[TheGreenMoose]

I have attached a PDF of the overall scope of my issue along with another example where I was able to get a calculation that matched my predecessor and a literary source for hexavalent chromium determination in a passivation process.

Hexavalent Chromium Determination (Alodine 600 & Permatreat 686A):

Pipette 5 Ml sample into a 250 ml Erlenmeyer Flask.
Add 50 Mls of Distilled Water.
Add 10 Mls of 10% Potassium Iodide
Add 10 Mls of Hydrochloric Acid
Let solution stand for 5 minutes.
Titrate with 0.1n Sodium Thiosulfate to a Straw Color
Add 3-5 drops of Starch Indicator
Continue Titration until Blue-Black color disappears.

My Predecessor’s Calculations:

Mls of 0.1n Sodium Thiosulfate x 0.21 = oz/gal Alodine 600
Mls of 0.1n Sodium Thiosulfate x 1.25 = % by volume Permatreat 686A

My calculations:

CA (oz/gal)=mL Thio*0.15=Concentration of Chromic Acid

Chemical Manufacturer's Calculations:

CA (oz/gal)=mL Thio*0.23=Concentration of Chromic Acid

Questions:

What are the balanced equations?  Is the 1 mole of Chromic Acid to 2 moles of thiosulfate the correct ratio?

Why are these calculations different?  Is it due to the fact that I would be titrating for hexavalent chromium concentration specifically?  Whereas my predecessor is calculating for the amount of a chemical as a whole.  Therefore if Alodine 600 is 50% chromic acid the final calculation would be twice the amount of ounces in a gallon of solution (i.e. mL Thio * 0.30 = Concentration of Alodine 600 in my calculation)?

Why do both my calculation and my predecessor’s calculation differ from the technical data sheet?

[Borek]

What are the balanced equations?  Is the 1 mole of Chromic Acid to 2 moles of thiosulfate the correct ratio?

Why don't you try to write them by yourself? There are two steps, first is the reaction between Cr(VI) and iodide, the other is the reaction between elemental iodine and thiosulfate. The latter is basis of the whole method, called iodometry.

[TheGreenMoose]

I think the Redox reaction and thiosulfate reactions are as outlined in the "Chromic Acid Redox Reaction.pdf" document.  Which provides the 1 mole of chromic acid to 2 moles of thiosulfate ratio.

[Borek]

The reaction of Cr₂O₇^2- listed in the pdf is wrong (check that the charge is not balanced). Plus, it doesn't support 1:2 stoichiometric ratio, 1:3 if anything. Which is not correct (1:2 is not correct either).

To be honest this is a mess. I can't even reproduce rather clear statement from the ALODINE document (the one about 1 mL of the titrant being equivalent to 1.73 mg/mL of chromium in the original solution). They say the titrant concentration is 0.1 N - no idea what they mean by N, it is ambiguous. I got either 0.26 mg/mL of Cr²⁺ or 0.54 mg/mL of Cr₂O₇^2- assuming 0.1 M solution of thiosulfate. Conversion between M and N requires typically multiplication or division by a small integer number, neither reproduces 1.73 mg/mL.