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topic 59980

Titrating to find ferrous iron in manganese phosphate bath

August 5, 2015

Q. Hello,

I am hoping one of you knowledgeable fellows can help me out.

I have recently taken over the heat treatment/phosphating facilities in my company. We have had a phosphate facility for around 12 years now. In that time, the facility has been run a certain way, however, I would like to put more controls in place in order to improve the quality of the components coming out.

At the moment we do not do any titrations to find ferrous iron. we only titrate to find the total acid / free acid / acid ratio.

Can anyone point me in the direction of which chemical & indicator I need in order to properly titrate to find the ferrous iron content? the specs of our bath at the minute are below.

Current Specs:

brand: Henkel Bonderite M-Mn117
Total acid: 30-40 pts
Free Acid: no spec.
Acid ratio: 12-22
ferrous iron: no spec/not checked.
temperature: 68-72 °C

I believe these specs are not ideal, and so as well as checking for ferrous iron, I want to adjust my bath to something like this:

brand: Henkel Bonderite M-Mn117
Total Acid: 50-65 pts
Free Acid: 3-7 pts
Acid Ratio: 10-25
Ferrous iron: max 2 g/l
Temperatue: 78-82 °C

does this seem like a sensible move?

Michael Whitworth
Heat Treatment Manager - Glasgow, Scotland, UK

simultaneous August 19, 2015

A. Hi Michael,
You can determine ferrous iron in a phosphate bath using a titration.
Take 10 mls of the phosphate solution, add a small amount of redox indicator powder (i.e., Barium diphenylamine-4-sulfonate) to help with the end point. Then titrate with 0.02 M potassium permanganate. Multiply the titration result by 0.558 to give the ferrous iron concentration in grammes per litre.

Mark Lees
Aerospace - Ballasalla, Isle of Man, Great Britain

August 13, 2015

A. Good day Michael.

I would directly contact Henkel for your queries.
I am not familiar with the M-Mn117 process.
I am using the M-Mn Lubrite 2 process @ 205-210 °F.
The temps do appear to be on the low side for phosphate- 154-180 °F.
My iron analysis is as follows;

10 ml sample - 250 ml flask
75 ml DI water
15 drops 50% H2SO4
Heat to near boiling-95 °C
Titrate 0.1N KMn04 to FIRST sign permanent pink
Fe% = mls 0.1N KMn04 x 0.056

There is VERY good info in this site concerning phoshate, as Robert Probert can attest to this.

Hope this helps.

Eric Bogner
Lab Tech. - Whitby, On., Canada.

August 20, 2015

thumbs up signThanks for the responses fellas.

Mark, that sounds a sort of similar process to how I find total acid/free acid (all be it with different chemicals and indicators). Ill give that a go as it sounds simple enough.

Eric, I did have a look around the site at some information on phosphating, which in truth was the original decision to alter my process towards the manufacturer recommended specifications. we have been running the phosphating process at 70 °C for years (We were originally using a product called Henkel Granodine LT10 - but it was discontinued due to the type of chemicals present in it), we brought in MN-117 as a direct replacement, although upon looking at the technical specs recently, I have noticed that the advised temp. for this product is 75-85 °C - hence my decision to move to 80.

Michael Whitworth [returning]
Heat Treatment Manager - Glasgow, Scotland, UK

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