522 



ILLUSTRA TI VE EXPERIMENTS 



22. Strength of Acids. 



(a) By Taste. Prepare a imniher of N/'lO solutions of acids, e.g. hydro- 

 chloric, acetic and boric acids. JJry the side of the tongue and apply a small 

 quantity of the acids in turn with a camel's-hair brush, rinsing the mouth 

 out after each application. Place them in their order of sourness. Dilute 

 each of them ten times and repeat the experiment. 



(6) By hydrolysing poiver. Take three test tubes and put 2 c.c. of a 0-5 

 M solution of cane sugar in each. Add to each respectively 5 c.c. of one of 

 the acids prepared above, e.g. N'lOHCl. N/IOHA and N/IOHBO.,, and place 

 all three in boiling water at the same time. Leave them there for IJ- to 

 2 minutes. Remove all together and cool. Add 5 c.c. N/lONaOH to each 

 tube, mix and then add 5 c.c. of Benedicf\s qualitative sugar reagent. Boil 

 and com[)are the amount of copper reduction in each tube. 



(c) By indicators, (a) Prepare a series of graded concentrations of a 

 strong acid like hydrochloric and a similar series of a weak acid such as 

 acetic, e.g. : 



Take seven test tubes, and in every tube except the first put 9 c.c. of dis- 



FiG. 107. — Diagram of conductivity apparatus. 



filled water. In the first tube put 10 c.c. of N acid. Transfer 1 c.c. of this 

 to the second tube, mix and transfer 1 c.c. of the N/10 acid so prepared to the 

 third tube, and so on, rejecting the 1 c.c. removed from the last tube. You 

 will then have a series of tubes containing 9 c.c. of (1) N, (2) N/10, (3) N/KM), 

 (4) N/1,000, (5) N/10,000, (6) N/100,000. (7) N/1,000,000 acid. 



To each tube add a few drops of B.D.H. " Universal " indicator, mix and 

 compare the colours produced with those of the standard labelled tubes 

 provided. Why does tube 3, which gives a reading of about _/jH 2 with 

 hydrochloric acid, only give a pH of between 3 and 4 when acetic acid is 

 used ? 



(^) Determine the same range more accurately, using thyniol blue {pK 

 1 •2-2-8), bro)iio-phei)ol blue (2-8-4-6), and methyl red (4 •2-6-3) in turn as 

 indicators. 



(d) By conductivity. Comparison of the electrical conductivity of equi- 

 molecular solutions of mineral and organic acids. Conductivity, being the 

 reciprocal of resistance, obviously can be measured by a resistance method, e.g. 

 by the Wheatstone bridge (Fig. 107). The current from b is divided between 

 two circuits (1) by B^ and R.^ and (2) by i?3 and R^ (where R = resistance in 

 ohms). The amount of current travelling by these circuits is such that the 

 drop of potential in both is the same. If then a lead be taken from the 



