Molecular Still Heads A. J. BAILEY University of Washington, Seattle, Wash.
mica and glazed or unglazed porcelain were unsatisfactory b e cause of the large quantities of gas given off when heated. Glass wool taken from a bat in the general laboratory supply and wrapped around the heater proved admirably effective as insulation. The various glass tapes, cloths, and sleeves supplied by the Owens-Corning Fiberglas Corporation, Toledo, Ohio, permit easy application. An input to the heater of about 20 watts sufficed for temperatures up to 300' C. The most satisfactory wax for completing the vacuum seal between the two bell jars was picein; it was equally effective whether built up as a dam on the outside of the joint or allowed to form a layer between the opposite joint faces, although it was usually easier to make a successful seal at the first attempt by the latter method. The condenser effectively cooled the wax seal so that the vapor pressure of the wax did not interfere. The details of construction are apparent in Fi ure 1. fVhile several types of vacuum systems were employed, best results were obtained by using a simple, short-coupled vacuum line. The still was coupled to a freezing trap which was attached to the mercury pump. A McLeod gage was also connected directly to the mercury pump. Between the mercury pump and the mechanical forepump a three-way stopcock was inserted. The third outlet led to the high-vacuum side of the mercury pump, thus making it possible to by-pass the mercury pump completely to prevent contaminating the mercury. On the low-vacuum side of the mercury pump, a liter flask was connected to serve as a reservoir. It was then possible to reduce the running time of the mechanical pump to two or three brief periods in 24 hours. Using the molecular still and vacuum system as described, a two-stage Kurth mercury ump and Cenco Hyvac fore-pump reduced the mm. of mercury in about an readin of the i&Leod gage to hour if the heater was not energized, and to the same level in about 2 hours with the heater and sample a t 200' C. The combination of McLeod ga e and freezing trap records the pressure of permanent gases onfy and these not in the distillation gap The total pressure of temporarily uncondensable molecules in the ap during distillation was probably higher than 10-8. The effectiveness of this still was demonstrated by successfully distilling lignin for the first time (1) and it has proved to be dependable, sturdy, and simple to operate.
ANY devices for molecular distillation have been developed and made available t o chemists in general through publication. Morton (3) reviewed a number of the types most useful in the laboratory, Detwiler and Markley (2) cited over a hundred publications, and others are readily found in the literature.
I
t
FIGURE 1. MOLECULAR STILL A , lead sample pan: B , brass hot p!ate; C, thermometer; D ,glass wool insulation; E , No. 30 Nichrome wire (20 ohms) insulated above and below by glass wool and wrapped on a brass bar; F , s ring bronze contacts insulated by mica wasgers: G , tungsten wires sealed in
Experience in this laboratory has shown that the use of standard glass parts contributes greatly t o t h e economy of construction, rigidity, and mechanical strength of the still, and interchangeability and general serviceability of the device. Several designs are of sufficient value t o justify more general use. Figure 1 shows one type of molecular still made from two standard Pyrex microbell jars. The parts are inexpensive, and the simple and rugged construction contributes t o rapid and safe manipulation. The design shown includes improvements indicated by many months of continuous operation, and is free from the usual vagaries such as temperamental heater, thermal control, and measuring devices, and erratic and undependable vacuum seals.
-
J
0
5
CENTIMETERS
FIGURE 2. LARGEMOLECULAR STILL A,, vacuum distilling dome: E , iron cover: C, oooling 0011; D . condenser late; E , stove plate; F, thermometer electrical lead: I , water jacket for well; G. heater; tapered joint; J , vacuum line
5,
A feature of early designs which caused endless trouble was the material which insulated the heating element. Materials such aa 171
178
Vol. 14, No. 2
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Large Molecular Still For larger capacity, a still of somewhat similar design was developed (Figure 2). The central unit of construction was a standard Pyrex vacuum distilling dome (Catalog No. 3480) which contained the heater, hot plate, condenser, and thermometer. The heater was an inexpensive commercial hot plate 11.25 cm. (4.5 inches) in diameter and was fastened to a circular brass plate 13.75 cm. (5.5 inches) in diameter and 0.625 cm. (0.25 inch) thick: these two constituted the s’ove. It was suspended by three machine screws from the condenser, and the length of the screws permitted varying the stove-condenser distance. The condenser consisted of a circular brass plate 15 cm. (6 inches) in diameter and 0.625 em. (0.25 inch) thick with a copper cooling coil soldered to the upper surface, and fastened rigidly to the iron cover. The electrical connections were made by wires, one threaded through magnesium oxide beads, soldered to a nickel bead fused to the tungsten wires sealed through the glass. An internal condenser was attached to the male ground joint to maintain the vapor pressure of the n-ax on the joint at a safe value. Picein wax was used to seal both the cover joint and the tapered joint. The
thermometers for both stills were made from the broken stems of regular thermometers, with bulbs of proper size blown, and stems curved to fit the walls of the stills. Glass plates were used to cover both stove and condenser when direct contact with metal was undesirable. This still had a heating surface of over 150 sq. cm. (23 square inches) and a condensing surface of over 180 sq. cm. (28 square inches). The rapidity of pumping systems is increased by large, short tubing, and elimination of constrictions caused by stopcocks, the latter also being a prolific source of leaks in high vacua. Satisfactory service can best be achieved b y a n allglass, sealed system; all joints should be regarded with suspicion.
Literature Cited (1) Bailey, A. J., Paper Trade J., 111, TS 73-6 (-lug. 15, 1940). (2) Detwiler, S. D., Jr., and Markley, K. S.,IND. EKG.CHEM.,AX.AL. ED.,12, 348-9 (1940).
(3) Morton, A. A., “Laboratory Technique in Organic Chemistry”, New York, McGraw-Hill Book Co., 1938.
A Special Slide Rule for Calcium Carbonate Equilibrium Problems In Corrosion Control of Water Supplies A. ADLER HIRSCH, State Department of Education, Baton Rouge, La.
P
ROTECTIOS of distribution systems against corrosion by means of a calcium carbonate film was first proposed
by Tillmans (4).but a comprehensive theoretical formulation of conditions for deposition in the system COrHC03--COs---Ca-n-as evolved recently by Langelier (7). His complete equation involves pH. titratable alkalinity as calcium carbonate. calcium concentration, ionic strength, and temperature: p ~ =, (PK;
- PK:)
+ p c a + p[
+
~ i k
(HT)
The influence of ionic strength and temperature is shown in the term (pKL - pK:). From the actual p H value of the sample is obtained: =
pH
- pH,
pH,
2K m] + log [I + &]
- K,
where pH, = the hypothetical pH a water must have, without other changes in composition, for calcium carbonate stability pK: = negative logarithm of the second dissociation constant of carbonic acid pK: = negative logarithm of the solubility product constant of calcium carbonate pCa = negative logarithm of the molal calcium concentration Alk = equivalent concentration of alkalinity by titration K, = dissociation constant for water
Langelier saturation index
B y showing, under conditions, that the effect of (H-) and (OH-) on the alkalinity term and the influence of the final log parenthesis may be omittably small, Langelier simplified Equation 1 with negligible error for ordinary waters whose pH, lies within the limits 6.5 and 9.5 to
(2)
a negative value of which indicates undersaturation and possible corrosion; a zero value denotes balance, and a positive value evinces supersaturation and possible scaling, the actual effects being dependent on numerical magnitude. DeMartini (2) concluded from a survey of California waters that a saturation index more positive than -0.5 generally implied freedom from serious corrosion.
(1)
=
(pK:
- pK:)
+ pCa + p d l k
(3)
A t the ordinary temperature of 20” C. (pKL - pK’) approximates a value of 2.3, which substituted in Equation 3 gives a short form: . pH, = 2.3
+ pCa + pAlk
(4)
F A further simplification of the Langelier formula is ultimately possible in certain restricted cases where, in the absence of a calcium determination, i t may be assumed that the calcium and alkalinity values are proportional. If calcium and alkalinity are taken as equivalent, Equation 4 becomes pH,
= 2.6
+ 2pAlk
(5)
which resembles in form the empirical equation of Baylis’ experimentally established calcium carbonate equilibrium curve (1) and is equivalent to Strohecker’s short formula (9). T o facilitate further the calculation of pH, Hoover and Riehl (6) prepared a nomogram to solve Equation 3 graphically. Their drawing provides for temperature and salinity influences but omits corrections for the final log term of Equation l and the effect of (H+)and (OH-) ions. The slide rule described below was designed primarily to provide immediate and easy solution to the full Langelier formulation with suitable provisions for all corrections; its application in solving the simplifications, Equations 3 and 4, ir especially rapid and effortless.
