Question 8.2: Estimate the viscosity of toluene at 20ºC.

Toluene

Table 8.1. Contributions for calculating the viscosity constant I in Souders’ equation
Atom	H	O	C	N	Cl	Br	I
Contribution	+2.7	+29.7	+50.2	+37.0	+60	+79	+110
Contributions of groups and bonds
Double bond			-15.5
Five-member ring			-24
Six-member ring			-21
Side groups on a
six-member ring:
Molecular weight < 17			-9
Molecular weight > 16			-17
Ortho or para position			+3
Meta position			-1
				OH			+57.1
				COO			+90
				COOH			+104.4
				NO _{2}			+80

\dagger X \text { is a negative group }.

Contributions from Table 8.1:

7 carbon atoms 7 \times 50.2=351.4

8 hydrogen atoms 8 \times 27=216

3 double bonds 3(-15.5) = -46.5

1 six-membered ring -21.1

1 side group

Total, I = 296.4

Density at 20^{\circ} C = 866 kg / m ^{3}

Molecular weight 92

experimental value, 0.6 cp =0.6 mNs / m ^{2}

Author’s note: the fit obtained in this example is rather fortuitous, the usual accuracy of the method for organic liquids is around \pm 10 per cent.

Variation with temperature

If the viscosity is known at a particular temperature, the value at another temperature can be estimated with reasonable accuracy (within \pm 20 per cent) by using the generalised

plot of Lewis and Squires (1934), Figure 8.1. The scale of the temperature ordinate is obtained by plotting the known value, as illustrated in Example 8.3.