COMMUNICATIONS TO THE EDITOR THE SOLUBILITY OF CALCIUM BICARBONATE' The idea that passing carbon dioxide into a suspension of calcium carbonate solubilizes part of the salt as soluble calcium bicarbonate is so plausible that no one seems ever to have questioned it. Professor Bancroft reports that he can find no account of an experiment performed t o determine whether calcium bicarbonate is in true solution. The point seemed worth considering, because there are natural streams of water which are green (apparently a structural green), yet every ordinary source of the color seems to be ruled out. If calcium bicarbonate were in colloidal suspension, this fact might account for the color. Actually, there are three, not two, possible cases in this problem: (1) carbon dioxide might peptize calcium carbonate as such; (6) carbon dioxide might form calcium bicarbonate as a colloidal suspension; (3) carbon dioxide might form calcium biprbonate in true solution. Accordingly, the following experiment in ultrafiltration was carried out. Carbon dioxide was bubbled through a solution of C.P. calcium hydroxide for several hours. A collodion bag, previously tested for soundness with a Congo red sol, was filled with a portion of the suspension containing 26.8 mg. of calcium, calculated as calcium hydroxide, and immersed in distilled water. After standing overnight, the diffusate and the residue in the bag were analyzed for calcium by titration with hydrochloric acid. Any soluble calcium impurities in the original hydroxide would not interfere in this procedure; moreover, these were negligible in amount. The residue contained 6.2 mg., again calculated as calcium hydroxide, and the diffusate, 20.4 mg. This accounts for 26.6 mg., which differs negligibly from the amount introduced into the bag. The calcium which did not diffuse through seemed to be all in the form of calcium carbonate, since a precipitate settled to the bottom of the bag during the experiment. The diffusate was examined with a Tyndall beam in direct comparison with an identical tube containing freshly drawn distilled water. The diffusate was optically empty, which made examination with an ultramicroscope unnecessary. Professor Mason of the microscopy staff supervised this examination. Evaporation of a portion of the diffusate gave a residue which effervesced upon treatment with hydrochloric acid. The diffusate remailled clear for several hours after the experiment, when it was discarded. Received June 13, 1938.
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The diffusion experiment was repeated with bags which were dried for a longer time after preparation than the first one before being wetted, and which were soaked in distilled water a shorter tiin? before use. (The first bag was dried for 10 min. and soahed for 12 hr.) This procedure should give smaller pores than the shorter drying period and/or a longer soaking period. Again the calcium diffubed through the collodion memIirane. Thc conclusion is drawn that carbon dioxide converts the major portion of the calcium carbonate precipitated from a solution of calcium hydroxide into calciuni bicarbonate, which is in true solution. Very reasonably, people have believed this all along, but it ib satisfying to have experimental evidence t o sipport the belief. S ~ E J ~ HS. EHUBARD S Department of Chemistry Cornel1 University Ithaca. New YorG
THE STASDARD BATTERY (A CORRECTION)1 R. B. Elliott and G. A. Hulett in a previous article (J. Phys. Chem. 37, 489 (1933)) hai-e described the "characteristics of standard batteries, but we find that the specifications given do not give a satisfactory electrolyte. The constancy of current and voltage of these batkries depends on maintaining the electrolyte wit,hin quite narrow limits. For the coinbination !CdS04 HgzSOajHg, Weston (U. S. patent 494,824:, April, Cdorng. 1891) found that a cadmium sulfate (CdS04 8/3H20) solution, saturated a t 4OC., was satisfactory. This combination has a practically zero temprrature coefficient of m u , for all ordinary temperatures and furnishrs a most satisfactory working standard of E.M.F. It is not a n easy matter t o niake a saturated solution at 4"C., especially with CclS01.8/3II10, which has an extraordinarily slow rate of solution. Solubility data show that a solution of CdS0,;.8,i3H20 saturated at 4°C. is 43.18 per cent Cd80, and 56.82 per cent H30,so that 100 g. or 100 cc. of n-ater dissolves 112.75 g. of CdSOa.8/3€110. For the 30-Crii. battery descri'ned (,J. Phys. Chem. 37, 492 (1933)) one needs 4 liters of water and 4.5 kg. of CdS04. 8/3H20. The liwt grade of commercial sulfate, or the cloudy crystals left over from recrystallizing the salt, are entirely satisfactory. Sel-era1 hours stirring will lie necessary to effect solution. Heating above 70°C. is not desirable. $'our litera of Tvatcr a n d 4.5 kg. of CdS01.8/3H?O will give 5 . 3 liters of solution, and t o this is added 25 cc. of c:.P. sulfuric acid (density 1.84). This will give a11acidity of 0.085 niolar sulfuric acid, 1
I?wt=ived April 18, 1938.
