fine anything wrong with the solution and the shopf loor are right when they say they cannot get a good job out of it.

Testing like described above will give a robust answer and take into account the effects of the.

A. Didn't you answer your own question? You can measure ferrous and you can measure total Fe. Well, the difference is ferric.

?. Where did you get your method for Ferrous Chloride?

A. Without knowing the composition of the solution you are analyzing, it is impossible to give a precise method. It is possible to measure Fe(III) with standard EDTA (Nitric acid interferes). You would adjust the pH of the sample solution with glycine, warm it and employ 5-Sulfosalicylic acid as the indicator. The color change is from red to pure yellow.
Determining the acid content may be a little more complicated. If there are no interferents, you should be able to do a total acidity titration using a standard base and phenolphthalein ⇦ on eBay & Amazon [affil link] indicator. Correct for the Fe(III) (1 mole = 3 moles acid) to find the HCl. I suggest you find the text by Schwarzenbach and Flaska for more information. I think the title is "Compexiometric Titrations" ⇨

A. A method that maybe can work for you :
First analyze iron. If you have nitric acid do this :
1- Pipet 1 mL to an erlenmeyer, add 10 mL d.i. water
2- Heat to dryness on an hot plate
3- Let cool a bit and add 5 mL concentrated HCl to dissolve
solid and add 30 mL d.i. water
4- add some KI and titrate liberated iodine with
1N sodium thiosulphate ⇦ on eBay or Amazon [affil link] with starch just before the end
point
1 mL Na2S2O3 1N = 0.05585 g Fe

For chloride, you can determine it by precipitation with silver nitrate ⇦ on eBay or Amazon [affil link] . Substract chloride linked to iron (3:1)

I'm wondering too if Mohr's method do the job as :
1- 1 mL sol'n + 50 mL d.i. water
2- neutralize with ammonium hydroxide
3- 1 mL indicator (4.2% K2CrO4 + 0.7% K2Cr2O7 in d.i.water)
4- Titrate with silver nitrate 1N to red-brown end point.
Subtract chloride linked to iron (3:1)

"The Chemical Analysis of Electroplating Solutions"
by Terrance H Irvine

on Amazon or eBay
or AbeBooks
(affil link)

"Analysis of Electroplating & Related Solutions"
by Langford & Parker

on Amazon
or AbeBooks
(affil link)

A. phenolphthalein is not a good indicator for your acid titration because of the high pH of 9 (plus or minus). At an alkaline pH, you are getting a false reading as metals start to precipitate out as hydroxides, using part of your titrant. I think that you can see the endpoint of methyl red ⇦ on eBay & Amazon [affil link] , pH of about 5. methyl orange ⇦ on eBay or Amazon [affil link] would probably be tough to see in a ferric chloride ⇦ on eBay or Amazon [affil link] solution.

A. Hmmm. It's a tough problem because the acidity doesn't change, just the solution metal loading.

You could try this: take a suitably sized sample (trial and error testing would be necessary to determine this) and dilute it up to 100 ml with water, then add aq. NH3 to pH 11 or so. Filter out precipitated metal hydroxides to get a solution that should only contain the nickel. I'm assuming you're processing a stainless steel with nickel in it - most do.

Now, add some murexide [affil link] indicator, and titrate with 0.1 M EDTA to a violet endpoint. This, after a simple arithmetical calculation, will give you the nickel loading, which should be correlatable to your etch test.

Or, wait a few days. Someone who actually does this type of work themselves may have a method all worked out. In that case, you should listen to them.

A. I'm not going to look up the fudge factors, etc., but if you do an acid/base titration for HCl and a total chloride titration (titrate w/AgNO3, chromate indicator), you could then subtract out the chloride from HCl. What's left comes from the ferric chloride ⇦ on eBay or Amazon [affil link] .

Were I in your shoes, though, I'd just use an ORP controller.

Q. Blake,
I don't have a response to your question, but I am interested in the FeCl3 etch solution you reference. I have some stainless steel parts that require temper etch inspection, and an etch solution very similar to the one you reference has been suggested. I would like to discuss this process.

A. Reduce your ferric chloride ⇦ on eBay or Amazon [affil link] (Fe3+) down to ferrous chloride (Fe2+) using tin chloride (SnCl2). Keep adding the tin chloride until the solution turns clear. Then precipitate out the excess tin chloride with mercury chloride (HgCl). Be careful because mercury can be toxic. Add the mercury chloride to the solution until it doesn't dissolve anymore. Let this solution settle out until you have a clear liquid on top, and precipitate at the bottom.

For the titration, siphon off a known quantity of the clear liquid and titrate with potassium permanganate ⇦ on eBay or Amazon [affil link] (KMnO4). Measure the ORP as you titrate - you should start out around 400-500 with Fe2+, and add KMnO4 until you hit 900 mV or so (Fe3+). Use the quantity of KMnO4 it took to hit 900 mV ORP as your titration quantity, and multiply the # moles of this additive by 5 to figure out how many moles of iron was in your original solution

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.

A. Hi Kaushik. While awaiting responses, you may wish to read the earlier entries on this thread. You may also find Barlow Campano's article "Kleingarn Regenerated Spent Acid at Increasing Ferrous & ferric chloride Content" in our on-line reference library interesting.

Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey