Solutions. Solution: A solution is homogeneous liquid mixture of two or more substances.

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1 Solutions Objectives: 1. Learn the various methods of expressing concentrations of solutions. 2. Learn to make percent and molar solutions from solids, liquids, and stock solutions. 3. Learn the various forms of solution recipes in making solutions with multiple s. 4. Learn to make and adjust ph of buffer solutions. Definitions Solution: A solution is homogeneous liquid mixture of two or more substances. Solute: A is a substance that is dissolved in a solution. Usually the is a solid, but it may also be a liquid. Solvent: The solvent is the substance that the is dissolved in. For solutions used in biology, the solvent is almost always water. Bringing to Volume (BTV): The procedure in which solvent is added to until the total volume of the solution is exactly the final volume desired. You may see the equivalent term QS from the Latin quantum sufficit (as much as sufficient). Please be sure that you have fully dissolved the before bringing to volume. There are many ways in which chemists and biologists express the concentrations of solutions. Most are basically represented by the equation: amount of Concentrat ion volulme of solution A. Percent solutions Percent means parts per hundred. When dealing with percent solutions, always start by putting 100 ml in the denominator. If you recall that one milliliter of water weighs 1 gram it makes sense that solutions will be described in units of grams (solids) or milliters (liquids) of per 100 ml of solution. Volume/Volume (V/V) solutions: Usually used when the is a liquid and therefore both the amount of and the amount of solution are expressed in units of volume. 5ml 5% v / v 5ml acetic acid made to 100ml with water 100ml solution Weight/Volume (W/V) solutions: Usually used when the is a solid and therefore measured in units of grams. 7g 7% w/ v 7 g NaCl made to 100ml with water 100ml solution Weight/Weight (W/W) solutions: This usage is relatively uncommon. It would be used when the weight of the solvent is considered rather than the volume of the solution. 30g 30% w/ w 30 grams of sucrose dissolved in 100 g water 100g solvent A difficulty with this usage is the volume is not defined. 1

2 B. Molar solutions Molarity is a measure of the number of molecules in a liter of solution. For solutions with concentration expressed in units of molarity (M), the denominator is one liter (1L) and the numerator indicates the number of moles of in one liter of solution. A one molar solution is defined as one mole (mol) of in one liter of solution. A mole of a substance is equal to the molecular weight (M.W.) expressed in grams. The number of molecules in a mole is referred to as Avogadro s number X mol 3.5M 1 L solution C. Direct Notation: concentration is expressed as a given number of grams or milliters of in a given volume of solution. 20 mg/ml Solution Key: The following process can be used to make up any volume of any solution you may be asked to prepare. Always be extremely careful with units. You should always indicate the units corresponding to any numbers you write down. 1. Redefine the concentration in terms of 100 ml of solution for percent solutions or 1 L for molar solutions. 2. Set up a proportion indicating that the ratio of to solution in the volume of solution you are asked to make is the same as the ratio of to solution that you defined. 3. Solve the equation for the amount of required to make the requested volume of solution. 4. In the case of molar solutions, you must then convert moles to grams, that is, calculate the weight of equal to the number of moles of calculated in step #3. Examples Prepare 25 ml of a 4% (w/v) solution of sucrose 1. redefine the concentration 4% = 4g/100ml 2. Set up a proportion?g/25 ml = 4g/100ml 3. Solve for? = 1 g Add 1 g of sucrose to less than 25 ml of water, dissolve, BTV 25 ml D. Parts: Prepare 200 ml of a 3M solution of NaOH (MW = 40.0) 1. redefine the concentration 3M = 3mol of /1L of solution 2. set up a proportion? mol/200 ml = 3mol/1L 3. Solve for? = 0.6mol 4. Convert to grams 0.6mol X 40g/mol = 24g Dissolve 24g NaOH to a final volume of 200ml. 2

3 Parts solutions tell you how many parts of each component to mix together. The parts may be in any units but must be the same for all so that the proportions remain the same. Example A solution that is 3:2:1 ethylene:chloroform:isoamyl alcohol is 3 parts ethylene + 2 parts chloroform + 1 part isoamyl alcohol and could be 30 ml ethylene + 20 ml chloroform + 10 ml isoamyl alcohol. E. Parts per Million: Parts may be used in the expression of concentration parts per million (ppm) It is the parts of per 1 million parts of solution. 3 ppm sodium might be: 3 g sodium in 1 million g solution 3 mg sodium in 1 million mg solution. Parts per Billion (ppb) would similarly be the parts of per 1 billion parts of solution. F. Multiple Solutes Laboratory solutions are often mixtures of several s. These solutions may be made by following recipes. Below are three examples of recipes for solutions with multiple s. 1 M MgCl 2 5 ml 0.4% thymidine 10ml 20% glucose 25 ml This recipe tells you exactly how much of a previously made solution to add. You would have to first make these individual solutions. This recipe could also be expressed in parts as 5:2:1 Phosphate Buffered Saline (PBS) KH 2 PO g Na 2 HPO g, NaCl 7.5g BTV 1L This recipe tells you the amount of each to weigh out and add to the solution bringing to a designated final volume Tris/EDTA/SDS 0.1M Tris 0.01M EDTA 1% SDS This recipe tells you what the final concentration of each in the final solution. You must determine the weight of each to give you that concentration in your desired volume. Biologists often use highly concentrated stock solutions as a way to avoid frequent solution making. The working solutions are made by diluting the stock solution. These dilution problems can be solved by using the following equation: V 1 C 1 = V 2 C 2 or V S C S = V D C D Where S = stock and D = dilution That is the volume of the stock solution x the concentration of stock solution equals the volume of the diluted stock times the concentration of the dilution. 3

4 How do you make 400ml of 1.5M NaCl it you have a 5.0M stock solution? V S X 5.0M = 400ml X 1.5M V S = 400ml X 1.5M/5.0M = 120ml Take 120 ml of the stock and dilute to 400 ml G. Making buffers of a given ph. There are three methods of adjusting a buffer to a given ph. I. The buffer is titrated with a strong acid or base. The buffer is prepared in water but is not BTV. If the ph is lower than desired, a base (often NaOH) is added to raise ph. If the ph is too high, an acid (often HCl) is added to lower ph. II. The buffer is prepared from two stock solutions, one more acidic and one more basic. You will have a table which indicates what percent of each to add together to make a buffer of a given ph. The concentration of each must be at the final desired concentration. Example: phosphate buffer III. The buffer is prepared from two forms of a reagent, one acidic and one basic. An example would be Tris base and Tris-HCl. You would have a table indicating the weight of each to add to make a solution of given concentration and ph. Example Problems How would you make 250 ml of an 8.5% Sucrose solution? 8.5% = 8.5 g/100ml 8.5g/100 ml = X/ 250 ml X = 21.3 or 250 x = 21.3 How would you make 300 ml of 0.5M NaCl? Find M.W. Na = Cl = NaCl = g/mol Then 1M = g/l 0.5M = 0.5 X g/l = 29.22g/L but you want 300ml so 29.22/1000 ml = x/300ml = 8.76 gm Method: Lab Exercises (Actually make up these four solutions in lab- record detailed procedures and calculations in your lab notebook. ) 1. Make up 300 ml of a 4% sucrose solution 2. Make up 100 ml of 3 M NaCl 3. Make up 250 ml of PBS adjust ph to Using stock phosphate buffers, make up 100 ml of a ph 6.4 buffer. Use table on page 515 of the Seidman text) Problems (work out on paper) 4

5 How much CaCl 2 do you need to make 500 ml of a 4M solution. How would you make 3L of 4M NaOH from 12M NaOH? If you weighed 150g of NaCl and BTV 500 ml, what is the molarity of the solution? What would the %W/V be? How much CaCl 2 do you need to weigh to make 1500 ml of a 3M solution? If you weighed 160g of NaOH and diluted to 1.5 liters with water what is the molarity? Prepare 250 ml of a 0.9% sodium chloride solution. Prepare 500 ml of a 1.0 M calcium chloride (CaCl 2 ) solution. Prepare 500 ml of 1X TBE using a 5X stock solution. Prepare 400 ml of a 50 mm calcium chloride solution. Prepare 100 ml of a 1ug/ml ethidium bromide staining solution using a 5 mg/ml stock solution. Prepare 100ml of 4% V/V isopropanol Prepare 100 ml of 4%W/V CaCl 2 5