Aqueous Solutions, Solubility, Diffusion and Osmosis - West Virginia University - PDF

Aqueous Solutions & Concentrations; Solubility, Diffusion, and Osmosis NSG 741: Genetics, Chemistry, and Physics of Anesthesia School of Nursing Terminology Solute—the material that got dissolved; the component of the solution present in the sm...

Aqueous Solutions & Concentrations; Solubility, Diffusion, and Osmosis NSG 741: Genetics, Chemistry, and Physics of Anesthesia School of Nursing Terminology Solute—the material that got dissolved; the component of the solution present in the smaller quantity. Solvent—material that does the dissolving. Solution—homogeneous mixture that consists of one or more solutes uniformly dispersed at the molecular or ionic level throughout a medium known as the solvent. Homogeneous mixture—not possible to discern phase boundaries between the components of the mixture. Phase boundary—separates regions of a mixture where the chemical or physical properties of the mixture change. Solutions aren’t necessarily liquids. Air is a solution of nitrogen, oxygen, and a few other minor gases. School of Nursing Mixtures of oxygen and nitrous oxide are also solutions. Solution Concentrations Molarity Molality Also called molar concentration Also known as molal Moles of solute per liter of concentration solution Expresses concentration in Common concentration unit in terms of moles of solute per chemistry kilogram of solvent Abbreviated with a capital M Can be used as a conversion Because molarity has units of factor between moles of solute and kilograms of moles per liter, molar solvent concentrations are conversion factors between moles of Abbreviated with a lowercase material and liters of solution m School of Nursing Solution Concentrations: Molality Because solutions in the laboratory are usually measured by volume, molarity is very convenient to employ for stoichiometric calculations. However, since molarity is defined on moles of solute per liter of solution, molarity depends on the temperature of the solution. Molar concentration will decrease as the temperature increases. Molality finds application in physical chemistry, where it is often necessary to consider the quantities of solute and solvent separately rather than as a mixture. Mass does not depend on temperature, so molality is not temperature dependent. School of Nursing Solution Concentrations: Molality Much less convenient in analysis because quantities of a solution measured out by volume or mass in the laboratory include both the solute and the solvent. When doing stoichiometry, molality requires an additional calculation to take this into account. Molality is never equal to molarity, but the difference becomes smaller as solutions become more dilute. To convert between molarity and molality, we need to know the density of the solution. School of Nursing Solution Concentrations Percent Percent by Weight to Volume (%w/v) The percent of concentration you encounter in a clinical setting when measuring out a volume of medicine in a syringe. Defined as grams of solute per 100 ml of solution. There are two mathematically equivalent statements of this definition: The first formulation is useful as a conversion factor between grams of solute and milliliters of solution. School of Nursing Solution Concentrations Percent Percent by Weight (% w/w) – e.g., topical creams Exactly analogous to the definition of percent weight to volume, except the denominator expresses the quantity of solution in terms of grams, not milliliters. To relate percent by weight to percent weight to volume, we need to employ the density of the solution. Percent by Volume (% v/v) Never used in an analytical laboratory, because volumes are not additive. Because of strong intermolecular interactionsSchool of Nursing between ethanol Solution Concentrations Normality and Equivalents An equivalent (abbreviated Eq) is analogous to a mole. Normality is analogous to molarity (acids and bases). One equivalent of a substance contains 1 mole of chemical reactivity. Normality is equal to the equivalents of solute per liter of solution. Unless the context of the chemistry is specified, normality is ambiguous. School of Nursing Solution Concentrations The concentration of extremely dilute solutions is often expressed as parts per million. A ppm concentration is analogous to a percent concentration, except you are comparing the amount of solute to a million grams of solution, rather than 100 grams. A safe exposure concentration for any halogenated anesthetic agent is less than 2 parts per million (ppm) collected over a one-hour period, or 25 ppm of nitrous oxide over an 8-hour time weighted average. So, 2 grams in a million grams of air. When nitrous oxide is used in combination with halogenated gas, control of nitrous oxide to 25 ppm during anesthesia should School limit of Nursing concentrations of the halogenated gases to less than 0.5 ppm. Solution Concentrations 100% N2O = 1,000,000 ppm 1% N2O = 10,000 ppm 0.01% N2O = 100 ppm 0.0025% N2O = 25 ppm School of Nursing Epinephrine Epinephrine concentration is measured differently. A 1 mg ampule of 1:1000 epinephrine means that the solution contains 1 mg of epinephrine per ml. Epinephrine is commercially available in two ampule sizes: A 1 ml ampule containing 1 mg (i.e., 1:1000 or 1000 mcg per ml) A 10 ml ampule containing 1 mg (i.e., 1:10,000 or 100 mcg per ml) So, 1:10,000 = 1 gram/10,000 = 1000 mg/10,000 or 1 mg/10 ml or 1000 mcg/10 ml = 100 mcg/ml OR a 1:100,000 solution contains 10 micrograms per ml OR a 1:200,000 solution contains 5 micrograms per ml OR a 1:400,000 solution contains 2.5 micrograms per ml  (Avoid in locations lacking collateral vessels – fingers, nose, toes, ears.) School of Nursing Epinephrine How do you make up 25 ml of 2% lidocaine with 1:250,000 epinephrine? Take an ampule of 1:1000 epinephrine (i.e., 1 mg/ml). Add 10 ml of saline to give 1:10,000 (i.e., 100 micrograms per ml). Add 1 ml of this epinephrine solution to 24 ml of 2% plain lidocaine. (i.e., 100 micrograms per 25 ml = 4 micrograms per ml). How do you prepare a 1:200,000 solution of epinephrine in 20 ml of 1% lidocaine? Take 0.1 ml of epinephrine from a 1:1000 ampule and add it to 19.9 ml of 1% plain lidocaine (100 mcg/20 ml = 5 mcg/ml) An epidural test dose most often consists of 3 ml of 1.5% School of Nursing lidocaine with 1:200,000 epinephrine. How much of each drug Solubility Some solutes are much more soluble in a given solvent than others. The solubility of a solute is the amount of the solute that will dissolve in a given amount of solvent at a given temperature. Increased water solubility allows medications to reach the bloodstream more quickly. Factors affecting solubility can be intermolecular interactions between the substances, temperature, and pressure. A medication able to form hydrogen, dipole-dipole, or ion-dipole and not covalent bonds will be more water soluble (alkanes and alkenes cannot form hydrogen bonds with water). School of Nursing Solubility A saturated solution contains the maximum amount of a solute, as defined by its solubility. A supersaturated solution contains more solute than allowed by the solubility of the solute. Not a stable system, because there is more solute dissolved in the sample than the solvent can accommodate. The excess solute will come out of solution, crystallizing as a solid, separating as a liquid, or bubbling out as a gas. Two liquids are miscible if they are soluble in each other in all proportions. School of Nursing Solubility Solubility is enhanced by intermolecular interactions between substances that have similar electron configurations. “Like dissolves like” Polar solutes are more soluble in polar solvents, while nonpolar solutes are more soluble in nonpolar solvents. Salt (NaCl) solubility in water is an example. The similar polarity of water and salt's constituent parts promote dissolving. Substances like nitrogen, carbon dioxide, and oxygen are nonpolar. They are typically insoluble in a polar compound like water. School of Nursing Effect of Temperature on Solubility The solubility of solid and liquid solutes in liquid solvents generally increases with increasing temperature (with a few exceptions). Gas solubility in liquids is inversely related to temperature. As temperature increases, less gas is able to dissolve into a liquid. An increased temperature represents greater kinetic energy. Greater kinetic energy allows dissolved gas molecules to escape and prevents further dissolving. Lower temperature slows the kinetic energy of gas molecules, allowing them to dissolve into liquids. A clinical example of temperature affecting solubility is seen with the slower emergence of hypothermic patients receiving School of Nursing volatile agent general anesthetics. Energy Changes & the Solution Process When a solute dissolves in a solvent, there is an associated energy change, and there is often times a noticeable change in the temperature of the solution (bond breakage). The energy change when using hot or cold packs is called the heat of solution or the enthalpy of solution: hsoln. Defined as the energy change that accompanies dissolving exactly 1 mole of solute in a given solvent. Enthalpy H is equal to the heat Q as long as the pressure remains constant. School of Nursing Energy Changes & the Solution Process The energy change may be endothermic or exothermic. If the solution process is exothermic, energy flows out of the system (solvent and solute) into the surroundings, resulting in a temperature increase in the solution. If the solution process is endothermic, energy flows from the surroundings into the system, resulting in a temperature decrease in the solution. Whether the heat of solution is endothermic or exothermic depends on the relative magnitudes of the lattice energy and the heat of solvation. If tearing the ions apart requires more energy than is released by solvation, then Hsoln is going to be positive (endothermic) – consumes heat. School of Nursing If the energy released by solvation is greater than the energy Energy Changes & the Solution Process Energy is absorbed to break bonds which is an endothermic process. Energy is released when new bonds form which is an exothermic process. Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break bonds and the energy released when new bonds form. If more heat energy is released when making the bonds than was taken in, the reaction is exothermic. If more heat energy was taken in when making the bonds than was released, the reaction is endothermic. For example, if the temperature increases with an endothermic reaction, it is essentially like adding more reactants to the system. Then, if the temperature increases with an exothermic reaction, it is essentially like adding more products to the system. School of Nursing Energy Changes & the Solution Process Solubility of a solute decreases with increasing temperature if ΔHsoln is negative (exothermic). Solubility of a solute increases with increasing temperature if ΔHsoln is positive (endothermic). According to Le Châtelier’s Principle, the system will attempt to restore/maintain equilibrium. More reactants (endothermic) which would favor a shift in the equilibrium to the right (products). More products (exothermic) will shift the equilibrium to the left (reactants). School of Nursing Factors Affecting Solubility Effect of Pressure on Solubility As pressure increases, the solubility of a gaseous solute in a liquid solvent increases. Since solids and liquids are not very compressible, at least not compared to gases, pressure has very little effect on the solubility of solid and liquid solutes. Gas solubility in a liquid is directly proportional to pressure and is described by Henry's law. The quantitative relationship between pressure and solubility is given by Henry’s law: S = kHPgas where S = solubility, kH = Henry’s law constant, and Pgas = partial pressure of the gas School of Nursing Henry’s Law The main applications of Henry’s law in anesthesia pertains to calculating how much dissolved O2 and dissolved CO2 is in the blood. Henry’s law states that at constant temperature, the amount of gas that dissolves in a liquid is directly proportional to the partial pressure of the gas in the gas phase above gas-liquid interface. p=kc (p is pressure, k is Henry’s constant, c is concentration) Overpressurizing is the process of significantly increasing the concentration of volatile anesthetic (partial pressure) delivered to a patient to increase the alveolar concentration, thereby increasing the amount dissolved in the blood and speeding uptake. School of Nursing Henry’s Law The amount of O2 that dissolves in blood is 0.003 ml/100 ml blood/mmHg partial pressure. To calculate the amount of O2 dissolved in the blood, multiply the partial pressure of O2 by 0.003. So, how much O2 is dissolved in arterial blood when the PaO2 is 300 mmHg? 0.9 ml O2 /100 ml blood dissolved How much does dissolved O2 in the blood increase when the PaO2 increases from 100 to 500 mmHg? 0.3 versus 1.5 ml O2 /100 ml blood dissolved or a difference of 1.2 School of Nursing Henry’s Law If the inspired ml O2 is given, estimate the PaO2 by multiplying the inspired concentration by 5. FiO2 is 40%? 40 x 5 = 200 mmHg x 0.003 = 0.6 ml O2 /100 ml blood dissolved How about CO2? The amount of CO2 that dissolves in blood is 0.067 ml/100 ml blood/mmHg. How much CO2 is dissolved in arterial blood when PaCO2 is 50 mmHg? 50 x 0.067 = 3.35 ml CO2/100 ml blood. School of Nursing Colligative Properties of Solutions The vapor pressure of a liquid results from the most energetic molecules near the surface of the liquid escaping into the gas phase. The likely escape sites for the liquid molecules are at or near the surface of the liquid. As we begin to introduce solute molecules, some of these escape sites are occupied by the solute molecules, so fewer solvent molecules can escape into the gas phase. Therefore, the vapor pressure of the solution is less than the vapor pressure of the pure solvent. Raoult’s law states the vapor pressure of a volatile component of a solution (P) is equal to the vapor pressure of the pure substance (Po) times the mole fraction (χ) of that substance. School of Nursing P = χPo Henry’s Law vs. Raoult’s Law Overall, the difference is that Henry's law takes care of what happen IN the solution when you have gas over it. Raoult's law looks at what is happening OVER the solution when you mix a non-volatile solute to a solvent that has a known vapor pressure when it's pure (e.g., water). School of Nursing Colligative Properties of Solutions Boiling Point Temperature at which the vapor pressure of the material is equal to the ambient pressure. The boiling point of a solution increases as the concentration of solute(s) increases. The change in boiling point is directly proportional to the molal concentration of the solute particles. School of Nursing Colligative Properties of Solutions Freezing Point Temperature at which the liquid phase of the material is in equilibrium with the solid phase. In order to enter into the solid state, the molecules (or ions or atoms) of the sample need to settle into an orderly, crystalline lattice structure. The presence of solute particles interferes with this process by getting in the way. It is necessary to cool the sample to lower temperatures, thereby lowering the kinetic energy of the molecules even further, before they will settle into the solid phase. School of Nursing NACL Example The more salt (or any solute) added to water, the more you raise the boiling point. Freezing point is another colligative property that works the same way: If you add salt to water, you lower its freezing point as well as raise its boiling point. School of Nursing Colligative Properties of Solutions Diffusion is the net movement of one type of molecule through space as a result of random motion to minimize a concentration gradient. Temperature is directly proportional to kinetic energy. Kinetic energy allows molecules to move freely in a fluid, and therefore mixtures of fluids tend to evenly distribute. The velocity at which a molecule may distribute is determined by its molecular weight. If the mass of a molecule is changed, there must be an opposite change in velocity. Greater velocity correlates with faster diffusion. Thus, molecules with smaller mass will diffuse faster. KE = (½)mv2 School of Nursing Colligative Properties of Solutions Graham’s Law: The rate of effusion (gas diffusion through an orifice) of a gas is inversely proportional to the square root of its molecular weight. The formula for this relationship is as follows: r = 1/√mw where r is the rate of diffusion and mw is the molecular weight. Graham's law describes the faster diffusion of smaller molecules compared to larger molecules. Graham's law is helpful in understanding the effect of molecular weight on diffusion but is limited in fully describing all the factors influencing diffusion. School of Nursing Colligative Properties of Solutions Permeable/Semipermeable Membrane: Diffusion may occur through open space or through permeable membranes (tissues). Diffusion of a fluid (gas or liquid) through a permeable membrane is dependent on five factors: Concentration gradient, tissue area, and fluid tissue solubility are directly proportional to diffusion. Whereas membrane thickness and molecular weight are inversely proportional to diffusion. School of Nursing Colligative Properties of Solutions Osmosis is the movement of water across a semipermeable membrane to equilibrate a concentration gradient. The relative concentration of solutes in osmotic systems is called the tonicity. Two solutions are isotonic if they contain equal concentrations of particles. Entropy demands that osmosis occur between two solutions of unequal tonicity until the concentrations of the two solutions are equal. Semipermeable membranes are permeable to water only and not to solutes. School of Nursing Colligative Properties of Solutions Osmotic pressure is the force needed to stop osmosis from occurring. Oncotic pressure is the osmotic pressure exerted by plasma proteins and electrolytes in capillaries (e.g., colloid osmotic pressure). Oncotic pressure balances the hydrostatic pressure tendency to push water out of capillaries. Normal oncotic pressure is approximately 28 mmHg. The vascular system is a semipermeable membrane that responds to intravascular delivery of colloids by sequestering fluid. School of Nursing Colligative Properties of Solutions Osmotic Pressure: Osmotic pressure (symbolized as capital pi, Π) results from the potential drive for the concentration of water to equalize. Osmotic pressure is a colligative property, and the osmotic pressure of a solution increases with increasing solute concentration. Most capillary walls are permeable to small solutes and do not exert an osmotic effect. Albumin (MW is 69,000) does not penetrate the capillary wall and does provide osmotic pressure. Albumin is the major determinant of intravascular volume. School of Nursing School of Nursing School of Nursing Diffusion and Anesthesia Diffusion is a passive process driven by entropy. A gas or liquid will become uniform over time. The diffusion rate varies depending the medium. The diffusion of oxygen and nitrous oxide represents both positive and negative consequences of this process. Nitrous oxide diffuses into air-filled cavities; therefore, delivery of nitrous oxide is contraindicated in patients with pneumothorax or where air-filled cavity expansion is undesirable. Nitrous oxide expansion of endotracheal cuffs may cause tracheal mucosal damage. Distention of the bowel during nitrous oxide delivery has also been documented. School of Nursing Diffusion and Anesthesia Apneic oxygenation is well known and exemplifies the beneficial process of diffusion. The diffusion of gases across biological tissues is expressed by Fick’s Law: Fick's law states that diffusion of a gas across a semipermeable membrane is directly proportional to the partial pressure gradient, the membrane solubility of the gas, and the membrane area, and is inversely proportional to the membrane thickness and the square root of the molecular weight of the gas. Fick's equation allows determination of pulmonary gas exchange. The diffusion hypoxia that occurs after the delivery ofNursing School of nitrous oxide is discontinued, and low inspired oxygen is Fick’s Law of Diffusion Diffusion Rate (J) = (P1-P2) x (Area) x (Solubility) (Membrane thickness) x (√MW) Fick’s law of diffusion for gas explains: Diffusion hypoxia COPD patient and slow gas induction Cardiac output calculation N2O leads to increase volume (or pressure) in gas spaces of the body Graham’s law explains why smaller substances diffuse in greater quantities. Second gas effect (interrelated with concentration effect) High fresh gas flow turbulence (density) School of Nursing Need to Know Even though CO2 is larger than O2, CO2 diffuses 22 times faster across the alveolar and capillary membranes than O2 because it is much more soluble in fluid than O2. Equilibration of an inhalational agent or any gas occurs in the body when the partial pressure of the gas is the same everywhere. The process by which the fetus receives O2, and medications is simple diffusion. Diffusion of a gas from alveoli to blood or the reverse requires a difference in partial pressure. School of Nursing Need to Know Non-gases? The main factors determining diffusion rate across membranes for non-gases is the concentration gradient for nonionized substances or electrochemical gradient for ions, lipid solubility, and size. Agents that poorly penetrate the blood-brain barrier or placental barrier are: Lipid insoluble (ionized substances are lipid insoluble) Large (high molecular weights) Why do ions like Na NOT penetrate lipid bilayers? Ionized particles are hydrophilic and lipophobic! School of Nursing Colloids Colloids are similar to solutions in that they consist of one phase uniformly dispersed in a second phase. Examples: milk, blood, paint, and jelly. Not true solutions because the particles in the dispersed phase are not the size of molecules or ions. Particles in a colloid range in size from 10 nm to 200 nm Colloidal particles cannot be filtered and do not settle out of solution. Colloids can be stable for years if they are stored under controlled conditions. Colloids exhibit the Tyndall effect, whereas solutions do not. Particles of a colloid are large enough to scatter light School of Nursing passing through. School of Nursing