[gat]

Hello,

I am looking for a correct method to titrate a bath consisting of the following chemicals:

            H3PO4: 30%

            HNO3: 10% and

            HCl: 15% to determine the concentration of each acid.

The reason of this strong acid bath is because of seamless tubes that we need cleaned inside (used in food industry).

This is imperative for us!  If after a while of using the bath the concentrations drop, we need to be able to check the solution and be able to tell which one and how much we need to add to readjust the bath.

Anything can help, even a separate titration method for each acid.

Thank you a million!

gat

[Borek]

This is not as simple as determining amounts of three separate acids.

I have added 100 g NaOH to 1L of your solution. What is the concentration of H₃PO₄ now? All I can tell is that 2.5 mole of H⁺ was neutralized, no idea how to define which acid is "left" and which is "spent". Properties of your solution must be defined in some other way.

[AWK]

Why not using a combination of different volumetric analyses, eg:
1.all three
2. Cl^- + PO₄^3-
3. Cl^-

[Arkcon]

I would probably titrate the chloride ion with silver nitrate.  There may be an indicator for phosphate quantization, and  like AWK says, once you know those two, the remainder is nitrate.  But I have other questions:

Why does your application need to know how much of these particular acids?  If it's just for cleaning, isn't it total acidity what does the cleaning? 

Also,  building on what Borek said, as the cleaning bath is consumed, isnt it acidity that's being lost?  That is, the counterions would likely aways be there, just the acidity is neutralized, so quantifying them might tell you nothing about the cleaning ability of your bath.

[Borek]

I would probably titrate the chloride ion with silver nitrate.

That was my first idea, but I wonder if phosphate will not interfere.

[Arkcon]

That was my first idea, but I wonder if phosphate will not interfere.

See, I don't even know.  I only ever did chloride in a HCl, H2SO4, CuSO4 plating bath.  I also did total acid of the same bath, even though there are 3 (technically only 2) different acids present.

It's just that the HCl is usually present at 50 ppm, and the contribution of Cu2+, in a large excess of H2SO4 is not really significant.

gat:

I wonder about your cleaning application.  I recognize that H3PO4 is a good pickling agent for steel, and nitric may passivate the steel, and is good for removing the organic contaminants.  But the HCl, that's always bad for steel.

Unless you're not cleaning steel. Heh.

[gat]

I wonder about your cleaning application.  I recognize that H3PO4 is a good pickling agent for steel, and nitric may passivate the steel, and is good for removing the organic contaminants.  But the HCl, that's always bad for steel.

Unless you're not cleaning steel. Heh.

Thank you so much for all the replies!

Knowing the particular acids is a standard requirement for the maintenance of the bath to provide SPC data for control, which is a must in large industries.

This particular pickling is for removal of free iron on the surface of the stainless steel 304 (I forgot to mention this! Thank you for pointing that to me!) which is again a requirement so that the taste is not affected.