Richard A. Dursf Boston College Chestnut
Hil!, Massachusetts
Porous Glass Salt Bridges
Altliougll witlely used by nlany c~cctrorhcmists and clectro>~nalyticalchemists for a number of years, the pornus glass salt bridge has not been employed extensively in other areas where its convenience, ruggedness, arid reliability make it especially useful. In secondary school and undergraduate college teaching, the agar-gel salt bridge is still the universally used conncct~ion between electrochemical half-cells. One rannot deny its effimcy in this respect, but in most cases the porous ghss salt bridge works cqrlally well while in numerons olher applications it is superior in many ways to tho conventional bridges. Porous glass salt bridges arc made from Corning Code 7930 Porous Glass' which is the unfused form of the 7900 Vycor glass. Its chemical composition is 96% silica (SiO?) with the remainder mostly boric oxide (B20r). This glass can be obtained as tubing, rods, and flat plates, or blown on request into any of the standard glass fornls. Since the porous glass shatters when placed in a flame, special shapes must he fabricated prior t,o the leaching step in its production. The average pore diameter is 40 Angstrom units, nud 28y0 of its total volnme is void space. The large void space allows a large amount of the electrolyte solution to be contained in the glass, while the very sm:ill pore size prwentz significant flow of solution 'Coming Pmdnrt Inforrnslrion Brochure IC-21 (R/R/60), Corning G I m Works, Corning, New York.
(flow rate is 0.65 p1 of water per sq. cm area per atmosr>here iwessnre ner hr throueh a 2 mm thickness of the &s). ' Although the salt bridge designs in which the porous glass can bc used are too numerous to be treated in detail, two of the most common types are shown in Figure 1. These bridges are connected between the half-cells in the same may as the conventional agartype bridges. Bridge l3 in Figure 1 consists of a short 1)iece of the porous Vycor rod inserted into the end of a length of Tygon tubing. The fit between the glass
-
Figure 1. Porous glass salt bridge.; a, electrolyte d u t i o n ; b, Tygon tubing '/1 in. id; c, porous Vycor (A, tubing 7 mm od; B, rod 7 mm od, -7 mm long); d. Pyrex rod plug 7 mm od
Volume 43, Number 8, August 1966
/
437
and tubing must he very tight to prevent excessive electrolyte flow along the side of the porous rod. Bridge A is somewhat more difficult to construct and requires a larger volume of sample solution to make proper contact. However, the area of contact of the pomus glass with the solution is determined by the length of the glass tubing used (not by its diameter as with the rod), and its wall thickness is less than the normal rod length used in the B-type hridge (5 to 8 mm), therefore the resistance of the A-type bridge can be made considerably smaller. The bridges can be either a single unit made by closing the other end of the tubing with the porous glass, or it may be left open and connected directly to one of the half-cells, e.g., as is often the case for a reference half-cell. The reference half-cell designs can be as varied as the bridge types, and Figure 2 shows a very convenient self-contained, dip-type saturated calomel electrode.
Figure 2. Porous gloss ratvrotcd colomel refarencs electrode; o, soturoted KCl; b, colomel (HgFI1l; r, mercury; d, PI wire contact; e, Pyrex tubing sealed into test tube bottom; f, Tygon tubing; g, porous Vycor rod.
438
/
Journd o f Chemical Educofion
The advantages of the porous Vycor salt hridge over the agar-gel type may be summarized as follows: (1) more chemically and electrically inert; (2) easier to construct (no weighing, mixing, heating, or gelling time required), thereby allowing more time for the actual experimental work; (3) can contain nonaqueous electrolyte solutions for studies of nonaqueous systems; (4) can use any electrolyte solution (agar does not solidify well in some electrolytes); (5) not affected by elevated temperatures (agar gel would redissolve); (6) more durable than conventional bridges (storage in a saturated solution of the electrolyte permits use for several months with little or no change in characteristics). When discussing the merits of any salt hridge, its electrical resistance is of considerable importance, since the iR drop introduced is a factor which must eventually be corrected for in any careful electrochemical study. The resistance of the 7 mm od porous Vycor glass rod containing the saturated KC1 electrolyte is on the order of 10 ohms/mm. Consequently, the resistance per unit length of porous glass containing a saturated KC1 solution is greater than a 1% agar gel saturated with KC1, which in turn is greater than a saturated KC1 solution (all of equal cross sectional area). However, since most of the length of a porous glass bridge is made up of the lower resistance saturated KC1 solution, the contribution of the porous glass to the total resisb ance becomes less significant especially as the total bridge length increases. For bridges less than a foot in length, the resistances of the porous glass and agar-gel bridges are comparable, while for longer bridges, the porous glass type is of significantly lower resistance. In spite of its slightly higher resistance, the porous glass hridge would also be the practical choice for very short salt bridges, such as those shown in Figure 2, because of its superior mechanical properties. The above considerations would seem to indicate that the convenience and highly desirable mechanical, chemical, and electrical properties should make the porous glass salt hridge very popular in both teaching and research laboratories.
