STRENGTH OF ACIDS 523 



juiictioii hetween R^ and R., mid couiioction iiiailc with ttic //.,, /i*, route 

 so that R^IR., = JisJJ^A, then im curroiit will How throiiiih the lead as shown 

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

 i?j, R2 and R.^ being known, R^ can be found. In |)ractice, the lead fixed 

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

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

 constituting R^, R.^- The length of R^ 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 em[)loy 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. 



Experiment (a). Rough Demonstration. Fit up a Wheatstone bridge, 

 using a 4-volt accumulator with switch as h, and a pea lamp as G. R^ is a 

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

 containing 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 



i?3(100 - x)' 



and the molecular conductivity is equal to C(j), where ^ is the volume in 

 cubic centimetres in which 1 mole is dissolved. Suitable solutions = 1/16 

 molar hydrochloric, acetic, and benzoic acids ; 1/8 molar NaCl and glucose. 

 {N .B. — Keep the switch from h open as much as possible to prevent polarisa- 

 tion of the electrodes of x.) 



(|8) 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^) so that the bridge reading is about 50 cm. 

 From a burette run in a standard (N/50) solution of HgSOj 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 w^ill move in the reverse direction. The point at w^hich 

 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. 



23. Principle of Measurement of H-ion Concentration by Potentiometer. 



The principle is much the same as that of the conductivity measurements 

 (Fig. 107), only instead of having a single source of potential and an unknown 

 resistance, one has known resistances and tw'o sources of E.M.F., one of 

 which is of unknown value. 



In Fig. 107, 6 may be taken as a cell of standard E.M.F. sending its current 

 through the wire bridge, R^ — i?._,. If we lead into i?j — R.^ the wires from 

 another battery x, taking care that the direction of the difference of potential 

 is the same, e.g. both negative poles leading to 7?j, as the fall of potential 

 along R.^ — i?2 is regular, we can readily divide the wire so that the ditTerence 

 of potential between the ])oint of entrance and exit of the current from x 

 is equal and opposite to the difference of potential between the same points 

 caused by the standard cell, i.e. the galvanometer or electrometer will indicate 

 no E.F.D. The cell x is of interest. It consists of two half-cells or electrodes. 

 One of these is the ordinary standard calomel electrode, i.e. an electrode of 



