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. I n 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 n i t h 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. I n 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
V O L U M E 19, NO. 1 0
812
part of the distilling flask and evaporated to dryness on its overheated surface. Consequently, in spite of the fact that antimonious chloride does not volatilize in an aqueous hydrochloric acid solution, its real vapor is formed and carried with the outgoing vapor stream. The liquid condensed in the interior of the. droplet’ catcher is sufficient i o dissolve this antimonious chloride and carry it back to the boiling solution. The passing of a stream of hydrochloric acid through the liquid and the use of any of tht. other precautions formerly suggested for this purpose arc thus eliminated and a sharp separation of arsenic and antimony can be performed in a single distillation. h former model of the device described ( 1 ) had an inner steam conducting tube which did not affect the efficiency of the droplet catcher, but causrd difficulty in filling it with the small glaFs rings. So, although highly appreciat,ed by the chemists who used it, the device has not been fabricated in quantity. The new modificat.ion overconies thew technical difficulties in the fahrication. ’
TESTISC, EFFICIENCY O F DROPLET CATCHER
I n a 1-liter flask provided with a connecting bulb 500 ml. of 1 -I’sodium hydroxide and a small quantity of zinc powder (to generate hydrogen) were boiled. The pH of the distillate was found to be 8 to 9, proving that one part of dispersed liquid was carried along with 100,000 to 1,000,000 parts of distilling liquid. When the assay was repeated, the interior droplet catcher replacing the exterior bulb, the p H of the distillate \vas 7.0 to 7.2. At the same time, if the position of the distilling flask and the height of the gas flame were maint’ained exactly, the distilling rate, in milliliters per minute, increased 50%. It could have been further improved up to 100% by increasing the intensity of heating. On the other hand, when the connecting bulb was used, i t ivas not possible to speed the distillation by more intense boiling because in this case the condensed liquid closed the inner tube, so that the passing steam threw quantities of it towards the condenser. Exactly the same relations were observed in the distillation of an acid ammonium chloride solution with zinc powder and measurement of the ammonia in the distillate iTith Kessler reagent; no coloration a t all \vas found with the droplet catcher, while 10-6 t o part of the ammonia content of the boiling liquid was observed in the distillate when the conventional connecting ,bulb via3 used. -in example may be cited as nonanalytical proof of the efficiency of the droplet catcher. I n 1942, while in charge of a n agricultural
Figu
1.
Droplet (:atc*her
institute, where it was not possible to acquire a modern still, the, author was in immediate need of reIat,ively large quantities of distilled water, pure enough t o be used in soil analysis. d n oltl still of some 30-liter capacit,y, regarded as unserviceable, wai found in storage. A droplet catcher was installed in the interioi. of the ret,ort and the still 17-as fed with tap water to which potassium permanganat,e and sodium hydroxide were added continuously. During two years, the outgoing distilled water x a > always colorless and practically neutral. I n this case the uw of the droplet, catcher proved that it, is possible to obtain n-atcbi, with the purity of “redistilled” water in a single distillation. LITERATURE CITED
(1) Hahn, F . L., Ber., 57, 1858 (1924). RECEIVED October 23, 1946
Apparatus for Rapid Electrometric Titration of Acid Determination of p H and Measurement of Turbidity in Microbiological Assays I.0UIS B. ROCKL,IND AND 3IAX S. DCNN Chemical Laboratory, Cnicersity of California, Los Angeles, Calif.
apparatus shown in Figure 1 has been employed more than a year in the writers’ laboratory for the determination of turbidity, pH, and titratable acid of solutions in the microbiological assay of amino acids and vitamins. These measurements may be made simultaneously by an experienced worker with the aid of a technician at the rate of about 90 per hour and independently a t rates of about 150, 300, and 200 per hour for turbidity, pIi, and titratable acid, respectively. Reeves ( 2 ) has described an apparatus for multiple p H determinations and Silber and Nushett ( 5 ) have pointed out the convenience, speed, objectivity, and applicability of the p H procedure in the determination of pantothenic acid with Lactobacillus casei. A spread of about 2 pH units was obtained over a working range of 0.02 to 0.10 microgram of pantothenic acid per tube. The precision and accuracy attained by the present authors in microbiological assays of histidine with Leuconosfoc mesenteroides P-63
have been reported ( 1 ) . I t has been found that the acid produc,tion of organisms in microbiological a s s a y may be deterniinc~tl more rapidly, conveniently, and accurately by means of the dcxscribed apparatus than by titration using bromothymol blue indicator to determine the end point. Some of the shortcomingwhich are eliminated or minimized include eyestrain, general fatigue, and end-point errors caused by indicator fading, turbitlity, and colorations. ’ APPARATUS
The titration cup is a Pyres funnel, -4 (Corning 6110, ESPGY , and 2- to 3-mm. bore Pyres stopcock, b. The outlet, a , of the cui) under stopcock b is clamped to a ringstand and is connected to :L water aspirator with rubber pressure tubing. A 25-m1. automatic zero Kimble Blue Line Exax buret, B , with a three-way stopcock is clamped t o the ringstand in such a mannc-i’ that the buret is just above and in the center of the titration cup.
