416 ILLUSTRATIVE EXPERIMENTS 



so that R^Rz = Rs/Rt, then no current will flow through the lead as shown 

 by the indicator (G), a galvanometer, lamp, telephone receiver, etc. Thus 

 R v R 2 and R 3 being known, R can be found. In practice, the lead fixed 

 between R 3 and R terminates in a jockey-piece which slides along a 

 platinum, iridium, or nickelin wire of uniform resistance and a metre long, 

 constituting R lt R 2 . The length of R l is read from an underlying scale 

 when G indicates that no current is passing. 



Owing to polarisation occurring at the electrodes when a steady current 

 is passed through an electrolyte, it is necessary to employ an alternating 

 current which renders a galvanometer useless as an indicator. The alter- 

 nating current is usually got from the secondary terminals of an induction 

 coil. As an indicator, one may use a telephone receiver or a small A.C. 

 pea lamp. 



Expt. (a). Rough Demonstration. Fit up a Wheatstone bridge, using a 

 4-volt accumulator with switch as b, and a pea lamp as G. R 3 is a resistance 

 box and R^ or x is a simple type of conductivity cell, e.g. a beaker contai tii ug 

 the solution to be tested with two sheet-silver electrodes. Find the point 

 on the metre wire when the light from the lamp is at its minimum = x. 

 Then conductance 



c = 



and the molecular conductivity is equal to C<f>, where </> is the volume in 

 c.c. in which 1 mole is dissolved. Suitable solutions =1/16 molar hydro- 

 chloric, acetic, and benzoic acids ; 1/8 molar NaCl and glucose. (N.B. 

 Keep the switch from b open as much as possible to prevent polarisation 

 of the electrodes of x.) 



(b) With the same apparatus the neutral point of a titration may be 

 determined. Place 20 c.c. of N/50NaOH in the conductivity cell and 

 arrange the resistance box (R s ) so that the bridge reading is about 50 cm. 

 From a burette run in a standard (N/50) solution of H 2 S0 4 and mix as in 

 titration, determining the point of balance on the wire after each addition. 

 It will be found that the balance point will first tend towards the zero end 

 of the scale and later will move in the reverse direction. The point at which 

 it changes direction is the electrotitrametric neutral. 



For any but very rough readings, many precautions have to be observed. 

 These will be found in any book on practical physical chemistry. Instead 

 of a lamp, the capillary electrometer may be used as an indicator. 



40. (i) Buffer Solution. S^rensen. (a) M/15 solution of KH 2 P0 4 

 (9-078 grams per litre). The salt should be chemically pure, and its solution 

 should be water claar and free from even traces of chloride and sulphate. 



(b) M/15 solution of Na 2 HP0 4 . 2H 2 (11-876 grams per litre). The 

 salt is prepared by exposing to the air the recrystallised Na 2 HP0 4 . 12H 2 0. 

 After two weeks' exposure on a porous plate, dissolve the salt in water and 

 test for chloride and sulphate. 8 c.c. of (a) KH 2 P0 4 + 2 c.c. of (b) Na 2 HP0 4 

 2H 2 gives a solution of ptt 7-381. 



(i) To demonstrate the acid-combining power of a " buffered " solution. 

 Take 50 c.c. of the above mixture or prepare a solution containing about 

 0-25 per cent, sodium bicarbonate. Place this in a tall cylinder and in 

 another similar vessel place the same quantity of water. Add about 

 20 drops of neutral red (0-05 per cent, in alcohol) to each. Add N/10 NaOH 



