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Preparing Solutions that Contain More than a Single Solute study guide
In the biotechnology laboratory, you will often have to prepare solutions that
contain multiple components. Consider, for example, the following buffer:
Klenow buffer
(concentrated stock; http://wheat.pw.usda.gov/~lazo/methods/lazo/buffer.html)
7 mM Tris-HCl
7 mM MgCl2
50 mM NaCl
50% glycerol
This is the recipe for making up a commonly used buffer in molecular
biology procedures. Note that this buffer has 4 components (Tris-HCl,
MgCl2, NaCl and glycerol). The concentrations indicated are the
concentrations that each will have in the prepared Klenow buffer. There are
two ways to prepare this buffer. You could prepare the buffer by weighing
or measuring out the appropriate masses or volumes of individual reagents.
You could also prepare this buffer by diluting out more concentrated stock
solutions of each of the individual components. Let’s consider both ways on
the following pages…
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PREPARING MULTICOMPONENT SOLUTIONS FROM INDIVIDUAL
REAGENTS
Sometimes you will prepare multicomponent solutions by individually
weighing or measuring each component from a stock reagent (often a
powder, or pure liquid). This is usually the method if the buffer is not often
used, or is made from unusual chemicals for which concentrated stocks are
not readily available.
Ex:
Calculations to make 250 mL of Klenow buffer from:
Reagents:
Tris-HCl (MW 157.6)
MgCl2 (Hydrate) (MW 203.3)
NaCl (MW 58.44)
Glycerol comes as a 100% pure liquid
Procedure:
Calculate how much of each reagent you will need.
Dissolve all components together in the SAME container in about 80 mL of
water, and bring the final volume to 250 mL with water.
Calculations:
Amount of Tris-HCl:
0.25 L(7 mmoles Tris)(1 mole Tris)(157.6 g Tris) = 0.28 g Tris-HCl
(1 L) (1000 mmoles)(1 mole Tris)
Amount of MgCl2:
0.25 L(7 mmoles MgCl2)(1 mole MgCl2)(203.3 g MgCl2) = 0.36 g MgCl2
(1 L) (1000 mmoles)(1 mole MgCl2)
Amount of NaCl:
0.25 L(50 mmoles NaCl)(1 mole NaCl)(58.44 g NaCl) = 0.73 g NaCl
(1 L) (1000 mmoles)(1 mole NaCl)
Amount of glycerol:
(100%)(V1) = (50%)(250 mL)
V1 = (50%)(250 mL) = 125 mL glycerol
100%
So, dissolve 0.28 g Tris-HCl, 0.36 g MgCl2, 0.73 g NaCl and 125 mL of glycerol
in about 80 mL of water. Then, bring the final volume to 250 mL with water.
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PREPARING MULTICOMPONENT SOLUTIONS FROM STOCK
SOLUTIONS
Now, instead, consider the situation where you are working in a lab that
commonly uses the four components of your desired solution in other
contexts, so there are concentrated stock solutions existing of each of the
components. Although the calculations required will be different, making
solutions from concentrated stocks often saves considerable time and effort.
Ex:
Calculations to make 250 mL of Klenow buffer from:
Reagents:
1.5 M Tris-HCl
5 M NaCl
100 mM MgCl2
80% Glycerol
Procedure:
Calculate how much of each reagent you will need.
Mix all components together in the SAME container and bring the final volume to 250
mL with water.
Calculations:
Volume of 1.5 M Tris-HCl:
(1.5M)(V1) = (0.007M)(250 mL)
V1 = (0.007M)(250 mL) = 1.17 mL 1.5M Tris-HCl
1.5M
Volume of 100 mM MgCl2:
(100 mM)(V1) = (7 mM)(250 mL)
V1 = (7 mM)(250 mL) = 17.5 mL MgCl2
100 mM
Amount of 5M NaCl:
(5M)(V1) = (0.05 M)(250 mL)
V1 = (0.05 M)(250 mL) = 2.5 mL 5M NaCl
5M
Amount of glycerol:
(80%)(V1) = (50%)(250 mL)
V1 = (50%)(250 mL) = 156.25 mL glycerol
80%
So, measure out 1.17 mL of 1.5 M Tris-HCl, 17.5 mL of 100 mM MgCl2, 2.5 mL of 5 M
NaCl and 156.25 mL of 80% glycerol and bring the final volume to 250 mL with water.