Page 67 - Essentials of physical chemistry

Viscosity of Laminar Flow 29

Upper
mark

Lower
mark

FIGURE 2.3 A typical Ostwald viscometer. (From Gohel, M., Parikh, R., Popat, A., Mohapatra, A., Barot, B.,
Patel, C., Joshi, H., Sarvaiya, K., Baldaniya, L., Mistry, P., Parejiya, P., Parmar, R., Nagori, S., and Patel, T.,
Pharmaceutical suspensions: A review, Pharmainfo.net, http:==www.pharmainfo.net/free-books=pharmaceutical-
suspensionsa-review).

larger bore viscometers are necessary in order to obtain convenient run times (private communica-
tion from Prof. MacKnight, Figure 2.4). One merely uses a pipette to standardize the amount of the
liquid used and it is drained into a clean viscometer. Then a suction bulb is used to pull the liquid
into the reservoir bulb at the top of one arm of the tube and the time it takes for the reservoir to drain
past the lower mark on that arm is recorded. Multiple runs can be averaged to improve precision for
liquids of low viscosity but for very viscous liquids it is not unusual for run times over 150 s to be
reproducible to within 0.1 s, so precision is very good for liquids of higher viscosity (Table 2.1) [6].

FIGURE 2.4 William J. MacKnight is an American polymer scientist at the University of Massachusetts at
Amherst. He is a member of the National Academy of Engineering and has received the Ford Prize in High
Polymer Physics, the American Chemical Society Award in Polymer Chemistry (Mobil Award), the Distin-
guished Service Award in Advancement of Polymer Science from the Society of Polymer Science, Japan, and
the Herman F. Mark Award from the Division of Polymer Chemistry of the American Chemical Society.