Mixing your own reagents?

[eugene.rogers]

Now I now chemistry is rather exact science, but being more like an hobbyist, I'd like to approximate a little bit. I don't remember much from my chemistry classes, except the safety regulations of protecting eyes and wearing protective clothing (something I always do in my little kitchen lab). So, maybe someone with more skills in chemistry could say am I even getting close here.

The Wikipedia page (and several other sources) cite Marquis reagent to be 20:1 95-98% sulfuric acid and 40% formaldehyde. This gives me an the idea of a final solution of Marquis reagent being 1.9% formaldehyde. From a textbook example I see that the recipe for Marquis reagent is given as 100mL of 95-98% sulfuric acid added to 5mL of 40% formaldehyde ie. the final strength of formaldehyde is 1.9% (= 40% x (5mL/105mL). Should I have at my hands 37% (w/w) formaldehyde and 98% sulfuric acid, mixing those into a solution of 5.5mL of 40% formaldehyde and 99.5mL of 98% sulfuric acid would give me something quite close to 1.9% formaldehyde in the final solution, but is this close enough to be considered as working Marquis reagent?

And then there is also Erlich's reagent (1:1 ~95% ethanol mixed w/ 2mg p–dimethylaminobenzaldehyde and concentrated hydrochloric acid)? Now the ingredients aren't too exotic either, except maybe p–dimethylaminobenzaldehyde, but concentrated hydrochloric acid is a task definitely not suitable for my amateur kitchen lab and a 50% solution of hydrochloric acid would probably boil at my face too. But the final solution of Erlich's reagent is 50% of hydrochloric acid, right? If I mix concentrated hydrochloric acid in ratio of 1:1 with anything, the final solution would be something close to 50%, assuming that my concentrated hydrochloric acid was actually somewhere near 100% to begin with. I'm really confused here now, as I see that Erlich's reagent is being sold on the internets and thus the product can not be something that boils in around 40°C, right? Thus I've misunderstood something and the final solution of hydrochloric acid in the Erlich's reagent is not 50% but something else, I guess?

 

[Transform]

Yes, reagent tests are qualitative reagents and as a result they do not have to have to be incredibly accurate mixtures.

Concentrated HCl is only 37% solution in water. For someone with enough common sense this is not excessively dangerous. You should always work in a well ventilated area with a lab coat/apron, gloves and safety glasses when using conc. HCl.

Diluting it down from this would result in a low concentration and may not be as effective as required but hopefully this clears your confusion and you'll see it doesn't require dilution.

 

[eugene.rogers]

Thanks, indeed that did clear my confusion somewhat. The concentrated was translated in my mind to something around >= 95% which seems to be 100% wrong in the context of HCI.

As probably might be guessed from my somewhat vague understanding of concentrated on concentrated HCI, english is not my first or even second language.

 

[Transform]

I wouldn't be surprised if a native speaker made the same mistake. Only chemistry knowledge would have helped you here - your English seems perfect to me.

 

[sekio]

is this close enough to be considered as working Marquis reagent?

Yes, you could even just use the 37% in place of the 40% and it would work.

And Ehrlich's reagent is made with 37% hydrochloric. That's what's commonly known as "concentrated" hydrochloric acid.

Please be very careful handling sulfuric acid, formaldehyde, and hydrochloric acid. You can give yourself serious burns, or in the case of sulfuric acid, light things on fire if you aren't careful.