Refined Analysis of Gases Using the Orsat Apparatus

G. I. Fuks

Of all the gas analyzers that have been proposed, in practice the Orsat apparatus, or devices similar to it, are used most widely. The reason for this is their simplicity, reliability, and also the possibility of carrying out the analysis as rapidly as possible. This is especially important, for example, when testing a boiler installation, where what we have, in essence, is fluctuation around a state of equilibrium, and only the performance of many analyses during the test guarantees the reliability of the mean value.

But for all its merits, the Orsat apparatus has the substantial drawback of low analytical accuracy. The measuring burette is graduated in divisions of 0.2%, so that, it would seem, a reading accuracy down to 0.1% is guaranteed (estimating half-divisions by eye). In reality this accuracy is unattainable in an Orsat-type apparatus.

Indeed, as is known, in the usual arrangement of the Orsat apparatus the gas being analyzed is drawn through the manifold when the sample is taken into the measuring burette. At the same time a certain amount of gas remains in the tubes of the manifold, forming a kind of “dead” volume that substantially affects the accuracy of the readings. We checked the magnitude of this dead volume on several specimens. It turned out that it varies within the limits of 2.5–3.0 cm³.

A simple calculation shows that, with a content of some constituent of 10% in the gas under investigation, the reading will be (if it is assumed that all the gas in the dead space is delivered into the absorption burette) 10.3%, and at 20%—20.6%. Thus, the error in the reading on the apparatus may reach 0.5–0.6%. Moreover, one must suppose that in reality the error in measuring the $O_2$ content is especially significant, since the sum $CO_2 + O_2$ is close to 20%, and pyrogallol with alkali, of course, absorbs $CO_2$.

It follows from this that readings to 0.2% by the divisions on the measuring burette are completely unreliable.

On the other hand, an accuracy of 0.2% cannot be considered entirely sufficient, for example, for determining $CO$ or heavy hydrocarbons¹. Therefore we set ourselves the task of increasing the accuracy of readings with the Orsat apparatus, without, of course, complicating the apparatus itself or the work with it. The following solution to this question suggested itself.

Reducing the influence of the dead space of the Orsat apparatus by decreasing the diameter of the opening in the manifold is, of course, permissible only up to certain limits. A calculation of the internal diameter gave an average value of about 3 mm. In reality this value is rather variable, ranging from 2 mm to 3–3.5 mm in individual sections. It appears entirely possible and necessary to reduce the internal diameter in the manifold to 1.3–1.5 mm, which gives a reduction of the dead space to 0.5–1.0 cm³ and by itself already raises the accuracy of the reading. But it is still more expedient, in addition to this palliative solution, to change

the arrangement of the manifold in such a way as to almost completely paralyze the influence of the dead space. For this purpose the branch for taking the gas sample must be made in the manifold on the same side as that on which the measuring burette is placed, according to the diagram (Fig. 1):

Fig. 1.

With this arrangement of the gas intake, as is not difficult to see, the main part of the manifold above the absorption burettes will, during operation, be filled with “spent” gas from the preceding analysis, i.e., with a practically inert gas, the presence of which should not affect the measurement of the absorption volume.

In order, at the same time, to make possible more accurate readings on the measuring burette, it should be modified according to the following scheme (Fig. 2):

Fig. 2.

With the dimensions (internal) indicated in the diagram, divisions from \(0\)–\(22\%\) should be marked on tube \(A\), and on tube \(B\)—\(22\)–\(44\%\). The zero reading must be in the capillaries, as must the \(100\%\) reading. If the dimensions are observed, the volume of the measuring burette will be \(100\ \mathrm{cm^3}\). The divisions corresponding to percentages (\(\mathrm{cm^3}\)) must be applied by empirical calibration. The length between marks of \(1\%\) will be \(1\ \mathrm{cm}\); thus, by applying millimeters, we can estimate by eye to \(0.05\%\).

If this accuracy in the analysis of flue gases does not satisfy the experimenter, the matter can be helped by a small change in dimensions. Thus, by increasing the length of the widened part to \(69\ \mathrm{mm}\), and decreasing the diameter of tubes \(A\) and \(B\) to \(8\ \mathrm{mm}\), we shall have a tube capacity of \(11\ \mathrm{cm^3}\), which, with millimeter divisions, will make it possible to estimate by eye \(0.025\%\).

Tubes \(A\) and \(B\), by means of rubber sleeves fitted over the capillaries, are connected with two metal stopcocks \(A_1\), \(B_1\), the opening and closing of which the experimenter performs with the left hand. After the stopcocks, both branches are connected through a tee and led to the usual Orsat bottle.

Work with this burette proceeds as follows: after the sample has been taken in the usual way, it is driven into the absorption burette; in doing so, with tubes \(A\) and \(B\) filled with confining liquid, stopcock \(A_1\) is closed and the further pumping of gas proceeds through tube \(B\).

When absorption is complete, then, without opening stopcock \(A_1\), the gas is first drawn off from the absorption burette through tube \(B\). If more than 22% of the gas has been absorbed, the reading will immediately be obtained on tube \(B\). In the usual case, however, when analyzing the products of combustion of solid and liquid fuel, one may at once bring the level in tube \(B\) to the lower mark, close the corresponding stopcock \(B_1\), and then, after opening stopcock \(A_1\), draw the remainder of the gas out of the absorption burette. Thus, the only complication, as compared with the usual Orsat type, is the introduction of two metal stopcocks, \(A_1\) and \(B_1\). But they are considerably more convenient in operation than the rubber-tube clamping customarily practiced when working with the Orsat apparatus.

1. Here we do not touch upon the methodology of their determination. ↩