NOTE: This module is concerned only with routine lab procedures and ordinary chemicals and biochemicals.  FOR RADIOACTIVE OR OTHER PARTICULARLY HAZARDOUS CHEMICALS, SPECIAL TRAINING AND SPECIAL PRECAUTIONS ARE NECESSARY.


     The following exercises make primary use of two common chemicals, or, in more precise terminology, chemical "compounds": sodium chloride (also known as table salt), and sucrose (also known as table sugar or cane sugar).  A chemical compound is defined as a pure substance that is composed of two or more elements that are in fixed proportion.  There are two general categories of compounds: molecular compounds, which contain atoms linked by covalent bonds, and ionic compounds, which contain ions (electrically charged atoms) linked by charge attraction.  Sucrose is an example of a molecular compound, while sodium chloride is an example of an ionic compound.  In the material that follows, the term "chemical" will be used as a brief way to say "chemical compound".


     Many experiments in biosciences research laboratories begin with the preparation of "solutions" of various chemicals or combinations of chemicals.  A solution is normally made by dissolving a certain amount of a solid chemical in a liquid "solvent", or, if the chemical is a liquid, by mixing a certain amount of the liquid with the solvent.  In a biological research laboratory, the solvent is almost always water.


     The specific amounts of chemicals and of water to be used are determined from (A) the volume of solution needed, and (B) the desired concentration of the chemical.




     Review the module on measuring volumes, and the relationship between metric volumes and metric lengths.  Also, consult the wall chart on the metric system.  In making relatively large volumes of chemical solutions for use in research (say, 50ml - 1000ml), it is important to know what degree of accuracy is required.  If in doubt, it is safest to use a volumetric flask to bring a solution to its final volume, since a volumetric flask is typically more accurate than a graduated cylinder (lower percentage error).  However, in many cases this degree of accuracy is not required, and graduated cylinders are more convenient (easier to pour from and to wash, convenient because you can measure a variety of volumes over a given range).  Therefore, before preparing a chemical solution for research, you should inquire as to whether a volumetric flask should be used, or whether a graduated cylinder is satisfactory. 




     The term "concentration" refers to the amount of a given chemical per unit volume of the solution.  Concentrations can be specified in several ways: by molarity, by weight per unit volume, by percent, by "parts per million (ppm)",  and by units of activity.




     AMOUNT:  One "Mole" of any chemical is equal to its formula weight (or, its molecular weight - see below) expressed in grams, and that amount contains 6.02 x 1023 molecules (Avogadro's number).


     How do we know what the formula weight of a chemical is?  In general, there are three ways: (a) by reading it off the label on the container that the chemical came in, (b) by calculating it from the atomic formula for the chemical, or (c) by looking it up in a standard reference work, such as the "CRC Handbook of Chemistry and Physics".




     Take a commercial bottle of sodium chloride (as from Fisher Scientific) out of the chemicals cabinet.  Look for the chemical formula and the "Formula Weight" of sodium chloride (abbreviated as FW), somewhere on the label near the top.  Similarly, pick up a bottle of sucrose, and find its chemical formula and formula weight.  Write these into your Tech Facility lab notebook. Look at the Fisher ChemAlert Guide on the bottle, then go to the Fisher ChemAlert Chart on the wall. Make a note of all safety precautions to be observed when using these chemicals. When finished, return the bottles to the chemicals cabinet.


     The term "formula weight" is a more general one than "molecular weight".  It is used because a salt such as sodium chloride is an ionic compound rather than a molecular one.  In solid sodium chloride, the bonding between sodium and chlorine ions is based on charge (Na+, Cl-), and after dissolving in water there are ions present but not molecules of sodium chloride.  Thus, strictly speaking, sodium chloride has a "formula weight" but not a "molecular weight". 


     In sucrose, on the other hand, the bonding between all atoms is covalent, and not based on charge.  Thus, sucrose can be described as being composed of molecules that remain as such both in solid form and after dissolving in water.  The formula weight for sucrose is therefore also properly described as its molecular weight.


     Despite these definitions (in practice) the formula weights on commercial bottles of chemicals can be used in the laboratory as equivalent to the molecular weight for making solutions.  You do not have to worry about whether a compound is an ionic or molecular one, and in many laboratories, the term "molecular weight" is used routinely for both types of compounds.




     Check that the formula weights on the bottles are correct by calculating them yourself.  To do this, you must know the chemical formula (number of atoms of each kind making up one molecule of the chemical) and the atomic weight of each kind of atom.  Obtain the formula from the bottle, and look up the atomic weights on the Periodic Table of the Elements chart hanging on the lab wall.  Write your calculations in your Tech Facility lab notebook.


     One Mole, examples: sodium chloride has a formula weight of 58.4, and sucrose has a molecular weight of 342.3 (in the units known as Daltons).  Therefore, one Mole of sodium chloride = 58.4 grams, and one Mole of sucrose = 342.3 grams.


     Concentration, in Molarity:  A one Molar (1M) solution of any chemical = one Mole of that chemical dissolved in a total volume of 1 liter.  By "total volume" we mean the volume of the chemical and the water combined, not the volume of water alone.  1M sodium chloride = 58.4 grams/l, and 1M sucrose = 342.3g/l.


     To make a 1M solution of sodium chloride, 58.4 grams are dissolved in a total volume of 1 liter.  BUT -do you always have to make one liter of solution in order to make a 1M solution?  NO!  You can make less, so long as the concentration remains the same.




     Instead of dissolving 58.4 grams in 1 liter, we could dissolve half the amount in one-half liter, i.e., 29.2 grams NaCl in 500ml.  Similarly, we could dissolve one-tenth of 1 Mole in one-tenth of a liter, i.e., 5.84 grams NaCl in 100ml.  Or, if we needed a very large volume, 584 grams could be dissolved in 10 liters!  All of these solutions would be 1M NaCl.


     What is constant in all of these examples?  No matter what volume of solution is made, the ratio of salt weight to volume stays constant, and thus the concentration of NaCl remains at 1M. 

     To make a one-tenth Molar solution of sodium chloride (i.e., 0.1M NaCl), one-tenth of a Mole of NaCl (= 5.84 g) is dissolved in a total of 1 liter.  For 100 ml of 0.1M NaCl, which is one-tenth of a liter, one-tenth of 5.84g (= 0.584g) is dissolved.




     USING THE ELECTRONIC BALANCE AND A 100 ml VOLUMETRIC FLASK, PREPARE ONE MOLAR (1M) SODIUM CHLORIDE.  Before proceeding, watch the video tape on making solutions, if available.




     In most of the exercises on preparing chemical solutions in the Techniques Facility, you will be using sodium chloride (table salt) or sucrose (cane sugar).  However, note that, even for these ordinary chemicals that are eaten daily, there are precautions on the bottle!  On the reagent bottle of sodium chloride (Fisher Chem. Co.) you are warned of possible eye and skin irritation, and for sucrose, possible eye irritation.  The bottle labels recommend that you wear eye goggles when handling both!  How might these chemicals get into your eye?


            PUT ON YOUR GOGGLES FIRST AND THEN A PAIR OF GLOVES BEFORE PROCEEDING!  This will get you into a "good habit" mode before handling more dangerous chemicals.




(1) Prepare in advance a 100 ml beaker (obtained from Labware cabinet) containing a magnetic stirring bar (obtained from drawer labeled "Stirring Bars".

     The beaker and bar must be known to be clean ones, and the bar should be picked up using a lab wipe tissue (Kimwipe), not your fingers!  If there is any doubt about beaker or bar cleanliness, or if you drop the bar onto desk or floor, place it immediately in the dirty glassware buckets that are in or near the sink, and obtain a new one.


(2) Put about 70ml water (reagent grade!) into the beaker and place it on a magnetic stirrer.


(3) Weigh out 5.84 grams of sodium chloride.  Use the electronic balance as previously instructed, and use the bottle of sodium chloride labeled "TECH FACILITY - SODIUM CHLORIDE (TABLE SALT)".  Do not use the commercial bottle.  Follow instructions below.


     Remove the sodium chloride from its container using a clean spatula, and placing it into a weighing dish on the balance.  Did you remember to "pre-zero" the weight of the dish?



     What did you do with the lid of the sodium chloride bottle?  Did you put it upside-down on the table top (correct), or right-side up (incorrect).  If right-side up, you take a chance that (a) something picked up from the table onto the lid will contaminate the bottle, or, (b) some of the chemical adhering to the lid will fall onto the table.



     DO NOT PUT IT DOWN ON THE LAB BENCH TOP!!  If you do so, you will have contaminated the lab bench top with the chemical, and made the spatula unusable since it could have been contaminated by something already on the bench top.  In this particular exercise the chemical is harmless, but that will not always be the case.  Therefore, you must have a good place to keep the spatula during weighing, and when finished with it, you should put the spatula directly into a washing tray or basin (in or near sink).  Do not put spatula into a wash bucket; when other glassware is put on top of spatulas in buckets, they tend to poke holes through the bucket!  Of course, the spatula cannot be used to weigh out another chemical until it has been washed properly (see "WASHING LAB GLASSWARE" IN WATER QUALITY MODULE).



     If you removed excess sodium chloride from the container, should you put it back into the container?  In general, good lab technique says "No!".  If either the spatula or the dish (or perhaps weighing paper) has been accidentally contaminated with another chemical, you will have contaminated the whole bottle.  Although the chances of this occurring may be small, should this happen even once in a lab, the results can be devastating.  Solutions might be made for weeks, months, or even years from the contaminated stock bottle, affecting experimental results without anyone knowing that it is occurring.  Moreover, such effects will not be repeatable once a new container of the same chemical is used.


     Nevertheless, the general rule that excess chemical should not be returned to the stock bottle cannot always be followed, particularly when the chemical involved is expensive.  Thus, in a research lab, before weighing a chemical for the first time, you must ask your research sponsor or someone else in the lab whether excess removed from the stock container must be saved.


     For exercises in the Tech Facility, DO NOT return the excess to the stock bottle unless otherwise instructed.  Since we are using two inexpensive and harmless chemicals (sugar and salt), both highly water soluble, the excess can be poured into the sink and rinsed down the drain.


(4) Gently, slowly, transfer the sodium chloride into the beaker, with water stirring until all of it has been added.  The transfer is done by pushing the salt out using a spatula, or by gentle tapping of the weighing dish.

     Note that some of the salt, particularly fine particles, may stay behind in the weighing dish.  Using a deionized distilled water "squirt bottle", rinse the weighing dish into the 100 ml beaker.  This is a major advantage of weighing dishes over weighing paper.




            DO NOT TOUCH THE TIP OF THE SQUIRT BOTTLE TO ANY SURFACE AT ANY TIME!!  SQUIRT AT A SMALL DISTANCE FROM TARGET (or you may pick up chemical - here salt - contamination that is transferred to another solution later on).  If you do accidentally touch the tip of the squirt bottle, rinse it off immediately with reagent grade water, while making it squirt.


            NEVER UNSCREW TOP OF SQUIRT BOTTLE AND STICK A PIPETTE INTO IT!  You may contaminate the whole bottle, and thereafter every solution that it is used to make.


            WHEN RE-FILLING EMPTY SQUIRT BOTTLE: take extra precautions not to touch the internal delivery tube (now exposed by removing top of bottle) to any surface, in addition to the normally exposed tip.


(5) Obtain a clean 100ml volumetric flask from the glassware cabinet.  When the salt is fully dissolved (still in a total volume less than 100ml) carefully pour the solution into the volumetric flask.


(6) DO NOT FILL UP THE FLASK WITH WATER. Instead, use a reagent-grade water squirt bottle to rinse the sides of the beaker, and pour the rinse into the volumetric flask.  Repeat this to transfer any residue of salt solution into the volumetric.


            NOTE: you must be careful not to use too much rinse water per rinse, otherwise the rinses may exceed the volume of the volumetric.  You do not want to leave any rinse volume in the beaker, as that would mean that not all of the salt has been transferred into the volumetric.


(7) Now, complete filling the volumetric with water.  Use either a "squirt bottle" or a Pasteur pipette, plus more water in the original beaker.


(8) Place a piece of Parafilm over the top of the flask, stretching it to seal the flask.  Place thumb or other finger on top of parafilm, and invert flask several times to mix contents thoroughly.  Any volume of solution now removed from the flask will have a concentration of 1M.

     Note that, prior to mixing, the salt was not homogeneously dissolved within the flask volume, so that some of it (nearer the flask bottom) had a concentration exceeding 1M, and the rest (nearer the top) had a concentration less than 1M.  The concentration difference can be seen as waviness within the solution (due to refractive index differences) prior to mixing.




     Solutions ARE NOT stored in the volumetric flasks in which they were made.  These flasks are to be available at all times for making other solutions.  Capped bottles and tubes are used to store solutions.


     Obtain a plastic reagent storage bottle of appropriate volume (look at bottom of bottle).  Carefully pour contents of flask into bottle and screw on bottle cap.




     Use a piece of white lab tape from tape dispenser, and a "Sharpie" marking pen (once dry, this ink will not come off if label gets wet).


     What should you write on label?  Chemical name, formula, and concentration (1M NaCl; sodium chloride), your name or at least initials, and the date.  Include any other information you think may be useful at a future time.




     Where, how, and under what conditions will you store the solution?  Should you store it as a single large volume right in the bottle, or should you dispense it into smaller aliquots before storage?


     In general, solutions to be used immediately can be left at room temperature.  Solutions to be used over a long period (days, weeks) should be stored in the refrigerator, or possibly frozen.  The storage decision for a given solution rests on a number of factors:


(a) Can bacteria grow in the solution?


     Some kinds of bacteria can grow in nutrient solutions even at low temperature in the refrigerator, including solutions not designed for experiments on bacterial growth.  For example, a commonly used detergent called "Triton" is biodegradable, and bacteria will grow in solutions containing it, even in the refrigerator.  Therefore, these solutions are typically stored frozen.  Alternative methods involve either removal of all of the bacteria by filtration or killing all of them by autoclaving (heat and pressure) before storage.  Such methods are not usually employed unless the solution is to be used for cell culture.


(b) Will any of the solution ingredients deteriorate under certain conditions of temperature or light?


     Some chemical solutions may crystallize at low temperature, particularly ones that are nearly saturated when initially made up at room temperature.  Other chemical solutions deteriorate when exposed to light, and must be stored either in a light reducing bottle (usually dark brown) or in a bottle wrapped with aluminum foil.  Still others deteriorate too rapidly even when kept in an ordinary freezer (approx. -20 C), and must be kept frozen at lower temperature (usually in special freezers at -80 C).


(c) Can freezing the solution in one bottle be inconvenient and/or create problems?


Answers to both questions: Yes!




     If you have prepared a relatively large volume (say, 250 ml-1 liter), and it is stored frozen in one bottle, it can take a very long time to thaw out before you can use it.  Thawing is usually done by placing the bottle in a warm water bath (CAUTION: do not let bottle tilt sideways into bath, as water can seep in around cap).  Thawing is sometimes done by using a microwave oven (CAUTION: bottle caps must be OFF, otherwise bottle can explode!!).


     Can anything be done about the long thaw period?  Yes - before freezing, prepare aliquots of the solution in volumes convenient for use in experiments.  Use storage tubes that have tight caps, and label each tube with the same information as the original bottle.  This has two advantages: (a) only the amount needed for a particular experiment is thawed (certain biochemicals, such as antibodies, deteriorate with repeated freezing and thawing), and (b) thawing is much more rapid in tubes because of greater surface-to-volume ratio.  




     If you fill a bottle or tube too full (with a watery solution) before capping and freezing, the bottle/tube may crack during freezing.  The reason is that water volume expands at lower temperatures, so that frozen water occupies a greater volume than liquid water.  How full is too full?  In general, if you store 250 ml of solution in a 250 ml bottle (or 500/500, 1l/1l), that is too full!  Use a bigger bottle, or else store most of the solution in the large bottle, and some of it a smaller bottle or in aliquots (see below).




     Fill a 15 ml conical plastic centrifuge tube exactly to the 10 ml mark with deionized water.  Stand it vertically in a small test tube rack obtained from the shelf labeled "Test Tube Racks" and put rack into freezer.  Examine the tube containing frozen solution (after several hours or next day), and record approximate volume in Tech Facility lab notebook, using tube markings.




     Assuming that you have mixed the solution thoroughly after making it up, the concentration of chemical(s) in solution will be constant (homogeneous) throughout.  However, during freezing, the concentration of chemicals can become inhomogeneous due to temperature gradients in the container (outside surface initially colder than interior when container is put in freezer) and other physical processes.  The effect is sometimes referred to as "freezing out".  This means that, when a tube of a solution is thawed, one region within it (say, the upper half) may well have a different concentration than another (say, the lower half).  If you were to put a pipette into such a tube or bottle, the concentration of chemical in your sample would vary, depending upon where the pipette tip was!


     Can you do anything about this?  Yes -  thoroughly mix any solution after thawing it and before using any of it!  Try to develop this into an automatic "habit". 




     Experience is the key.  Before storing any chemical solution in a research laboratory, consult with others in the lab, or with the chemical company from which the chemical was purchased.  NOTE: most such companies have technical service departments that you can call for information (typically, these are free calls, 800 numbers).


     IN TECH FACILITY: Store all of your Tech Facility solutions in the refrigerator, unless otherwise instructed.  Add the information "store in refrig" to the label, so that you will not have to think about it next time you use the solution.


(12) ARE YOU FINISHED??  NO!!  You must now clean up!

     All used glassware must be put into the buckets in the sink, which contain detergent solution specifically designed for lab glassware.  DO NOT forget to wash it.  DO NOT put metal spatulas into buckets - they frequently poke holes in bucket, especially when other items are later put on top of them.  Instead, put spatulas into a separate detergent-containing graduated cylinder or a tray on sink.


     Clean off the weighing surface and the area around the balance if you have not already done so.  If you used gloves, discard them properly in the lab waste container labeled "Hazardous Waste" (turn gloves inside-out as you take them off, as previously instructed in the "Lab Safety" module).




     First, calculate the amount of NaCl needed to make 100 ml of 10M NaCl.  Then look up the "solubility" of NaCl in water, using the "CRC" HANDBOOK OF CHEMISTRY AND PHYSICS.  Is it possible to make 10M NaCl?




      Continue to read the label on the sodium chloride bottle.  Note that it says "certified A.C.S.".  The ACS stands for "American Chemical Society", and "certified ACS" means that this grade of sodium chloride meets certain standards of purity established for chemicals by the American Chemical Society.  Nevertheless, this is not absolutely pure sodium chloride.  Read the "Certificate of Actual Lot Analysis" on the label.  It tells you that this sodium chloride contains traces of other elements and ions, such as potassium and phosphate.


     Look for the "Lot Number" at the bottom of the analysis, and record it in your Tech. Facility notebook.  This is the manufacturer's code number for the particular batch of sodium chloride from which this came.  Different batches (bottles with different lot numbers) may contain somewhat different levels of these impurities.  However, for most purposes in the lab, all chemicals designated "certified ACS" contain tolerable levels of impurities that will not interfere with experiments.




     In the recipes for some solutions used in research laboratories, the concentration of a chemical may be specified simply in terms of weight per unit volume.  Usually the volume involved is 1 ml.




     100 ml of a sucrose solution is needed containing 1 mg/ml sucrose.  Here, for each ml you need 1 mg, so for 100 ml you need 100 mg sucrose.  The solution is thus made by weighing out 100 mg of sucrose, dissolving it in water, and bringing the final total volume to 100 ml.  For storage, the bottle would simply be labeled "sucrose, 1mg/ml" (plus other pertinent information, as above).




     Prepare 100 ml of sodium chloride solution containing 10 mg/ml sodium chloride.




     The term "percent" means, literally, per 100 ("cent" = 100).  Thus, a 10% solution means 10 per 100.  But - 10 of what per 100 what?!  Strangely enough, the "what's" here are frequently sloppily defined.  In its strictest definition, a 10% solution of a chemical means that 10 g of the chemical is mixed with 90g of water (or watery solvent).  The total weight of chemical plus water is thus 100g, and the solution is designated as 10% w/w (weight/weight).  Thus, 10% of the total weight is NaCl.  What volume of water weighs 90 g?  At room temperature, one ml (1cc) water weighs 1 g, and 90 ml water = 90 g.  So, to make a 10% w/w solution of sodium chloride in water, one would mix 10 g NaCl into 90 ml water.  Since the 10 g NaCl does not occupy 10cc volume after it has dissolved, the total volume of the salt plus water is less than 100 ml.


     In practice in the laboratory, "percent" solutions are not usually made as w/w.  Instead, they are made as weight/volume (w/v).  In this case, for a 10% solution, 10 g of a chemical are dissolved in a total volume of 100 ml, i.e., the volume of chemical plus water = 100 ml.




     10% (w/v) sodium chloride = 10 grams sodium chloride dissolved in water, total volume = 100 ml.


     5% (w/v) sodium chloride = 5 grams sodium chloride in 100 ml total.


     10% (w/v) sucrose = 10 grams sucrose in 100 ml total.


     5% (w/v) sucrose = 5 grams sucrose in 100 ml total.


     When the chemical is a liquid, there is still another option!  A 10% solution can be either 10 g/100ml (i.e., 10% w/v), in which case 10 g of the liquid is weighed out on the balance, or it can be made up as 10 ml per 100 ml total volume (denoted as volume/volume, or v/v).  It must be specified whether the solution is w/v or v/v, because a given weight of the liquid chemical, say 10 grams, will usually not be the weight of that volume of chemical (10ml).  In the case of water at room temperature, 10 ml weighs 10 g, i.e., the weight per unit volume, or "density" = 1 g/ml.  However, for other pure liquids, the weight per unit volume (density) is usually greater or less than 1 g/ml.


     So, before you can make up a "percent solution", you must know whether the desired solution is w/w, w/v, or v/v!  The important thing is that, if you are preparing a "percent" solution previously prepared for a particular purpose by others, you make it up the same way that they did.  In addition, by specifying that it is w/w, w/v, or v/v, others will be able to prepare it the same way that you did.  Repeatability is the key, in this and all other aspects of research.





     Prepare 100 ml of a 10% w/v solution of sucrose.  Use magnetic stirring and other methods as for 1M NaCl.  Label and store in refrigerator.




     Prepare a 10% w/w solution of NaCl in water using 10 g NaCl and a 100 ml graduated cylinder, and determine what the total volume is.




  Prepare 100 ml of a 10% w/v solution of NaCl in water.

Compare the concentrations of NaCl in the w/w and w/v solutions, by figuring out the weight of NaCl per 1 ml of each solution.  What is the difference in concentration per ml?




     Occasionally, when a chemical concentration is very low, it is specified in terms of "parts per million" by weight.  Example #1: a chemical company prepares a large batch of crystallized sodium chloride, and finds that it contains 2 milligrams of lead (Pb) as a contaminant per kilogram total weight.  One kilogram = 1000 grams = 1 million milligrams, and thus 2 milligrams of lead per 1 million milligrams total weight = 2 parts per million (by weight) or 2 ppm.  Example #2:  A solution of a chemical in water contains lead as a contaminant at a concentration of 2 mg per liter.  One liter of water weighs 1 kilogram = 1 million milligrams, and thus 2 mg/liter = 2 ppm.

     Note that the lead concentration in the solution in example #2 could also be given in percent: recall that a 1% solution (w/v) = 1 gram per 100 ml = 10 grams per liter = 10,000 milligrams per liter.  A 0.1% solution = 1000 mg/l; 0.01% = 100mg/l; 0.001% = 10mg/l; 0.0001% = 1mg/l = 1ppm.  If a solution contains 2 mg lead per liter (2ppm), that is 2/10,000 of a 1% solution = .0002%.  Usually it is to avoid the inconvenience of such calculations and the writing down of all the decimal places that concentrations are specified in "ppm".



     Obtain the bottle of sodium chloride (Fisher Scientific) from the chemicals cabinet.  Look at the "Certificate of Actual Lot Analysis" on the label, which is a list showing concentrations of contaminants.  Note the concentrations of "heavy metals (as Pb)", "iron", and "phosphate" in ppm.  What is the concentration of "phosphate" in (a) mg per liter, and (b) percent by weight?





     In solutions used in biomedical research, concentrations of certain biochemicals are frequently given in "units of activity" per ml, rather than in % or Molarity.  This is made possible by defining "one unit" of the biochemical as that amount which produces a specified level of a specific effect on a particular reaction or system under specified conditions.  The "units of activity" are thus standardized.


EXAMPLE: Heparin is an anti-clotting agent used routinely in research on blood, and employed therapeutically in hospitals.  Heparin is a powdery material that could be sold by weight, but because heparin preparations vary considerably in purity, the truly important factor for the user/purchaser is the anti-clotting ability of the material.  The precise definition of one "unit" of heparin is too complex to describe here, but in general it is defined as that amount which will block a certain level of clotting activity under specified conditions.


     A bottle of heparin containing dry powder reads:  100,000 units.  Using all 100,000 units at one time, how would one prepare a stock solution (in water) having a heparin concentration of 2000 units per ml (written as 2,000U/ml)?  If the 100,000 units were dissolved in a total volume of 100 ml, there would be 100,000 units/100 ml, = 1000 U/ml.  For double this final concentration (2000 U/ml), we must use half the volume, i.e., dissolve the 100,000 units in 50 ml total volume = 100,000 U/50 ml = 2000 U/ml.


EXAMPLE: Penicillin and streptomycin are anti-bacterial agents (antibiotics) frequently used in cell culture media to prevent growth of contaminating bacteria.  Both are purchased in "units" defined by their bacteria killing potency. 


EXAMPLE: Trypsin is one of the enzymes in the "protease" family of enzymes.  Proteases are enzymes that hydrolyze (break down) other proteins at specific structural sites, or other molecules that have a structure similar to these sites on proteins.  The molecules upon which any enzyme works are known as the "substrate" molecules for that enzyme.  In the case of trypsin (and enzymes in general), units of activity are defined in terms of the amount of substrate acted upon per unit time under specified conditions of pH, temperature, etc.