This preview shows page 1. Sign up to view the full content.
Unformatted text preview: ion
which is known to 0.1 mL or better. They have a narrow neck with a single mark. Solvent is added to the
Bure ts . These long, narrow, graduated tubes have a stopcock at the bottom. They are designed to
dispense variable amounts of liquid or solution. The volume of solution in the buret is recorded before
dispensing the solution and afterward. The difference between the two readings is the volume of liquid
dispensed. The volume readings should be to two decimal places. Burets are most often used in titrations.
Pipe ts . Pipets, like burets, are designed to dispense a known volume of solution. There are several
Volumetric, or transfer, pipets have a single mark on a narrow neck. They are the most accurate
pipets for delivering a known amount of liquid or solution. They are drained by gravity. Remaining
traces of liquid are not forced out, e.g. by blowing on the pipet.
Mohr pipets, or measuring pipets, have graduations and are used much like burets. They are not
Serological pipets are graduated and are designed to be drained completely. Most are designed to
have the last traces of liquid blown out.
If the concentration of a solution is unknown, the concentration can be measured by determining the amount
of light it absorbs (its absorbance, A) at a particular wavelength (λ), using a spectrophotometer. Absorbance and
concentration, c, are directly related by Beer's Law:
is the "molar absorptivity" (a constant unique to that solute at that wavelength) and l is the path length, or distance
the light travels through the solution. To evaluate c from absorbance measurements, one measures the
absorbance of several st andard solut ions (solutions of known concentration). A plot of A vs. c, which equation
1 shows should be linear, allows calculation of ε. In these measurements, l is held constant by using the same
sample holder (or a carefully matched set of them) for all measurements.
The plot of A vs. c is called a
calibration curve. The concentration of any other solution can be found with the calibration curve by measuring its
There are two standard ways to make solutions of a desired concentration. First, if the solute is
a solid, the appropriate amount of solid may be weighed out and then dissolved in enough solvent to make the
desired amount of solution. This is often done in a volumetric flask. The accuracy and precision of the mass and
volume determinations dictate the number of significant figures th...
View Full Document
- Spring '14