A Simple and Inexpensive Student Viscometer L. G. Daignaultl, D. C. Jackman, and D. P. Rillema University of North Carolina at Charlotte. Charlotte, NC 28223 We have assembled a simple Ostwald type of viscometer for use in hiah schools and college or university general chemistry lahoratorier that can be constructed from readily availablechemistry stockroom materials. As dbown in F~gure 1, the apparatus consists of two drying tubes (a 100-mm and a200-mm tubeand hulb) connected by approximately 12-15 rubber tuhine. in. - ~of '1.-in.-i.d. ." Inside the rubber tubing is a 3-in.-long piece of 1.0-mm-i.d., 0.26-in.-0.d. capillary tuhing. Since the outside diameter of the capillary tuhing is larger than the inside diameter of the rubher tubing, thk capillary tubing fits snuggly inside the rubber tuhing. In our heginning laboratories we used gum rubber tubing and nonhazardous solvent systems such as simple alcohols, glycol, and water. Since the contact time hetween the rubber tuhing and the samples is only a matter of a few minutes, we did not notice any deterioration of the rubber tuhing or discoloration of the liquid. The use of Tygon tuhing was tried but was rejected due to the increased difficulty the students had attaching the Tygon tubing to the drying tuhes and capillary tuhing, which, in this case, was inserted in series. This simple viscometer is capable of demonstrating to the student the basic principles of the measurement of viscosity. This type of viscometer operates on the principle that the time required for a given volume of a liquid to pass through a small capillary opening is directly proportional to its viscosity. The relationship for this proportionality is given by
Typical Student Data hom Slmple Vlscometer Viscosity (cp).
Measured
Densw
Sample
Time
g/mL
Calculated
EManoi(standard liquid)
22
0.789
1.1212
Literamre 1.1212
22 22
~
Elhylene glywl
180 171
1.113
12.9 12.3
15.0b
,
= Apt
where a is the viscosity of the liquid in centipoises, A is a constant for a aiven viscometer, p is the density of the liquid ing/mL and 1 is the time for a given volume of liquid to flow past any cross section." The value of the constant A was found a liouid of known viscosity (see the table). -. ~ ~hv -usine - "~ ~. The operation of this piece of equipment is very simple. (1)After assemblv of the a ~ p a r a t u sthe , student makes two marks above and below thebulb of the 100-mm drying tuhe, using a felt-tip glass-marking pen, as shown in Figure 1. A ~A~
0
2
4
6
8
1
0
1
2
Volume. mL Flgwe 2. Typical welghNvolurne plat used lo measure the density of a liquid. reference mark is also made ahove the 200-mm tube about halfway between the bulb and the end of the tube. (2) The student holds the two tuhes in the vertical position in one hand with the bulb of the 200-mm tube lower than the bulb of the 100-mmtube and fills the 200-mm tuhe with the liquid
' Author to whom conespondence should be addressed.
Figure 1. Schematic of a simple inexpensive vismmeter.
Daniel. F.; Williams.J. W.: Bender. P.: Alberly, R. A,; Harrlman. J. F. Emer;mntalPhvsicalChe~slry. 7th ed.; McGraw-Hill: New York. 1970; p 159. Volume 67 Number 1 January 1990
81
to he tested so that the bulh is just a little over half full. (3) The student then holds one tuhe in each hand and lowers the 100-mm tuhe, filling i t with the liquid to a point above the top mark. (4) The reference line on the barrel of the 200-mm tuhe and the top mark on the 100-mm tuhe are then leveled. (5) The leveling procedure causes the fluid to flow back into the 200-mm tube. The time is recorded for the liquid to flow between the two marks on the smaller tuhe. The procedure can he repeated as many times as desired. To change from one liquid system to the next the apparatus is merely rinsed, usually twice, with a small amount of the next liquid to be tested. The density of the liquid can he found in a standard handbook or can be measured in a number of ways. One experimental method suggested here is to use a buret, an Erlenmeyer flask, and halance for collecting data needed for Private communication with Joe T. Echols. Pfeiffer College, Misenheimer, NC.
82
Journal of Chemical Education
a weightlvolume plot as shown in Figure 2. Since d = mlu, the dove of the d o t is the densitv of the liauid and the intercept is zero.?'IJsing the measuied times and densities, the viscosity of a liquid can be determined.
Typical Student Results
We tried this amaratus in one of our eeneral chemistrv laboratories using&hanol as our standa;d reference liquk for the evaluation of the constant A. For the amaratus used in this work, the value of the constant A was-found to be 0.0646. Typical results are shown in the table. These data show good reproducihility for the lower viscosity liquids. The more viscous ethylene glycol shows an average time of 175.3 i 4.5 s. which amounts to onlv a 2.6% error. This is relatively good reproducihility for suph a simple instrument. While we have used s i m ~ l enonhazardous liauids. aaueous salt solutions of various bensities would a& work we11 for this experiment.
