Polymer solutions must be prepared with care. Special effort should be expended to ensure complete, homogeneous dissolution. The best method of obtaining correct concentrations is weighing the polymer and using volumetric flasks for solution volume. The second choice is weighing the polymer, then weighing the solution and using the density of solvent as an approximation for the density of the solution.
Determination of polymer and protein molecular weights requires preparation of solutions with accurately known concentrations. The accuracy of the obtained molecular weights depends directly on concentration. Correct concentration values are also needed for viscometry, calibrating UV and RI detectors, and measuring dn/dc values. Therefore, preparation of solutions with well-known concentrations is necessary for maximizing the performance of Brookhaven equipment. In order to aid the experimentalist, this note covers three topics useful for sample preparation. The first is practical discussion of the mechanics of preparing solutions with volumetric flasks. The second concerns preparing solutions by a purely gravimetric method. Finally, instructions for preparing dilutions of a stock solution are included.
The light scattering relationships (e.g., the Zimm equation) are written in terms of concentration as mass of solute per volume of solution. Therefore, the best method to prepare solutions is to weight a mass of polymer, then add solvent until the desired volume is reached. Alternatively, concentration can be obtained by weighing. In both cases, complete dissolution and mixing is critical to obtaining good results.
In practice, this is implemented in a multi-step process that takes hours, and can take longer for some macromolecules, particularly high molecular weight polymers. We recommend preparing solutions the day before making measurements. An important prerequisite is that the operation and calibration of the scale and glassware be well understood. The manual for your analytical balance is an excellent resource, as is a text on analytical chemistry such as “Quantitative Chemical Analysis” by Daniel Harris, W.H. Freeman, New York, 1999. It should be emphasized that while accurate weighing and volume determination will become routine, waiting and mixing for complete dissolution will continue to be a challenge for each new polymer sample.
There are two methods of preparing a sequence of solutions. The first is to make a stock solution, then dilute the stock solution to make a series of solutions. This method is convenient and fast. However, errors in preparing the stock solution will propagate to each of the diluted solutions. The other option is to prepare each solution independently, which is far more time consuming. But, the errors in calculated concentrations will be independent, which is useful for critical work. Good results can be obtained with either method and the choice will depend on the goals and resources of the laboratory making the measurements.
Preparing polymer solutions with volumetric flasks
Volumetric flasks allow one to measure solution volume precisely and therefore obtain accurate concentrations. However, flasks are expensive, time consuming to clean, and large volumes are required for high precision work.
Steps for preparing solutions volumetrically
1. Clean and dry glassware, taking care to ensure that no residues remain. Ideally, glassware is cleaned immediately after use, before polymer dries on the surface and makes cleaning more difficult. For example, if the flasks were used for polymer solutions, they should be rinsed with good solvent for these polymers at least twice. Next, the flasks should be rinsed with de-ionized (DI) water and completely dried. Note that if the flask is rinsed with a solvent such as tolulene that is immiscible with water, rinse with an intermediate solvent such as THF is desirable before rinsing with DI water. If a high vapor pressure solvent is desired for fast drying, a rinse with methanol is recommended. Acetone (particularly the grades chosen for washing glassware) tends to leave a residue. Since light scattering is a highly sensitive analytical technique, any residue in the glassware will be measured along with the sample. Needless to say, the glassware should be inspected for cleanliness before use.