V O L U M E 19, NO. 1 0
810 flask (19 cm. from A to B ) is made by attaching a 34/45 female standard-taper joint to the bulb of a 275-m1. Johnson flask. The thermometer, M (Fisher Scientific Co., Catalog KO.15-002), has a range of 0" to 250' C. PROCEDURE
Digestion. TISSUES.Quickly moisten 10 grams of prepared Aample, contained in a digestion flask, with 30 ml. of "starting solution" and 5 ml. of water. To prepare the starting solution, dissolve 1.6 grams of ammonium metavanadate in 300 ml. of water mixed with 1500 ml. of concentrated nitric acid. After seething stops, add 75 ml. of concentrated nitric, 5 ml. of 60% perchloric, and (carefully) 50 ml. of concentrated sulfur'c acids. Place a t'hermometer in the flask and slowly heat to 140" to 150" C. \Then nitrogen dioxide no longer is in evidence, slowly increase the temperature to 210" C., then cool. Wash the thermometer with 10 ml. of water. If the solution becomes green during digestion, add 1 ml. of perchloric and 10 ml. of nitric acids, then decrease the heating rate. SOILS.Heat 50 grams of soil wit'h 30 ml. of starting solution, until the foam breaks. Then add 75 ml. of nitric, 7 ml. of perchloric, and 100 ml. of sulfuric acids. Incline the flask in a 600ml. hforoney antibumping cup and digest as for tissues. SUTRIEKT SOLUTION.Evaporate an appropriate volume of solution to 30 or 40 ml., with 0.5 gram of sodium peroxide (8) in a digestion flask. .4dd 50 ml. of concentrated sulfuric and 1 ml. of 60% perchloric acids and 5 to 10 mg. of ammonium metavanadate. Add nitric acid only if the color due to vanadium changes from yellow to green. Heat to 210' C. Distillation. TISSCES . ~ N DSOLGTIONS. Apply silicone grease to the joints and assemble the distillation apparatus on a ring stand. Start air flowing into the tube, J , a t a rate such that 2 or 3 bubbles per second rise from outlet L, which is immersed in 50 ml. of 0.1% aqueous hydrazine sulfate solution, contained in a cooled 250-ml. Beraelius beaker. Run 5 ml. of 48% hydrobromic acid into the sample through the funnel, 1. Heat the flask until most of the bromine is driven from it, then allow 10 ml. more of hydrobromic acid to drain into it, at the rate of 1 ml. per minute, while a vapor temperature of 125" to 135" C. is maintained. Heat the thermometer hole, E , as required to remove condensate. SOILS.Distill as above, but thoroughly mix 10 ml. of hydrobromic acid with the sample before heating. Add only 5 ml. of hydrobromic acid a t the rate of 0.5 ml. per minute while heating. Titration. Add about 3 grams of urea and 2.5 ml. of 90% formic acid to the receiving beaker and heat until the bromine is reduced. Seutralize to phenolphthalein v-ith 4 5 7 , sodium
hydroxide solution. Add 13 ml. of 18 N sulfuric acid and cool Dissolve 1.0 gram of potassium iodide in 100 ml. of 0.1% wheat starch paste. ildd 5 ml. of this reagent to the sample and immediately titrate with 0.005 to 0.01 A' sodium thiosulfate. When the change from purple to pink is stable for more than 7 seconds, the titration is complete. Standardize the sodium thiosulfate by carrying a pure selenite or selenium dioxide through the appropriate steps of the above titration. h reagent blank should accompany a series of samples. After this paper had been completed, attention was called to an improved van der Meulen titration developed by XlcCullough, Campbell, and Krilanovich ( 5 ) . In the present method, which employs hydrobromic acid and about 1/200 as much potassium iodide as XIcCullough et al. use, only a fraction of the selenium being titrated appears as the element. The remainder combines colorless compound in the Sorris-Fay reaction. LITERATURE CITED
Coleman, W. C., and McCrosky, C. R., IND.ENG.CHEM., ASAL. ED., 9, 431-2 (1937). Hoffman, J. I., and Lundell, G. E. F., J . Research, Satl. But. Standards, 22, 465-70 (1939).
Klein, A. K., J . Assoc. Oficial A p . Chem., 24, 363-80 (1941). Lyons, R. E., and Shinn. F. I,., J . Am. Chem. SOC.,24, 1087-93 (1902).
RlcCullough, J. D., Canipbell, T. W.,Krilanovich, S . J., ISD.ESG.CHEM.,ANAL.ED..18, 638-9 (1946). Painter, E. P., Chem. Rem., 28, 179-213 (1941). Pavlish, A. E.. and Silverthorn, R. W.,J . Am. Ceram. Soc.. 23, 116-18 (1940).
Robinson, IT. O., Dudley, H. C., Williams, K. T., and Byers, H. G., IKD. EKG.CHEM.,ASAL. ED.,6, 274-6 (1934). Schemer, J. .4,*J . Research, S a t l . Bur. Standards, 16, 253-8 (1936).
Sherrill, >I. S.,and hard, E. F., J . Am. Chem. SOC.,50, 1665-74 (1928).
Silverthorn, R. W., Chemist-Analyst, 30, 52-4, 62-3 (1941). Smith, G. F., "Mixed Perchloric, Sulphuric and Phosphorio Acids and Their .4pplications in Analysis," pp. 10-51, Columbus, Ohio, G. Frederick Smith Chemical Co., 1934. R E C E I V E D October 2 , 1946
Saturated Potassium Hydrogen Tartrate Solution as a pH Standard JiAIES J. LINGAKE Department of Chemistry, Harvard University, Cambridge 38, .)lass.
SBTURATED aqueous solution of potassium hydrogen tartrate is a more convenient secondary standard for the calibration of pH-measuring' instruments than any of the buffer Jolutions generally used for this purpose. The preparation of the solution is extremely simple; it is only necessary to shake an excess of the pure salt wit'h distilled water of good quality for 2 or 3 minutes a t room temperature to obtain a solution whose p H is reproducible to 10.02 unit. Potassium hydrogen tartrate is available commercially in a high state of purity, and further purification is achieved easily by simple recrystallization from water. A sample of the C.P. commercial salt was recrystallized twice, and the pH values of eaturated solutions of the original material and the two recrystallizates agreed to * 0.005 unit. Since the pH of potassium hydrogen tartrate solutions is about 0.4 unit smaller than that of water saturated with carbon dioxide a t one atmosphere, the small amount of carbon dioxide dissolved from a normal atmosphere has no significant effect on the pH. Solutions of potaqsium hydrogen tartrate appear to be more stable than the commonly used potassium hydrogen phthalate solutions; a saturated solution showed an increase of only 0.03 pH unit after standing for a year in a stoppered Pyrex bottle.
However, since the preparation of the solution is so simple, it should be prepared freshly as needed, and not stored, so that the possibility of accidental contamination is avoided. Hitchcock and Taylor ( 2 ) determined the pH of an exactly 0.03 M potassium hydrogen tartrate solution by means of the hydrogen electrode and obtained a value of 3.567 a t 25" C. on the same empirical but thermodynamically consistent scale recently recommended by NacInnes, Belcher, and Shedlovsky (3, 4 ) and Bates, Hamer, Manov, and Acree (1). The writer found that the pH of a saturated solution of the salt (0.034 M a t 25') does not differ significantly from the foregoing value. For all pra,ctical purposes the value 3.57 * 0.02 for the pH of the saturated solution may be used. The influence of di!ution on the pH of a solution of potassium hydrogen tartrate, originally saturated at 25 was determined with the result shown by curve 1 in Figure 1. For comparison the dilution effect observed with 0.05 M potassium hydrogen phthalate is also included (curve 2). In this figure A pH i s the apparent difference in pH between the original and diluted solutions and it includes any effect resulting from changes in the liquid-junction potential between the saturated calomel reference electrode and the glass electrode half-cell. The dilution factor, O,
811
OCTOBER 1947
Oo2
lar solubility, S, of potassium hydrogen tartrate a t various temperatures have been computed:
I Temperature, 8,molar
' C.
30 10 15 20 25 0 0.0123 0.0190 0.0231 0.0283 0.0341 0.0405
On comparing these data with curve 1 in Figure 1it is evident that a solution saturated a t any temperature above loo,and then brought to 25" for measurement, will exhibit a pH within 0.02 unit of a solution saturated a t 25', so that no special care is necessary in preparing the saturated solution. The temperature coefficient of the p H of a saturated potassium hydrogen tartrate solution has not been precisely determined, but it is probably very nearly the same as that of 0.05 M potassium hydrogen phthalate (+0.0014 unit per degree a t 25", I), and hence negligible for all practical purposes. Since the ionization constants of tartaric acid are much closer together than those of o-phthalic acid, a potassium hydrogen tartrate solution has a greater buffer capacity, and therefore is less sensitive to adventitious acidic or basic impurities, than an equiconcentrated solution of potassium hydrogen phthalate.
0. I Q
a
LITERATURE CITED
DILUTION FACTOR, Figure 1.
V/V,
Influence of Dilution on pH
V/Vo, is the ratio of the diluted to the original volumes. In both cases the apparent p H increases on dilution, but the effect is much smaller with the potassium hydrogen tartrate than with the potassium hydrogen phthalate. From data given by Seidell (5) the following values of the mo-
(1) Bates, R. G., Hamer, W. J., Manov, G. G., and Acree, S. F., J . Research Natl. B u r . Standards, 29,183 (1942). (2) Hitchcock, D. I., and Taylor, A. C., J . Am. Chem. Soc., 59, 1812 (1937). (3) MacInnes, D. A, "Principles of Electrochemistry," Chap. 15, New York, Reinhold Publishing Corp., 1939.
(4) MacInnes, D. il., Belcher, D., and Shedlovsky, T., J . Am. Chem. SOC.,60, 1094 (1938). (5) Seidell, A., "Solubilities of Inorganic and Metal Organic Compounds," New York, D. Van Nostrand Co., 1940. RECEIVEDOctober 7. 1946.
Improved Trap for Analytical Distillations
'
F. L. HA",
Apartado Postal 9622, lMexico, D . F.
'TEAM leaving a distilling flask is supposed to contain only
those components of the boiling liquid which are volatile under boiling conditions-for example, ammonia but no sodium hydroxide in a Kjeldahl distillation, or arsenious chloride but no antimonious chloride in a strong hydrochloric acid solution containing these elements. Hon-ever, the outgoing vapor always contains dispersed liquid. To remove from the vapor stream these liquid droplets, formed by the bursting steam bubbles, connecting bulbs are used xvhich, although varied in some details of their design, are all based on the same principle: They impose a directional change on the vapor stream, projecting it against wet surfaces so that the droplets may be absorbed and carried back to the distilling flask by the condensed liquid. There may be one or more bulbs, round or oblong. connected by curved or T-shaped tubes, but in all cases these bulbs add to the air-filled volume which must be washed out by the passing stream, and they act as reflux condensers. Both factors prolong the distillation time, nhile the active surface and consequently the efficiency of these derices are limited. In Kjeldahl distillations, when the geneiation of hydrogen is used to avoid bumping 6f the solution, or in the reduction of nitrate by Devarda metal, etc., microscopic gas bubbles are projected through the surface of the boiling liquid and are not washed out completely by the connecting bulbs. Khen'the inner active surface is increased for the purpose of improving the efficiency of the bulbs, the outer cooling surface and consequently the distilling time increase at the same rate. Because of these limitations of the external connecting bulbs it would be useless to suggest any new form, but the aspect changes if the trap is installed inside instead of on the distilling flask neck. In the droplet catcher here described, the total air-filled volume
of flask and trap is less than that of the empty flask alone. Furthermore the active inner surface can be increased a hundredfold, while the outer surface (the cylindrical neck of the flask) always remains the same. CONSTRUCTIOF; AND OPERATION
The construction of the new droplet catcher is shown in Figure 1. (This apparatus is available from Scientific Glass Apparatus Co., Inc., Bloomfield, S. J. Specify joint and flask sizes when ordering.) The material used is Pyrex. Dimensions depend on the size of the distilling flask used. The three concentric tubesi.e., the neck of the flask and the two tubes of the droplet catcher-ought t o be as close as possible to speed the passing of vapors and leave the maximum space to the most important part, C, which is filled nith helices. The outer tube of the droplet catcher is equipped with two 4-mm. or three 3-mm. openings as vapor inlets and a small tip with a 1-mm. opening a t the bottom for the return of collected droplets into the flask. In use, the steam rises in the exterior passage, A , passes through the openings a t 0 down the middle tube, B , and a t last rises through the helices filling the wide central tube, C. The very small quantity of condensate containing all the washed-out droplets of bubbles drops back through the capillary point, P. The efficiency of this device in separating liquids from gases (vapors) in which they are dispersed has been confirmed in practice for many years. I n addition, it has proved very useful in the separation of the real vapors of a higher and lower boiling liquid. The separation of arsenic and antimony by distillation of a strong hydrochloric acid solution of the trivalent forms of these elements ib a very old analytical method which, however, presents a very difficult problem. It can be demonstrated that codistillation of antimony in this analytical operation is due to the fact that parts of the boiling liquid are projected t o the upper
