Clark’s soap solution

Clarke s soap solution (estimation of hardness in water). 

Determination of Hardness.—Hardness, whether temporary or permanent, is conveniently estimated by means of Clark s soap test, which consists m adding from a burette small quantities of standard soap solution (vide infra) to 50 c.c. of water which have been carefully measured out with a pipette into a 250-c.c. bottle. After each addition of soap solution the bottle is vigorously shaken, and the titration is complete when the lather remains unbroken for five minutes after laying the bottle on its side at rest. 

This can be prepared in several different ways. Commonly dry Castile soap is dissolved in 80 per cent, alcohol in such proportions as will yield a solution well above the desired final concentration 100 grams per litre is a convenient ratio. After allowing this solution to stand at rest for several days for the deposition of undissolved matter, a quantity of the clear liquid is withdrawn (usually 75-100 c.c. per litre of final solution), and so diluted with 80 per cent, alcohol as to produce a solution which on titration with a known weight of calcium chloride solution under the standard conditions will give results in accordance with Clark s table. The calcium chloride solution is best prepared by dissolving 0-2 grams of Iceland spar in dilute hydrochloric acid excess of acid is removed by evaporation on a water bath and the solution then diluted to 1 litre with distilled water. A mixture of 25 c.c. of this solution, mixed with 25 c.c. of water, should require 7-8 e.e. of Clark s standard solution for the production of a permanent lather. 

Clark [21] has measured the shear stress at static and dynamic state of a foam from hydrolysed keratin, and soap. The maximum static shear stress Toji was 35.0 Pa and the dynamic T0d was 23.0 Pa at 1% keratin concentration and 225.0 Pa and 110.0 Pa, respectively, at 10% concentration. According to Clark s data the To,, value is directly proportional to the surface viscosity. For a foam from 5% soap solution T0ii, and to,d coincided and were equal to 32 Pa. Being in qualitative agreement with Eqs. (8.14) - (8.16), this shear stress increased with diminishing foam bubble size. 

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