COLOR CHANGE OF CONGO RED
I277
The glass tubes fitted as described were kept in a glass thermostat a t the required temperature within *O.IOC. The small inner tubes were cut off a t an angle of 45' so that when the tubes were inverted and shaken, the Congo Red and acid solutions were rapidly mixed. The time taken t o the development of the first clear blue colour, a very marked step in the colour change was measured by means of a stop watch. Suitable standard tints were made up withBrom Thymol Blue and Brom Cresol Purple in a buffer solution of pH7.2. Results. The experimental results are given in series which show: I. The influence of hydrogen ion concentration and composition of the solvent when the Congo Red concentrattion is constant; 11. The effect of change in Congo Red concentration when the hydrogen ion concentration and composition of the solvent are kept constant. The values for a the degree of dissociation of hydrochloric acid in the acetone water mixtures are taken from a paper by Brownson and Cray.' SERIESI The Influence of Hydrogen Ion Concentration andComposition of the Solvent when the Congo Red Concentration is constant (a) Congo Red concentration in reaction tube:-2.7 x IO+ gram mols per litre. Temperature 25' C. 10 uolumes of water in 100 20 volumes of water i n I00 acetone water. acetone water. ~
~
Cone. of a From €IC1 l\Iols/ Condue[HI. Litre. tivitv Data 0.204X10-~ .70 o.142X10-~ 7 . 5 0.163 .73 O.IT9 11.; 0. I 2 2 ,775 0,094 23 0.I12 .79 0.088 33 0. I02 .80 0.082 44 0.092 .82 0,075 61 0.081 .83 0.067 104 0.071 .84 0.060 152
I
Cone. of CY From HCl Mols/ ConducLitre. tivit y Data 2.98X10-* .621 2.76 .630 2.55 ,640 2.34 .65 2.13 ,661 ,674 I .92 ,687 I . 70 ,702 1.49
SO volumes of water in 100
6.01X10-* 5.59 5.16 4.74 4.31 3.89 3.46 J. Chem. SOC.,127,
2 9 2 3 (1925).
I . ~ ; X I O - ? 9.; 1.71 1.63
13.5 I9 26 36
I.52
1.41 1.29 1.17
I21
1.0;
2 60
57
acetone water.
a: From
Conduc[HI, tivity Data. ,70 4.21X10-* ,707 3'95 ,713 3.68 ,721 3.42 .732 3.15 ,741 2.88 ,751 2.60
SW?.
35 volumes of water i n 100
acetone water. Conr. of HC1 NOIS/ Litre.
'r
[Hl'
Srcs. 11
15
I9 33
55 91 I j0
Cone. of HCl Mols/ Litre.
From Conductivity Data.
7.30X10-* 6.25 5.73
,745 ,758 ,766 ,772
5.44X10-* 4.74 4.39
I3
4.02
21
,782 ,791
3.67 3.30 2.93
34 61
5.21
4.69 4. I7 3.65
a:
.802
(HI'
T
Sees.
7 IO
I35
S E K I CI ~(C'ont inuerl I
'I'
,
