[Valboy]

Currently writing a validation protocol to validate the method detail given below – I would like to understand the chemistry, being a validation engineer me chemistry is a little rusty! –can anyone shed some light on this for me thanks.

Solution containing hydroquinone in water approx. 100ppm. To this solution 3mls of sodium phospo tungstate is added mixed and left for 10 minutes. Then a solution of sodium carbonate is added to produce a blue colour – this solution is then analysed at 710nm on a UV-Vis spectrophotometer.

I understand a colour complex is formed but what is it?

[Valboy]

I am currently validating a method for the determination on hydroquinone, within a  hydrocarbon matrix. The method works well but I am unsure of the exact chemical reaction at the 2 critical stages of the analysis. for example the standards are made up in an aqueous diluent and the steps are given below.

1. Solution containing hydroquinone in water approx. 100ppm.
2. To this solution 3mls of sodium phospo tungstate is added mixed and left for 10 minutes.
3. Then a solution of sodium carbonate is added to produce a blue colour – this solution is then analysed at 710nm on a UV-Vis spectrophotometer.

I understand a colour complex is formed, but would like to know exactly how the hydrquinone is converted to the coloured complex - any assistance would be greatly appreciated.

[Valboy]

After doing a little more background research I understand the Hydroquinone is reduced by the sodium phosphotungstic acid solution (Uric acid reagent), while in the acidic medium, the solution remains colourless, the addition od sodium carbonate produces the unstable blue salt (taken from OTTO FOLIN & W DENIS 'harvard medical school, bostan June 29th 1912) which is subsequenbtly analysed.

I'm still a little unsure on the reaction specific's , again any help would be greatly appreciated.

[vex]

Can you calculate the oxidation state of tungsten in the sodium phosphotungstate reagent? The chemical formula according to Sigma is:

2Na₂O·P₂O₅·12WO₃·18H₂O

Is this a high or low oxidation state for tungsten? What might that tell you about its role in the reaction? What would be the products?

[marquis]

Can't help with the reaction, but you may be able to get more method information from some polymer people.  Some fast polymer cures (I think they were acrylic based) use HQ and MEHQ as preservatives.  By the way, the level of HQ was about 100ppm.  They may be able to give more info.  Good luck.

[Ligte]

hello valboy

At first reading your post the question looked simple and from a chemist's point of view i probably should know this...Sounds like a typical exam question.
Finally i got hold of this patent number 5240860. I enjoyed looking and finally reading it because its "old school" chemistry dating back to 1910 before technology determined a scientist worth and there is no better way to scrub up on basics.

i hope you enjoy     
vrede