Topics in..
.
Chemical Instrumentation Edited by GALEN W. EWING, Seton Hall University, So. O r a n g e ,
feature
N. J. 07079
These articles are intended to serve the readers O ~ T H I SJOURNAL by calling affenlion lo new developments i n the theory, design, or availability of chemical laboratory inslrumenlation, or by presenting useful insights and ezplanations of topics lhat are of practical importance to those who use, or leach /he use of, modern inslrumenlalion and inslrumental techniques. The editm inviles correspondence from prospective conlributors.
LX. Step Perturbation Relaxation Techniques
Figure 13. The Durrum rtopped-flow-T-jump spectrophotometer. (Cowlow Duwum)
Icontinued,
2. A. Schelly, Department of Chemistry, The University of Georgia, Athens, Georgia 30601, a n d E. M. Eyring, Department of Chemistry, University o f Utoh, Solt Lake City, Utah 841 12 Subsequent to same preliminary studies (34, 35) Erman and Hammes (36) developed and successfully used a stoppedflow-T-jump apparatus, that led the Durrum Instrument Corporation to build a T-jump accessory for t,he DmmmGibson stopped-flow instrument (37). The functioning of this apparatus can he described using Fignre 12. Charge is stored in a 1 pF capacitor C a t a potential V , variable from 2 to 5 kV. The capacitor is discharged through a spark gap that is triggered by an SCIL circuit. The discharge through t,he eell is initiated either
permi& the choice of time delay Ah between mixing and discharge. The heating current is terminated hy firing a second triggered spark gap that bypasses the cell and rapidly drains the condenser. This is usually called a crowbar (38). The heating-pulse width is determined by the time delay Atz between briggering the two spark gaps, controlled by a. delay
circuit adjustable from 5 to 100 psec. The function of the shorting switch is to sholt out all disturbing signals for a selecled time interval Ata following the firing of the second spark gap and then to present subsequent signal changes t o the oscilloscope. The principle of the shorting circuit has been discussed by Cserlinski (10, 89). Figure 13 shows the photograph of the complete unit that can be used in the stopped flow or T-jump as well as in the combined mode. The stopped-flow part of the instrument is identical with that dexcribed in the fimt part of this nrticle, with the exception of the sample eell which is shown in Figure 14. The body and the electrode spacers are made of Kel-F and the electrodes, which are usually gold plated, of stainlesv steel. The windows are of quartz. Using this cell the instrument's mixing dead time is 10 msee. With a. sample resistance of 54 ohms, the voltage time ronstant is 54 psec and the heating time constant ro = 27 Fsec. Termination after 10 psec (triggering of the second spark gap) makes use of 31% of the stored energy. If initially the 1 FF condenser was charged up to T, kV, corresponding to 12.5 joules of stored energy, 311, of i t (3.88 joules) can raise the temperature of 200 p1 (that is, the volume of the eell) of water by 4.63'C.
the case) a. sudden 30-1.50 atmosphere jump of the pressure on a sample solution results in a chanee of eauilibrium concent,rstions detectable with sensitive conductornetric techniques, and the rate of re-equilibration can be measured. A much improved version of the first pressure-jump apparatus of Ljunggren and Lrtmm (11) was developed b y Strehlow and Becker (40) (Fig. 15). The jump of pressure is achieved by an abrupt depressurization of a previously pressurized autoclave, in which the sample snd reference cells are located. The depressurisstion is initiated by puncturing a thin metal rupture disc by means of a solenoid-activated steel needle. The pressure in the autoclave initially of the order of 50-60 atmospheres drops to 1 atmasphere within about 50 wet. The autoclave (Fig. 16) is built of stainless steel. In order to minimize the apparatus relaxation time (that is required by t,he eompressed gas to exhaust thmugh the mptured disc via a mufiler), the height of the pressurized gas layer, as well as the distance between the bottom of the conductivity cells and the Kel-F membrane are kept as small as possible. The solution under investigation is put into the miniature glass conductivity cell resting on a nylon base. The identical reference cell filled with a KNOs or KC1 solution provides compensation for ihe temperature
-
The Pressure-Jump Method The dependence of the equilibrium concentrations on pressure is given by the function
n
A,,
I
Figure 12. Schematic diogrom of the signal and control system of the Durrvm ,toppedRow-T-jump opporotus. iCourlesv Durrum)
where AVO is the standard volume change for the reaction in question. If the reaction AVO is large enough (which is seldom Volume
Figure 14. Cross section of the stopped-flowT.iump cell. (Courlerv Durrum)
(Continued on page A696)
48, Number 1 I, November 1971 / A 6 9 5
Chemical Instrumentation
Figure 15. The pressure-jump a p p a r a t u s (after Strehlow and W e n d t 141) and Takahashi ond
Alberty 15511.
drop associnted with t,he adiabatic expansion of the solutimt and of the prcssore transmitting inert liquid (e.g., paraffin ail) that envelops the cells and solutions. The observation is startcd by the ti.i~gering signal of n quarta prcssure transducer (e.g., Kistler No. 603, 01. eqnivalen0 that. senses the drop of presswe in the antaclave. Constant tempet.ature is mnintained by thermostntitig coils noldel.ed to the nutorlavc. The plastic mnlller on the top of the nulorlave allertuates the luud report of the shork waves lo a tolerable level, and the capillary in the lead for the compressed ail. reduces the rate of air flow after the expwiment, thus protecting the manometer from x heavy shock. Changcs of the roneentretiwls of reactants is followed nsiug conductivity measoremenls s t 3 frequency greater than the reciprocal of the shortest time r m stant to he measmed (50-100 kHz). The conductivity cells in the autarlave A form two arms of a n nc Whestslone bridge (Fig. 17) whose off-balance voltage is displayed on an oscilloscope OSC. Sinre stable bl.idge balance must be achieved to betler than I part in 10,000, the nc generator must have low hum, noise, and distortion ratings. Forthel.more, it is important that the oscillator 0 combined with the impedance matching t,rnnsformer Il' he free of overtones which is possihle only for bridge voltages less than 3 V ( 4 1). To reject any power line, broedcxsting, and high frequency noise R. filter P is inserted between the bl.idge xnd the preamplifier P. The triggering signal TS of the transducer amplified hy an amplifier A M P initiates the sweep of the oscillosmpe. Pains must he taken to eliminate stray capacitanc-, thus extensive shielding of the components is necessary. To our knowledge, no commercial Pjump apparatus is available, probably because of the inherent limitations of this technique. Chemical reactions are generally less sensitive to feasible pressure changes than to temperature changes, therefore, the highest possible sensitivity is necessary to delect dinplscements of the equilibrium. This need for sensitivity has been the main reason for t,he use uf (Continued on page Afi08)
A696
/
Journal o f Chemical Education
Chemical Instrumentation
Figure 16. The autoclove (after Strehlow and Beeker 1401 and Takohoshi and Alberty 1551): (01 reference cell, (b) chemicol system in rompls cell, (c) pressure tronrmitting inert liquid, Id1 Kel-F membrane, lei transducer, If) air inlet lglrupture dix, lhl to otcilloscope trigger, (;I to Wheotrtone bridge, li) thermostoting coils, ( k ) nylon gasket, (I) to air tank, (m)stainless steel autoclave body.
conductometric rather than spectrophotometric detection. Other closely related methods of producing jumps using shack waves (@), standing sound waves (45), and eleetromechanical techniques (44) have been reported.
The Electric Field-Pulse Method
relation
where AD* is the difference of partial molar "polarization" of products and reactants (46). This means that if the reartiou iu question involves a net change in the mnmber of ions or a net change in the dipole moments, sn electric field can be used to perturb the system. Generally, fields of the order of 50-100 kV/em mrmt be used to cause observable effects. If the system of interest is a week electrolyte, strong fields enhitnee dissociation. This is enlled the second Wien effect or dissociation-field effect. Onsager has
1 Figure 17. The Wheotrtone Strehlaw and Wendt 141)).
bridge
(Continued on page A700)
(ofter
given a theol.etical del.ivntkm of Lhis behavior based on detailed I
