The problem of gas drying
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Since gases (N2, H2, CH4) contain moisture, they need to be dried before being fed into the chromatograph. So what is the simplest, most convenient, and cheapest method for drying them? Common gas desiccants include: concentrated sulfuric acid, phosphorus pentoxide, soda lime, calcium chloride, and silica gel (add more if not all are listed). There is also the question of how to determine whether all the moisture in the gas has been absorbed, in other words, how to check when the desiccant has become ineffective. It should be a simple method that allows for an immediate assessment, such as using color-changing silica gel!
Of course, colored silica gel is the most cost-effective, and it’s easy to determine when it has lost its effectiveness; it’s harder to tell with other types, and they are generally used only for one-time or short-term drying purposes.
We have concentrated sulfuric acid in our analysis room, which is used to prepare 30% sulfuric acid for absorbing ammonia! I think water needs to be diluted anyway, so why not use concentrated sulfuric acid to absorb the water, and then use it to absorb ammonia afterward? I wonder if that would work Since it is unknown when the effect of concentrated sulfuric acid in absorbing moisture becomes poor, we are wondering whether it would be possible to add color-changing silica gel after the concentrated sulfuric acid; once the silica gel changes color, we could then replace the concentrated sulfuric acid. Is this feasible? Please give me some advice!
Concentrated sulfuric acid cannot be used for drying gas in chromatography, as the acid decomposes and its resulting gas is highly corrosive. What we can use for learning include silicone, molecular sieves, and activated carbon. Since you mentioned that the gas used for chromatography is CH4, activated carbon cannot be used in this gas path.
Molecular sieves and color-changing silica gel are usually used
I think you’re using it in a chromatograph, and the amount used should be small; you could consider using concentrated sulfuric acid followed by anhydrous copper sulfate – this will give the most obvious results. It seems like this topic is often tested in junior high school chemistry exams.
One more question: up to what point of dilution does concentrated sulfuric acid lose its ability to absorb water?
We have been using molecular sieves and color-changing silica gel, along with dedicated desiccation tubes, which are usually provided by manufacturers
The more concentrated sulfuric acid is diluted, the stronger its acidity becomes, while its dehydrating ability weakens. If you want to control its acidity and maintain its dehydrating properties, it’s better to keep it within the range of 93%…95%. Below that level, its acidic effect will be stronger than its dehydrating effect. This is why I think it should be used with caution even when it is available, as it is difficult to determine its acid concentration. If you don’t mind the hassle and really need to use it, measure it regularly.
For the gas phase, color-changing silica gel is used, as it is inexpensive and effective
The gases used in chromatographs need to be purified before they can be used; of course, the main task is to remove water. We use the Folie GPI-2 gas purifier, which contains color-changing silica gel. I’ve been in contact with Trap before as well; it’s an imported product, and I’m not entirely clear about its specific principles and structure (it’s completely enclosed in stainless steel). Its function is presumably to purify gases!