band is attached to hold the cell flush with the bottom spacer. The screw housing and the cell holder are made of brass, with the lower part of the cell holder blackened by a surface treatment. The standard cell compartment with the light-proofing assembly (Beckman Instruments, Instruction LIanual 305, page 35, Figure 23) was used, modifled only by displacing the two extra upper screws, outward and downward a few millimeters into the corner below the shoulder of the standard (Beckman 2390) sample compartment. There still was room to use the cell compartment with the ordinary cells of 1-em. path length. MOUNTING
OF
1
-
circular saw to height less than that of the cell holder. Thus a uniform height of the light beam is obtained for all measurements taken on a diffusion cell. To minimize reflections, the diaphragm and slit holder were blackened by a surface treatment. For evaluation of the light intensity over the light beam after passage through the two exit slits, a standard “slit block” was used. Two wellsquared pieces of brass with beveled ends were mounted on a brass bar, so that the beveled ends were 0.115 mm. apart, forming a narrow slit. With this block in the scanning device, the intensity of the incident light beam can be measured, for example, a t every 0.05 turn (0.03 mm.) over all the beam.
A-A
SECTION 1
I
i I
A SECOND LIGHT SLIT
The Beckman DU spectrophotometer has an adjustable exit slit which can be used a t small openings when the instrument is equipped with a photomultiplier tube receiver. This exit light beam, however, is divergent to the extent of about 2’ and its width does not decrease proportionately with a decrease in the exit slit width. A light beam in the visible range using exit slit widths of about 0.1 mm. had a width of 2 to 3 mm. in the middle of the cell compartment, with a light intensity varying horizontally over the beam. For determination of absorbance a t different positions in the diffusion cell, the width and intensity distribution over the light beam must be established to permit observations to be properly corrected. The exit light beam was made narrower by the installation of a second slit in the aluminum plate of the cell compartment. The cell holder is set in from the
Figure 4.
L,’”-l
Slit holder and slit
lower part of the carrier, leaving space a t the incident light side for another slit which can protrude slightly into the cell compartment. Figure 4 illustrates the second slit holder, F , and the type of diaphragm, E. The aluminum holder is inserted into the entrance hole of the aluminum plate of the cell compartment and aligned with the slit vertically by two pegs, G. The collar, L , of the slit holder is set in flush with the aluminum plate on the side of the incident light by lathing out the plate to a depth of inch centrally around the 1-inch opening. Holes to accommodate the two pegs in the collar of the slit holder are also provided. The slit was cut into diaphragm E by a thin
Such measurements were made after each refocusing of the hydrogen lamp light source, as well as with each set of diffusion measurements. Besides showing the light intensity of the incident beam over the slit opening, they gave accurate estimation of the width of the light beam passing through the diffusion cell. By comparing this estimation with the width of the second slit and the standard slit block slit as measured with a micrometer slide, and considering the slight divergence of the beam, the parallel alignment of the standard slit block and the diaphragm slit can be checked. Such a control is important t o assure that the light beam is perpendicular to the diffusion direction. Absorbance values obtained as described can be corrected for finite width of the light beam and for linear variation in intensity of the light across the beam according to procedures described elsewhere. John Sundling constructed the scanning device now in use.
Induction-Heated Ebulliometer Vernon A. Zeitler’ and Charles A. Brown,2 Department of Chemistry, Western Reserve University, Cleveland 6, Ohio
of a series of new inP organic compounds of moderately high molecular weights necessitated REPARATION
construction of a suitable apparatus for determination of molecular weights. The first Fompound prepared, tetrakistriphenylsiloxytitanium, [(CsHs)sSiOl4Ti,had such low solubility in the usual solvents a t the freening point that cryoscopic methods failed. The Rast method could not be used because this compound reacted with camphor. As the compound demonstrated some solubility in boiling ben-
Present address, Central Laboratory, T~~~~~ i v i ~The i ~ D~~ ~ , Chemical co., Free ort, Tex. * #resent address, Chemical Products Plant, General Electric CO.,Cleveland, Ohio. 1904
ANALYTICAL CHEMISTRY
zene and toluene, ebullioscopic measurements seemed promising. A sensitive vacuum-jacketed ebulliometer using Menzie-Wright differential vapor pressure thermometers has been described ( 3 ) . The internal heater of platinum wire requires several metalmetal and metal-glass seals. Replacement of the heating element \\-auld be a major repair job. The use of a ground-glass plug containing the heating element has been suggested (1). As this plug fits into the bottom of the solution well, two disadvantages are apparent: A tight seal must be secured without the aid of soluble lubricants; the plug provides an additional heat leak through the vacuum jacket. The vacuum-jacketed ebulliometer described (Figure 1) is similar to that of
Kitson, but modified to provide for inductive heating. An iron ring under the Cottrell pump serves as the secondary for a radio-frequency generator and provides heat for the solution in the ebulliometer. Induction heating eliminates the need for seals in the electrical heater circuit, It does not introduce a leak for either solution or heat. It permits thorough cleaning of the boiling chamber without possible damage to the heating element. The radio-frequency generator (Figure 2) is an adaptation of a Tesla coil (4). The primary coil of the generator is wound on a form constructed from 1/8-inch Lucite and has an inner diameter of 70 mm. This coil form consists of four coaxial Lucite rings supported by six vertical strips notched to hold 33 turns of No. 12 wire. These strips are made from three pieces of Lucite (1 x 6 inches), clamped
together, with 33 holes bored inch apart with a No. 36 drill. The three strips are cut lengthwise through the holes to provide the six strips with correctly spaced notches. This modification is the only change in Robberson’s radio-frequency generator. The sample is added through the opening to the side arm closed by cap A . Reflux of the solvent in the side arm dissolves the sample without disturbing the Menzie-Wright thermometer (E-G). Water at 35’ from a constant temperature bath circulates through the two condensers. Hose connections (two not shown) to these condensers are provided by ball and socket joints, D, reducing the danger of breakage and permitting rapid disassembly of the apparatus. The Cottrell pump, H , is ground to a slip-fit. The pump rests on an iron ring, J , which serves as the secondary of the induction heater. I n the apparatus of Kitson the pump is an integral pnrt and cannot be removed. -1 64-mm. vacuum jacket, K , surrounds the lower portion of the ebulliometer and just slips into the primary coil, L; 70 mm. in inside diameter (Figures 1 and 2). A benzene or toluene bfenzie-Wright differential vapor pressure thermometer ( 2 ) is used. The Tis? of liquid in the capillary is measured
Figure 1. heater
by a cathetometer. The three-way stopcock of the ebulliometer is connected to atmospheric pressure through appropriate drying tubes. A well, 2 cm. deep, is attached to the bottom of the inner jacket of the ebulliometer. J , a bicycle bearing set, rests on the bottom of the inner jacket. The sharp points which hold the balls in the bearing prevent superheating of the solution by providing sites for the formation of small vapor bubbles. The stem of the pump unit extends down through the inner hole of the bearing set and returns the drippings from the thermometer to the sump below the iron ring. This permits mixing and complete circulation of the solution. The pump unit is formed by flaring a piece of 7- or 8-mm. tubing until the flare is perpendicular to the tubing and larger in diameter than the inner jacket of the ebulliometer. Four holes are formed by heating with a small flame and piercing the hot spot with a sharp carbon rod. The pump arms, prepared in advance, are then attached. The oversize flare of the pump unit is then ground to slip into the inner jacket of the ebulliometer. The pump unit rests directly on the iron ring. The inner jacket is slightly tapered, with its smallest diameter a t the bottom. This
prevents the pump unit from sticking in the upper portion of the ebulliometer while being inserted. The inner shield, F, has four short legs (not shown) attached to the bottom edge. When the inner shield is in position, the legs rest on the flat floor of the pump unit equidistant between the arms of the pump. These legs are made so that the inner shield just clears the top of the pump arms and prevents the weight of the inner shield from damaging these arms. The condensate from the upper portion of the ebulliometer flows down on the outside of the inner shield and does not mix with the ascending vapors. The boiling solution is driven up through the pump arms and sprays the lower bulb of the thermometer with boiling solution and vapor. The upper bulb of the differential vapor pressure thermometer is in contact with the hot solvent vapor. Operation of the ebulliometer is as described by Kitson. Pure solvent, 5.5 ml., is pipetted by hypodermic syringe into the side arm. The radiofrequency generator and the circulating pump for the condensers are turned on. The equilibrium state for the apparatus is determined when the vapor pressure thermometer stabilizes a t the lowest capillary height, usually requiring about 2 hours. A sample of a compound of known molecular weight is added, and the increase in height of the thermometer liquid is determined. Approximately 20 minutes are required for the instrument to re-establish equilibrium after the last portion of the pill dissolves. Then a sample of the unknown is added, and the same procedure is followed to determine the change in the height of the capillary thread. Each sample of unknown is bracketed by runs with samples of material of known molecular weight. The unknowns and standards are pelleted in a pill press. Pellets range in mass from 8 to 25 mg. The molecular weight is calculated according to the equation:
Vacuum-jacketed ebulliometer and induction
A . 19/38cap B. 40/50 cap C. Vents to drying tube D. 18/9 ball and socket joint to condenser E. Upper bulb of MenzieWright thermometer F. Inner shield G. Lower bulb of Menzie-Wright thermometer H . Cottrell pump J . Iron ring secondary of induction heater K . Vacuum jacket L. Primary of induction heater $1. 811-A vacuum tube N . CI mica capacitor 0. RF generator case
Figure 2.
Radio-frequency generator
L1. RF transformer primary (33 turns No. 12
enameled wire, tapped at 11th turn) C1. CZ,700-mmf., 5000 volt-peak, mica capacitors Cs. 500-mmf. 5000 volt-peak, ceramic capacitor c4. 5000-mm!., 5000 volt-peak, mica capacitor Ri. 2300-ohm, aO-watt, wire-wound resistor Ti. 6.3-volt, 4-ampere, 2500-volt insulation, 115-volt primary, filament transformer T2. 750-0-750-volt, 250-ma., 115-volt primary late transformer PST toggle switches S1,SZ. V,. 811-A vacuum tube G. Ground clip
B
VOL. 29, NO. 12, DECEMBER 1957
1905
Table I, Typical Molecular Weight Determinations
where Mu and M,refer to the molecular weights of unknown and the standard, w,, and w , t o the weights of the unknown and standard samples, and Ah to changes in height of the thermometer thread. The accuracy of the instrument was checked by determining the molecular weight of hexaphenyldisiloxane (melting point 224-.2O, literature value 222-5 O), using triphenylsilanol as the standard. Two determinations of the molecular weight of triphenylbutoxysilane with hexaphenyldisiloxane and triphenylmethane as standards cross-checked the accuracy (Table I). Consistent molecular weight values viere determined for compounds having molecular weights from 194 to 2317. The measured molecular weight of tetrakistriphenylsiloxytitanium n-as consistently higher than the theoretical value for a monomer. Whether this is a real effect or merely experimental error has not been determined. The average difference from the theoretical molecular weight of the compounds listed in Table I is 395. When the compounds possess reasonable solubility, the average difference from theoretical is less than 1.5%. This method of heating the ebulliometer yielded experimental results compar-
Compound
siloxytitanium, [(C6H6)3Si0]4Ti Tetrakistriphenylsiloxysilane, [(C6H6)3 Si0 1,Si 16-Ph&yloctasiloxyspire( 9.9)titanate, Ti [06Sia(C6H&)8 12 Condensed butoxvtitanium polymer o-phenvlenediamine-
Molecular Weights Found Calcd. 534.8 526, 528
Reference Standard Triphenylsilanol
