Solutions | ||

MAKING SOLUTIONS You should become completely comfortable making up solutions. To do so, you need to to familiarize yourself with Molarity, Buffers and practice basic calculations.This provides you with a step-by-step guide. Eventually you should be able to dispense with this and do the calculations automatically. Before you start: List all of the ingredients that will be needed; note what the final concentration of each should be. Also note the total volume of the finished solution and what pH it should be. The steps to follow are: A) Measure out the appropriate amount for all the ingredients B) Dissolve all of the ingredients in about 75% of the required final volume
C) Check the pH and correct if needed
D) Bring to the correct volume
E) Label the container and store as needed
Beginning with the first ingredient, follow the flow chart below. 1) Are there any more ingredients to be added?
For example, do the directions say something like 'add 50 grams of sucrose'
3) Is the needed concentration a molarity?
4) Weigh out the appropriate amount and set aside. If you don't know how to calculate molarity: Click Here
5) Is the needed concentration a percentage?
6) Measure out the appropriate amount and set aside. If you don't know how to calculate a percentage t Click Here
7) Is this a dilution of a concentrated stock?. Typical concentrates are running buffers or destaining solutions. They will be labeled 'Running buffer 10x' or 'Destaining 4x'. Measure out the amount needed If you don't know how to calculate the amount Click here Note: if none of the above choices apply to your situation, check with the instructor
Molarity: Calculating the amount needed First: are you working with the pure chemical (solid or liquid) i.e you have a bottle of solid NaCl; or are you using a concentrated solution, such as a 1.5M stock solution of NaCl? Working from the pure chemical You need to know three things The Molecular (or Formula) Weight of the compound (MW). This is usually printed on the label of the bottle The final Molarity needed (M). Remember milliMolar mM is 1/1000 Molar, so a 50 mM solution is 50/1000M, = 0.05M The final Volume needed in liters (V) Remember milliliter ml is 1/1000 Liter, so a 200 ml is 200/1000M, = 0.20 Liter The weight you need in grams is MW x M x V Note: this is true whether the material is a solid or a liquid, you need to weigh out the correct amount Examples: 1) You need to make 400 mls of a 2.2 M solution of Tris (solid) Molecular weight of Tris is 121 Therefore the weight you need is 121 x 2.2 x 0.4 = 106.48 grams
2) You need to make 1.5 liters of a 75 mM solution of NaCl (solid) Molecular weight of NaCl is 58.5 Therefore the weight you need is 58.5 x 0.075 x 1.5 = 6.58 grams
3) You need to make 750 mls of 100 mM glycerol (liquid) Molecular weight of glycerol is 92 Therefore the weight you need is 92 x 0.1 x 0.75 = 6.9 grams Note: do not measure out 6.9 mls, this will not weigh the correct amount
Dilution to a known molarity from a concentrated stock. In some cases the stock solution is made up at a specific molarity and will be labeled as such. i.e. 2.5M NaCl. You can make dilutions of these as needed. The three things you need to know are: the starting concentration (S), the final concentration needed (F) and the final volume (V). The amount of concentrate that you will need is V x F/S Example: you need to make 500 mls of 50 mM NaCl and you have on hand a 2.5M stock Volume needed is 500 x 0.05/2.5 = 10 mls of stock solution
Creating a percentage solution Nearly all percentage solutions will be either weight:volume (w:v) or volume:volume (v:v) For a w:v solution you dissolve the appropriate weight of material into the desired final volume of solution. So a 10% w:v NaCl solution would have 10 grams of NaCl dissolve in every 100 mls of final solution. By contrast, a v:v solution contains the appropriate volume of material in the desired final volume of solution. For example a 25% v:v solution of methanol will have 25 mls of methanol in every 100 mls of final solution. 1) making a percentage solution from a solid or pure liquid, i.e. it is 100% First, check the desired final volume (V)and the final percentage (P) of your finished solution. The amount of material you will need is V x P. Example: you need 500 mls of 30% ethanol (v:v). The amount of ethanol needed is 500 x 30% = 150 mls. Measure this out in a graduated cylinder Example: If the same recipe had called for 500 mls 30% ethanol (w:v) Then the amount of ethanol needed is 500 x 30% = 150 grams. Weigh this in a beaker
2) Making a percentage solution from a pre-existing stock solution. This will be labeled as such: i.e. 60% sucrose (w:v) or 90% ethanol (v:v). You can make dilutions of these as needed. Note: if the stock solution is not labeled w:v or v:v generally assume that solids are dissolve as w:v and liquids as v:v The three things you need to know are: the starting concentration (S), the final concentration needed (F) and the final volume (V). The amount of concentrate that you will need is V x F/S Example: you need to make 500 mls of 50 mM NaCl and you have on hand a 2.5M stock Volume needed is 500 x 0.05/2.5 = 10 mls of NaCl stock Example: you need 1.5 liters of 10% sucrose and you have a 60% sucrose stock Volume needed is 1500 x 10/60 = 250 mls mls sucrose stock
Some stock solutions are made up at a high concentration and can simply be diluted as needed. In such cases you do not need to know the content, or the molarity of the ingredients, that has already been taken care of. For example, running buffer for electrophoresis might be made up 25 times stronger than needed. To use it, you will need to dilute it 25 times. To determine the volume of the concentrate that you need, you need to know the final volume needed (V) and the concentration of the stock (C). The volume of the stock you will need is V/C Example: You have a 5x stock solution of transfer buffer, and want to make up 2 liters of the final buffer. You will need 2000/5 = 400 mls of the concentrated stock solution.
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