8006-28-8
Action and use
Used to absorb carbon dioxide.
DEFINITION
Soda Lime is a mixture of sodium hydroxide, or sodium hydroxide and potassium hydroxide, with calcium hydroxide.
CHARACTERISTICS
White or greyish white granules, or it may be coloured with an indicator to show when its absorptive capacity is exhausted. It absorbs about 20% of its weight of carbon dioxide.
Partially soluble in water; almost completely soluble in 1M acetic acid.
IDENTIFICATION
A. When moistened with hydrochloric acid and introduced on a platinum wire into a flame, imparts a yellow colour to the flame.
B. A solution in 1M acetic acid yields reaction C characteristic of calcium salts, Appendix VI.
C. A suspension in water is strongly alkaline to litmus paper.
TESTS
Hardness of granules
Shake 200 g on a sieve no. 2000 for 3 minutes using a mechanical sieve shaker that reproduces in a uniform manner the circular and tapping motion given to sieves in manual use and has a frequency of oscillation of 282 to 288 cycles per minute. Place 50 g of the retained material in a hardness pan 20 cm in diameter having a concave brass bottom, 7.9 mm
thick at the circumference, 3.2 mm thick at the centre and with an inside spherical radius of curvature of 109 cm. Add 15 steel balls, 7.9 mm in diameter, and shake on the mechanical sieve shaker for 30 minutes. Remove the steel balls, transfer the contents of the pan to a sieve no. 2000 and again shake on the mechanical sieve shaker for 3 minutes. The material retained by the sieve weighs not less than 37.5 g.
Size of granules
Shake 500 g on a perforated plate of nominal pore size 6.70 mm; not more than 5 g is retained. Then shake on a sieve no. 4750; not more than 50 g is retained. Shake the unretained material on a sieve no. 1400; not more than 20 g passes through. Shake the unretained material on a sieve no. 600; not more than 7.5 g passes through.
Loss on drying
When dried to constant weight at 105°, loses 14.0 to 21.0% of its weight. Use 1 g.
Moisture absorption
Place 10 g in an open glass dish about 50 mm in diameter and 30 mm high in a desiccator over sulfuric acid (14%) and allow it to remain for 24 hours. The increase in weight is not more than 7.5%.
Carbon dioxide absorption
The activity is not less than 120 minutes when determined by the following method.
Use a vertically-clamped tube of glass or other suitable transparent material about 25 cm long and 29 to 31 mm internal diameter with closely fitting rubber bungs at each end; the bungs are bored to receive polythene or glass tubing of about 8 mm external diameter, the tubing being flush with the inner ends of the bungs. With the lower bung in position, place sufficient nylon mesh support on top of the bung to produce a bed of mesh about 10 cm deep and press a closely fitting
disc of stainless steel gauze of nominal mesh aperture about 500 μm on top of the nylon mesh so that its surface is at right angles to the axis of the tube. Introduce 59.8 to 60.2 g of the substance being examined onto the steel gauze in three portions, tamping lightly after the addition of each portion. Place a second disc of steel gauze on top, followed by a
sufficient quantity of nylon mesh such that the soda lime is kept consolidated by slight pressure when the second bung has been inserted. The exit tube is connected to a condenser, consisting of two 50 mL separating funnels, leading to a drying tube packed with anhydrous calcium chloride and then to a carbon dioxide analyser sufficiently sensitive to detect 0.2% v/v of carbon dioxide. A katharometer, calibrated for carbon dioxide and preferably used in conjunction with a chart recorder, is suitable.
Using the gas analyser in accordance with the manufacturer’s instructions, accurately determine the carbon dioxide content, p, as a percentage v/v, of a nominal 5% carbon dioxide mixture, the balance gas being oxygen, air or nitrogen as appropriate to the type of gas analyser being used. Suitable compressed gas mixtures are available commercially.
Assemble the apparatus described above and pass the gas mixture downwards into the absorption tube at a rate of 900 cm per minute until such time that the gas analyser shows the content of carbon dioxide in the effluent gas to have risen to 0.2% v/v. Steps should be taken to vent the effluent gas if an oxygen-carbon dioxide mixture is being used. Record the time taken, t, in minutes. The activity of the soda lime is given, in minutes, by the expression tp/5.
