Estimation of Carbon and Hydrogen

Carbon and hydrogen are always estimated together, by the same underlying combustion method: a precisely known weight of the organic substance is burnt completely in an excess of oxygen, converting every carbon atom it contains to carbon dioxide and every hydrogen atom to water (CxHy + O2(excess) -> x CO2 + (y/2) H2O). The masses of CO2 and H2O actually produced are then measured directly, and those two masses are enough, by simple stoichiometry, to back-calculate both percentages.

The apparatus needed has three distinct units working in series. First, an OXYGEN SUPPLY: oxygen gas is bubbled through concentrated sulphuric acid to strip out any moisture, then passed through a U-tube packed with sodalime to strip out any trace CO2, so that only dry, CO2-free oxygen reaches the combustion tube. Second, the COMBUSTION TUBE itself: a hard-glass tube, open at both ends and heated by a gas burner, packed (from the inlet end) with an oxidised-copper gauze (to stop combustion products diffusing backward), a porcelain boat holding the accurately weighed sample, a length of coarse copper oxide (which oxidises any organic vapour passing through it), and a second oxidised-copper gauze near the outlet. Third, the ABSORPTION APPARATUS, which the combustion products pass through on their way out: first a pre-weighed U-tube packed with pumice soaked in concentrated H2SO4, which absorbs the water formed; then a set of bulbs containing strong KOH solution, which absorbs the carbon dioxide formed; and finally a guard tube of anhydrous calcium chloride, which simply keeps atmospheric moisture from creeping backward INTO the absorption train and contaminating the readings.

In practice, the combustion tube is first heated strongly on its own to drive off any residual moisture, then cooled slightly and connected up to the absorption apparatus; its far (inlet) end is then briefly opened just long enough to insert the weighed sample boat, before being sealed again and heated strongly until the sample is fully burnt away (this typically takes about two hours). A final strong current of oxygen is passed through at the end to sweep any last traces of CO2 or water vapour out of the tube and into the absorption apparatus, after which the U-tube and the potash bulbs are detached and reweighed, giving the exact mass gained by each.

From those two mass gains -- x g of water and y g of carbon dioxide, from a starting sample of w g -- the percentages follow directly from the molar composition of water and CO2: since 18 g of water contains 2 g of hydrogen, %H = (2/18) x (x/w) x 100; since 44 g of CO2 contains 12 g of carbon, %C = (12/44) x (y/w) x 100. A fully worked example in the text (0.26 g of sample, giving 0.039 g H2O and 0.245 g CO2) works out to %H = 1.66% and %C = 25.69%. …