I am facing problem in making Method calibration curve (0-100 micro gram) by chromotropic method.
First three points get linear but other points have absorbance lower than the former. Sequence of method as follows:
1-In Standard solution 2,4,6,8,10 ml methanol (1 ml = 10ug) in 50 ml vol flasks.
2-Add demin/de-ionized water to make final volume to 10 ml of all stds.
3- Add 0.5 ml KMNO4 (1% in 10% H2SO4)
4- Excess neutralize with Sod. Metabisulfite
5- 1 ml Chromotropic Acid (2%w/v)
6- 10 ml Conc. Sulfuric acid
7 Heat on water bath (95 °C) for 20 min
then cool and make up the volume with water and take absorbance at 570nm in 50mm cell.
Please analyze and point out the probable source of error.
And share any better method by ISE, Ion Chromatography or other colorimetric...
Between step I and step 2, add a step to evaporate off all the methanol under a stream of nitrogen; then you should be go to go with the procedure as it is given. The methanol will react with the KMnO4; it would be best to remove the methanol before the oxidant is added.
But if we evaporate off methanol then its concentration in the solution will reduce (same to be determined)???
Did you tried with another absorbance or another length (to cell)?
I thought you were looking at a compound (like 2,4D) dissolved in methanol not methanol itself! After step 3 how long do you wait for the reaction to complete before adding the sodium metabisulfite? The reaction to generate formaldehyde from methanol might not be going to completion in the higher concentrations of the methanol standards. In step 7 were the tubes sealed tightly so no formaldehyde could escape when you heated them?
Dear Bruce you have rightly pointed out the issue.
Actually yesterday, we had figured it out that we were not giving any time for complete oxidation of MeOH to formaldehyde. We gave around 2 min. for oxidation and better curve was obtained.
In point # 7 is it safe to seal the flasks??
Thanks a lot for your help.
@Anna Thanks
I would not recommend heating a sealed flat bottomed volumetric flasks to 95C. The link below indicates that the optimum temperature for the chromotropic acid reaction is 60C. If you are developing the method, why not test if using this lower temperature with the volumetric flasks stoppered gives better results?
http://cool.conservation-us.org/jaic/articles/jaic33-01-004_3.html
Dear Bruce,
I have checked with heating at low temperature. But the issue is that when we add Conc. H2SO4 10 ml in the solution the temperature raises to 100°C . It cannot be controlled even under ice bath (can be reduced).
We have check the reproducibility of the method with 60C & 95C, in both cases the response is not reproducible.
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