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Introduction & Theory
For a fluid in which small rigid spheres are uniformly distributed at low concentrations, the viscosity is related to the viscosity of the pure fluid 0 and the ratio of the total volume of the spheres v to the total volume (V) by
The single bonds of polyvinyl alcohol (PVOH) allow its molecules to fold in on themselves to form string that meanders randomly within a roughly spherical shape. The length of the molecule will then determine the size of the effective sphere, and the length is directly related to the molecular weight. As a result, the following relationship exists between intrinsic viscosity and molecular weight:
The goal of this experiment is to use measurements of the intrinsic viscosity of PVOH to determine its average molecular weight.
Ostwald viscosimeter; four volumetric flasks (two 100 mL and two 250 mL); two pipettes (one 10 mL and one 50 mL); pipette bulb; 250 mL glass stoppered flask; three beakers (two 250 mL and one 100 mL); stirring rod; bunsen burner; tripod stand; wire gauze; 0 to 100C thermometer; rubber tubing. If the polymer solution must be prepared, a weighing bottle, funnel, and Pyrex wool will also be needed.
Glass walled thermostat bath at 25C; polyvinyl alcohol (M.W. ~60 000 to 80 000; 5 g of solid or 200 mL of solution containing 18 g L-1); 1 g KIO4; 50 mL cleaning solution (chromic acid or nitric+sulfuric acid); and a Westphal balance.
The flow rate of pure solvents and PVOH in solvent through a capillary will be measured to determine the viscosity and the intrinsic viscosity of the compounds.
The coefficient of viscosity will be measured by determination of the time t required for a given volume V of liquid of density through a vertical capillary. For an incompressible liquid, can be determined by
The viscosimeter must be thoroughly cleaned with chromic acid, 50/50 nitric acid/sulfuric acid solution, or equivalent. After rinsing with distilled water, dry with acetone and allow to equilibrate with a 25C temperature controlled bath. In addition, add a small flask of distilled water to the bath and let sit for at least 10 minutes.
If it is necessary to prepare a solution of PVOH, the following method should be used. Between 4.0 and 4.5 g of dry polymer should be gently stirred into about 200 mL of hot distilled water in a beaker. In this and all steps of this experiment, foaming must be avoided and glassware cleaned promptly as it can be very difficult once the solution has dried. After dissolving the powder, filter the solution through Pyrex wool if insoluble particles remain. Transfer the solution to a 250 mL volumetric flask and fill to the mark with distilled water. Gently invert the flask several times to mix, being careful to avoid foaming. If possible, calibrate the viscosimeter while this procedure is being carried out to save time.
Pipette 50 mL of the PVOH solution into a 100 mL volumetric flask and fill to the mark with distilled water. Place this flask (containing uncleaved PVOH) in the bath to equilibrate.
Pipette 50 mL of the stock solution into a 250 mL flask. Add 0.25g of solid KIO4 and up to 25 mL of distilled water and warm to 70C. Stir until the salt is completely dissolved, then cool the mixture (containing cleaved PVOH) in the bath until it equilibrates.
Both solutions should additionally be diluted 50%, and if time permits be diluted again to 25% of the original concentration.
With a viscosimeter immersed in a water bath of 25±0.1C, begin by pipetting the required amount of liquid into the apparatus. The pipette should then immediately be rinsed and dried with acetone and air. The solution should then be drawn up above the upper fiducial mark and then release the suction. The flow time between the upper and lower fiducial marks should be measured with a stopwatch. Repeat this with each solution until three readings within about 1 percent of each other are obtained. Thoroughly rinse and clean the viscosimeter immediately after use.
If possible, measure the densities of the solutions should be determined, otherwise they may be assumed to have the same density of the pure solvent.
Ensure that the solutions are not allowed to dry in the capillary tube and that acetone is not introduced until all traces of the polymer have been removed. Otherwise, residue may form, changing the size and coefficient of friction inside the capillary tube.
Foaming must be avoided in order to make accurate volume determinations. Bubble on the surface of the liquid make measurements difficult and the dilutions inaccurate.
By solving eq(4) for the empirical parameter B and substituting a known value for the viscosity of water at 25C, the instrument may be calibrated.
Many solvents in a lab are highly flammable. Be sure to keep fumes away from lit burners and other sources of ignition. The vapors of solvents are generally heavier than air; keep burners at a reasonable distance from where vapors may collect.
Mercury and mercury vapor are highly poisonous. Inhalation of vapor may lead to fever, nausea, vomiting, diarrhea, headache, chest pain, and possibly death. Skin contact may lead to a rash or allergic reaction, and if extensive may also cause the same effects as inhalation of vapor. Affected persons should be removed to fresh air and contaminated clothing removed. Necessary first aid techniques should be performed. Seek medical attention immediately.
Polyvinyl alcohol is a mild skin and respiratory tract irritant, as well as a fire hazard. Keep fumes away from burner. Fumes may cause asphyxiation if allowed to displace oxygen.
Sulfuric acid and nitric acids are strong corrosives; avoid contact with liquid and inhalation of vapors. Both acids react violently with water. In case of contact, remove affected clothing and flush area with copious amounts of water. In case of contact with eyes, seek medical attention and flush with water immediately. If high concentrations or large amounts of acid are to be handled, do so under a fume hood while wearing protective gloves.
Chromic acid is a known carcinogen and is highly toxic. Avoid direct contact with chromic acid in any form. Contact with chromic acid can cause system toxicity. As with all oxidizing agents, heating of chromic acid in the presence of combustible materials poses a serious fire and explosion hazard.
Potassium periodate is an oxidizing agent. Avoid contact with skin and inhalation of dust. Contact can cause irritation of skin and mucous membranes.
1. Safety data taken from the internet at "http://hazard.com/msds", "http://http://www.enviro-net.com/technical/msds/", "http://www-chem.ucsd.edu/Courses/CoursePages/Uglabs/MSDS/".
2. D. P. Shoemaker, C. W. Garland, J. W. Nibler, Experiments in Physical Chemistry, 6th ed., chapter XII, The McGraw-Hill Companies (1996).