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In Duma's method, we need to collect Nitrogen gas over water and subtract the aqueous tension. But why can we not just collect it over the air?

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    $\begingroup$ Ever seen any air? Other gases, maybe? Guess what: they mix. $\endgroup$ Commented Apr 29, 2021 at 12:46
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    $\begingroup$ It is possible only if density difference is huge, and even then it is just matter of time. $\endgroup$
    – Poutnik
    Commented Apr 29, 2021 at 12:48
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    $\begingroup$ Perhaps more importantly, you won't be able to see the border between the two, because it won't be there. $\endgroup$ Commented Apr 29, 2021 at 12:49

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Dumas's method for determination of nitrogen (and usually then for estimation of protein content) is fairly old (1831 Ref 1) and has been modified to be quick and automatable. In the original method, the equipment was arranged to collect the gaseous nitrogen after passing thru various traps. The picture (Ref 2) shows a modern arrangement of the original equipment used by Dumas. The inflow marked CO$_2$ represents the gaseous product of the ignition of the unknown specimen, and N$_2$ is collected over aqueous KOH (to remove CO$_2$).

enter image description here

In the picture below, modern, automated, Dumas nitrogen determination takes well-homogenized samples, heats them in a high-temperature furnace where combustion takes place at over 1000 °C in the presence of pure oxygen. This produces mostly H$_2$O, CO$_2$ and N$_2$ as several oxides. This gas mixture passes through a reduction chamber containing copper heated to around 650 °C. This stage converts nitrogen oxides into N$_2$ and collects excess O$_2$. Various traps remove the residual H$_2$O and CO$_2$. Total N$_2$ content is measured by a thermal conductivity detector. The N$_2$ gas is not collected over water - it is essentially measured (not collected) over air, because the flow thru the TCD (thermal conductivity detector) can measure how much N$_2$ is going by.

enter image description here

Now if you don't have one of these (expensive?) automatic measuring devices, you can go back to the older way and just collect the N$_2$ over water (actually over KOH solution).

Ref 1. https://www.velp.com/en-ww/dumas-method-1.aspx

Ref 2. https://www.brainkart.com/article/Dumas-method---Estimation-of-nitrogen_36468/

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  • $\begingroup$ then why does nitrogen gas come out to be moist? $\endgroup$
    – Neha
    Commented Apr 30, 2021 at 10:29
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    $\begingroup$ In the first diagram, the exit gas is collected over KOH solution. The vapor over that solution contains H2O molecules, which exert a vapor pressure. If you collect over plain water, there will be a higher vapor pressure, which you can correct for. If you had efficient water traps in the line, and collect over mercury, you could assume that there is no water in the collected gas. In the second diagram, there are 2 water traps (WT1 and WT2), so the TDC doesn't measure any H2O. $\endgroup$ Commented Apr 30, 2021 at 13:05

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