[pcm81]

I am trying to measure free acid and total acid in my home made manganese phosphate bath.

In all technical manuals about phosphating baths the titrations are done using indicators. while that works, i'd like to understand the chemistry as well as the pH curves for free acid vs total acid calculations. I tried googling, but the results come up 99% with titrations for wine making, rather than technical data for free acid vs total acid calculations.

For total acid in literature phenolphthalein is used, this is why i am GUESSING this is an equivalence point titration
But how to I titrate for free acid? In industry the bromphenol blue is used as the ph indicator. Is this just a pH=4 titration by definition or is there some science behind it, like equivalence point is the highest slope point, hence need to find derivative of pH curve etc etc etc?

Thanks ahead

PS. This is why i was asking about standardizing NaOH solution earlier.

[Borek]

How are "free acid" and "total acid" defined?

[pcm81]

How are "free acid" and "total acid" defined?

I was actually hoping that those were standard chemistry terms and you could tell me a more in-depth answer than google.
The most "scientifically sounding" answer I found so far is here: https://www.pfonline.com/articles/free-and-total-acid-values

Total and free acid values are often measured and used to control phosphate systems, which are acidic by design (to initiate the phosphate reaction). The first titration would be for free acid. This would involve taking a bath sample and titrating it with a known standard such as sodium hydroxide (probably 0.1N or 1.0 N). An indicator is added to the solution that changes color when the titration reaches a certain pH. A common indicator for this would be bromphenol blue which changes from yellow to blue when passing through the pH range around 3.5-4.0.

To determine total acid content, you would repeat most of the same steps above, instead this time using a different indicator, possibly phenolphthalein. This will change from clear to pink in the pH range of 8.5-9.

The free acid value is used to tell you how much acid is available to initiate the phosphate reaction and exists in its original “active” state. Too low a value may indicate that the bath would have difficulty initiating the phosphate reaction. The corrective action would be to add phosphoric acid to bring it in line with the supplier’s recommendations. Too high a value could indicate too much phosphoric acid in the system which will not “build” a coating since it would tend to strip it as quickly as it forms. The supplier may have a chemical to adjust for this (possibly mono- or disodium phosphate), or may just tell you to run some scrap parts or steel wool through the system to use some of the available acid. This value will tend to rise and fall around some centerpoint that you will control around (i.e., five plus/minus 1 point).

The total acid value is meant to indicate the total amount of acid that has been put into the system and is a combination of both free acid and that which has been “neutralized” through reaction and combination with iron. In general, this number will tend to climb over time as the bath ages and, when combined with a known production quantity, may be able to give you an indication as to when to dump the bath.

These measurements are most often performed with a zinc phosphate line. An iron phosphate line will typically have a titration for total acid and a pH measurement (which basically is another way to determine free acid)

The problem i am facing is 2-fold.
#1. All industry papers / recommendations are based on total acid and free acid measured in "points" which are ml of 0.1M NaOH when titrated with bromphenol blue and phenolphthalein. As usually the case, I am sure that these testing procedures are "short cuts" to real science behind the bath chemistry. Just as folks use phenolphthalein to find equivalence point, while really the EQP is the steepest slope, not just a ph value and it's ph differs from weak acid - strong base titration to strong acid strong base titration...
#2. so, i am basically trying to reverse engineer what these titrations are actually trying to measure and then gain deeper understanding of it, to be able to know what they are measuring and how the "recommended" values would apply to my own bath concentrations; given that i might change things up with bath chemistry to experiment a bit.

[Arkcon]

I think its jargon, and has meant something and still means something, to people who aren't chemists, and are plating bath engineers.  That happens, in industry, without apology.

We're lucky on this forum, its rare that we have to fight people over "alkaline diet" or "alkaline water", which are bogus terms that even clogged Wikipedia for a while.

Good for you for trying to reverse engineer the procedure to a meaningful topic.

You can tell, this is a simple titration, the reactant 0.1 N standardized NaOH is used in the same way in both cases, just the endpoints are different for the different indicators.  You get two numbers, and they tell you bath "health."

This is done to determine two separate reagents with similar properties.  Example: http://www.chemicalforums.com/index.php?topic=66432.msg239084#msg239084 This uses two titrations, with different indicators, to determine an unknown mixture.  The explanation there clearly states that the two products become another one in the process, and then that is titrated a second time.  That's likely what's happening here.

If we really understood these reactions, we could just say, "Submit sample for Ion-Chromatography" determine ratio of anions phosphate and eka-phosphate(I made that one up) and you'll know everything about your bath to two decimal places.

In fact that may be possible, but its likely the plating bath companies trade secret.

[pcm81]

[MOD Edit -- snip monster quote]

There is a separate titration for iron content using KMnO. But i got that under my belt already.
I think the total acid and free acid titrations in the case of phosphate bath are independent of each other. That is a different, clean, sample of the bath is used for each one. It's just the pH end points that i am struggling with to determine. Some people titrate total acid using pH meter to pH=7, others to pH=8.2... Free acid titrations i've only seen 1 "scientifically looking video" (not a guy with $10 ph meter doing wine titration) and the person was shooting for pH=4.

[Arkcon]

.. Free acid titrations i've only seen 1 "scientifically looking video" (not a guy with $10 ph meter doing wine titration) and the person was shooting for pH=4.

See, and that actually makes sense.  Comparing the endpoints of two indicators, we get two pH targets:

Bromnophenol blue:  pH 3.0 to 4.6 https://en.wikipedia.org/wiki/Bromophenol_blue#Acid%E2%80%93base_indicator

Phenolpthalein: 8.2−12.0 https://en.wikipedia.org/wiki/Phenolphthalein#Indicator

You can easily use a pH meter to find those points without using indicators.  You don't really have to use a new sample your each, because that's just what's going to happen anyway after you've titrated the first one.

[pcm81]

.. Free acid titrations i've only seen 1 "scientifically looking video" (not a guy with $10 ph meter doing wine titration) and the person was shooting for pH=4.

See, and that actually makes sense.  Comparing the endpoints of two indicators, we get two pH targets:

Bromnophenol blue:  pH 3.0 to 4.6 https://en.wikipedia.org/wiki/Bromophenol_blue#Acid%E2%80%93base_indicator

Phenolpthalein: 8.2−12.0 https://en.wikipedia.org/wiki/Phenolphthalein#Indicator

You can easily use a pH meter to find those points without using indicators.  You don't really have to use a new sample your each, because that's just what's going to happen anyway after you've titrated the first one.

But that is where the problem is..
I can titrate for pH=3 and pH=8.2 and get some number.
I can titrate to pH of 4.6 and pH=12 and get drastically different numbers.
How do i know which of these sets of numbers is "most in-line" with online documentation available for phosphating baths recommending a certain free acid and a certain total acid...
In all titration videos the titration is done when colour change starts, so i'd guess pH=3 and pH=8.2 would be my targets. But that disagrees with pH target of 4 i see in an actual "phosphate bath acid titration" video.
https://www.youtube.com/watch?v=pglorkLU4tM
In this video the lady titrates to 4.0 and 8.2... And since she is using gloves and a ph meter about 10x more expansive than mine, i can only hope that she knows her stuff since she appears to be in a commercial environment.

[pcm81]

I did a test titration and the pH curve i am getting has 2 rises in it. Kind of like a titration curve for acid with 2 pKa values. One "jump" is around ph=4 and the other is around ph=7.8.
I need to get better data set with more points in those ph regions. My GUESS would be that the free acid and total acid are the 2 "equivalence" points, basically where ph curve has the steepest slope.

Does this sound reasonable?

[Borek]

Try here: http://www.titrations.info/acid-base-titration

Sounds like the first end point is for H₃PO₄ and the second for H₂PO₄^-.

[pcm81]

Got a better dataset. The first steep rise appears to have a critical point at pH=4.2. Still need to investigate the 2nd jump range to get better resolution and determine 2nd critical point.

Here is the dataset in case if anyone wants for their own research. I did several titrations all with 10ml starting sample of the bath with addition of H2O to cover the ph meter.
The NaOH was standardized with KHP and was terermined to be 0.0937M
0.1M equiv ml of NaOH in H2O   pH
0   2.96
0.465   3.03
0.93   3.1
1.86   3.27
2.325   3.41
2.79   3.58
3.255   3.88
3.72   4.31
4.65   5.04
7.44   5.64
8.37   5.73
9.3   5.8
10.23   5.9
11.16   5.98
12.09   6.06
13.02   6.15
13.95   6.25
14.88   6.33
15.81   6.44
16.74   6.52
17.67   6.61
18.6   6.7
20.46   6.87
21.855   7
22.32   7.03
22.785   7.08
23.25   7.14
24.18   7.24
25.11   7.35
28.83   8.11
29.295   8.3
29.76   8.5
0   2.79
0.465   2.83
0.93   2.94
1.395   3.04
1.86   3.13
2.325   3.27
2.511   3.33
2.79   3.43
2.976   3.52
3.162   3.65
3.441   3.84
3.72   4.14
3.813   4.29
3.9525   4.37
4.185   4.56
4.4175   4.62
4.65   4.75
4.929   4.86
5.1615   4.91
5.487   4.97
5.673   5.02
6.045   5.07
6.603   5.17

[qwerty009]

I'm also in process of trying to make successful and repeatable manganese phosphate path for coating various steel parts. I ordered Bromothymol Blue and Phenolphthalein Solution as indicators for titration (should arrive within a week).
Formula i'm going to try is:

9.26 g/L H3PO4
12.61 g/L HNO3
3.3 g/L Mn

Source: http://www.scielo.org.mx/pdf/jmcs/v57n4/v57n4a10.pdf

I assume that those concentrations are for 100% acids? So when my H3PO4 is 85% and HNO3 is 52% and my manganese is in form of Mn02 i would have to use following amounts for 1 liter of water?:

10,9 g/l H3PO4 (85%)
25,2 g/l HNO3 (52%)
5,2 g/l MnO2 (Mn 63% O2 37% mass %)

Next question i have is also how to determine total and free acid in solution. Everywhere i've read TA & FA is in "points", what does it mean exactly?
Best explanation i've found is in here: http://imghost1.indiamart.com/data2/FJ/TM/MY-1736813/zinc-phosphates.pdf (explains how to maintain some commertial zinc phosphate bath).

Total Acid Points (TA)
reagents: 0.1 N sodium hydroxide solution
indicator: phenolphthalein
procedure: 1. Pipette 10 ml bath sample into a 250 ml Erlenmeyer flask.
2. Dilute approx. 50 ml deionized water.
3. Add 5 drops of indicator.
4. Titrate with 0.1 N sodium hydroxide solution from colorless to
    light pink.
calculation: consumption in ml = TA Points
correction: for each missing Total Acid Point = addition of  5 ml/l ZP­5
Free Acid Points (FA)
reagents: 0.1 N sodium hydroxide solution
indicator: bromphenol blue
procedure: 1. Pipette 10 ml bath sample into a 250 ml Erlenmeyer flask.
2. Dilute with approx. 50 ml deionized water.
3. Add 5 drops of indicator.
4. Titrate with 0.1 N sodium hydroxide solution from yellow to blue.
calculation: consumption in ml = FA­Points
correction: To neutralize 1 Free Acid Point add 0.4 g/l Na OH (pre­diluted in
water, 10 %).

So even if i have maybe very different composition (HNO3+H3PO4 vs ?) i could still take 10ml sample of my path and add X ml of 0.1N NAOH to get TA & FA points?
Therefore: X ml 0.1N NAOH it takes / 10ml path sample  = total acid points or free acid points

[Borek]

9.26 g/L H3PO4
12.61 g/L HNO3
3.3 g/L Mn

(...)

I assume that those concentrations are for 100% acids? So when my H3PO4 is 85% and HNO3 is 52% and my manganese is in form of Mn02 i would have to use following amounts for 1 liter of water?:

10,9 g/l H3PO4 (85%)
25,2 g/l HNO3 (52%)
5,2 g/l MnO2 (Mn 63% O2 37% mass %)

Sounds OK, with the exception of the Mn. Even after browsing the paper I have no idea in what form Mn(II) was put in the solution. I strongly doubt it was metallic Mn as it will consume some of the acid, and using MnO₂ doesn't sound OK, as it will not only consume acid but it will also introduce Mn(IV), not Mn(II).

Best thing you can do it to try to contact authors and ask.

i've read TA & FA is in "points", what does it mean exactly?

That you are reporting the result using an artificial unit:

calculation: consumption in ml = FA­Points

[qwerty009]

Tnx for info. I was going to try MnO2 since it's cheaply available and i've found it used in DIY phosphating recipes. What Mn compound would be preferred (that can be actually purchased from chemical supplier). MnCO3 is also smth i know i could purchase.

Regarding titrating i found patent from 1961 where it's written:
. The term points acidity as employed in the phosphating art represents the number of milliliters of 0.1 normal sodium hydroxide solution required to neutralize a milliliter sample of a phosphating solution in the presence of phenolphthalein as an indicator.

So no matter what mixture of acids i use i can take 10ml of path solution and titrate it with 0.1N NAOH. If it takes 100ml of NAOH solution to neutralize solution if would indicate that my solution has 10 points of total acid (100ml/10ml=10)?

PS since English is not my first language i have hard time understating scientific text.

[Borek]

. The term points acidity as employed in the phosphating art represents the number of milliliters of 0.1 normal sodium hydroxide solution required to neutralize a milliliter sample of a phosphating solution in the presence of phenolphthalein as an indicator.

So no matter what mixture of acids i use i can take 10ml of path solution and titrate it with 0.1N NAOH. If it takes 100ml of NAOH solution to neutralize solution if would indicate that my solution has 10 points of total acid (100ml/10ml=10)?

Yes, looks like that's how it is intended to work.

Beware: dissolution of MnCO₃ will consume the acid as well.

[qwerty009]

Since putting readily available Mn compounds to solution will consume acid it's probably best to dissolve for example Mno2 (that i have at hand) in H3PO4, reaction should be following?:
4H3PO4 + 3MnO2 = 6H2O + Mn3(PO4)4

H3PO4 98g/mol (82% = 120g acid).
MnO2 87g/mol

4x 98g (=392g (480gr @ 82%) + 3x 87g (=261g) = water + Mn3(PO4)4 (i assume that it's insoluble in water?)

After reaction has taken place i can add Mn3(PO4)4 as source of Mn to phosphating solution?

If i mix water, h3po4, Mno2 ... solution will be very dilute and MnO2 will have hard time dissolving IMO + it will gradually reduce acidity as it reacts.