V O L U M E 2 6 , NO. 4, A P R I L 1 9 5 4 A further advantage may be obtained from this procedure in the preparation of dithizone solutions of reproducible concentrations. This is readily accomplished through the use of a stock solution prepared by purifying 100 mg. of solid dithizone, dissolving in 100 ml. of chloroform, extracting as indicated with the exception t h a t the ammonia concentration is increased tenfold (1 to lo), and re-extracting into 500 ml. of chloroform a t 20.0' C. Utilizing 25.0- and 40.0-ml. portions of the stock (and 1 to 100 ammonia) produced standard dithizone solutions of 4.4 and 7.1 mg. per liter, respectively, when the final volume n-as 1.0 liter a t 20.0 ' C. These doubly purified solutions were stable, not easily oxidized, had a very narrohv range of variation in concentration,
733 and proved t o be highly satisfactory for use in the determination of cadmium, lead, and mercury. LITERATURE CITED (1) Bambach, K., and Burkey, R. E., IND.ENG.CHEW,ASAL. ED., 14, 904 (1942). (2) Cooper, S. S., and Sullivan, 51.L., ;IN > 1 and
where
N, N,
number of moles adsorbed per gram of adsorbent number of moles needed to form a monolayer p pressure a t which adsorption is measured Po saturation vapor pressure of the adsorbate a t the temperature of measurement RT log C C = a constant defined by the equation E1 - E2 = = = =
where E1 is the average heat of adsorption in the first layer and EPis the heat of liquefaction of the adsorbate. This plot gives a c1 straight line, the slope of which is -and the intercept, -. NmC N mC Equating these quantities to the experimentally determined slope and intercept of the straight line gives two equations which may be solved for N,. From this value and the crosssectional area of the adsorbate molecule, the specific surface area of the absorbent may be calculated directly. I n order to perform the indicated calculation it is necessary to make an ex-
Now the quantity of gas in the dead space may be estimated by assuming the relationship q = pzh
(3)
where q is the number of moles of gas in the dead space a t equilibrium pressure p2 and b is a constant which is a function of the size of the sample bulb. Let pl be the initial pressure and so chosen that upon exposing the sample to the adsorbate (without changing the volume of the measuring system) an equilibrium pressure pz is obtained within the relative pressure range 0.15 to 0.35. Then the number of moles of gas adsorbed per gram of adsorbent ( N ) . is given by (4)
