2.1 Introduction
The properties of molecules determine how they will behave in a certain environment or microenvironment. The environment is the surroundings in which the molecules exist. For example, in an Aspirin® tablet, acetyl salicylic acid, the active pharmaceutical ingredient (API), remains surrounded by inert solid material like starch or microcrystalline cellulose. The molecules are probably at room temperature at a relative humidity of say 35%. These factors are called the environment. When the acetyl salicylic acid molecule is in the blood-stream, it is a solute molecule that is in a solvent, that is water in this case. The API is also surrounded by a host of electrolytes like Na+, K+, H,2PO4–, H3PO4 etc. at a physiological pH or 7.4 along with dissolved oxygen, carbon dioxide. The API is also surrounded by a number of protein molecules that are constituents of blood plasma like albumen, globulin, fibrinogen etc. There are also cells like red blood cells, white blood cells and platelets around the acetyl salicylic acid molecules. All these components constitute the environment around the API. When the API is within the cells, the localized environment is called microenvironment. The microenvironment can be quite different from the environment. How the API behaves in its environment depends on its properties.
Some of the properties of the molecules are grossly defined by their molecular weights. The smallest molecules often tend to remain in the most dispersed manner; the molecules that are a little bigger in size in molecular weight tend to remain in a more consolidated manner; while the bigger molecules tend to remain in the most consolidated manner at room temperature. These sets of behavior are known as the states of matter. The state of matter of a molecule is important in that it indicates some common properties of that molecule. State of the matter also makes it easier to understand and predict some behavior of a family of molecules.
Some of the properties of the molecules, on the other hand, are unrelated to the state of the matter of the individual molecules. These properties of the molecules are related to the composition of atoms in the molecule and on how they are associated with the other atoms of the molecule. The association between two or more than two atoms is called bonds or intermolecular forces.
The study of states of matter, bonds and intermolecular forces helps us understand how a particular molecule behaves in a particular environment. In the context of pharmaceutical sciences, physical pharmacy and pharmaceutics, behavior of molecules in a particular environment determines the safety and efficacy of the API. The molecular structure, bonds etc. of an API determines whether it is an electrolyte or a non-electrolyte, its chemical stability in a particular state of the matter and in a particular dosage form. The molecular structure, and bonding also determines the partitioning of the API between various tissue of the body, its ionization status and distribution of the drug throughout the body, its metabolism and excretion and their respective rates. In short, the entire therapeutic use of a particular API, its safety and efficacy depend on the molecular structure and properties of the API. The study of the states of the matter, the properties related to the states of the matter, the bond or the intermolecular forces gives us the tools to understand the clinical and therapeutic issues with a particular API and how to approach some problems associated with them.
2.2 States of Matter: Solids, Liquids and Gases
There are three states of matter that are traditionally recognized; solid, liquid and gas. There is a fourth state of matter called plasma. Plasma is composed of ionized gas in which the electrons are ripped apart from the atoms. Plasma is formed when sufficient energy is supplied to the gas molecules generally in the form of heat. The stars are composed of plasma; so is lightening. The aurora in the north and south poles and the neon street lights are also composed of plasma. Plasma, as a state of matter, is not encountered in the materials used in pharmaceutical sciences and thus will not be covered in this discussion. Most of the APIs remain in the solid state at room temperature. A great number of pharmaceutical formulations are solid and are composed of solid ingredients. Thus, solid state is most important in the discussion of physical pharmacy. Very few APIs exist in the liquid state at room temperature. The API like glyceryl trinitrate (nitroglycerin, Nitrostat®; used in angina or chest pain) that remains in liquid state at room temperature is one such example. A lot of pharmaceutical formulations (syrups, suspensions, emulsions, lotions, creams etc.) are prepared and used in liquid state. Additionally, only a few gases are used in medicine. That is why all three states of matter are relevant to physical pharmacy.
The solid state is a state of matter in which the molecules are very closely packed in space. Intermolecular forces of attraction dominate in solid state. The molecules in solid state have less energy and their movement is restricted. The molecules in solid state can vibrate in one place but they cannot move around or move away with respect to their neighboring molecules. Solids, thus, have a specific shape and volume.
Figure 2.1a:
Molecules in liquid state have more energy. When heat energy is applied to the molecules in solid state, they gain energy and transform to liquid state, the associated energy is called heat of fusion. Conversely, when liquid molecules give off energy (negative heat of fusion), they can transform to solid state. The liquid molecules are further apart with respect to their neighboring molecules than the molecules in solid state. That is why, same mass of molecules in liquid state have higher volume in liquid state than in solid state. The volume of water is though, less than the same mass of ice. This is
Figure 2.1b:
an exception and happens due to more extensive hydrogen bonding in liquid water than in ice. Hydrogen bonding and its effects will be discussed later in the chapter. Liquid molecules can vibrate in a place and also can move around their neighboring molecules without going farther away from their neighbors. Molecules in liquid state, thus, have a constant volume but not a fixed shape; liquids take the shape of their containers.
Gaseous molecules have even more energy than liquid molecules. The intermolecular forces of attraction are non-existent or negligible in this state. Molecules in gaseous state do not have a fixed shape or even volume. The molecules in liquid state take up the entire volume and shape of the container in which they are kept. Energy has to be supplied to the liquid molecules for their change of state to gas molecules. This energy is called the heat of vaporization. When the gas molecules give off energy (negative heat of vaporization) they change their state to liquid molecules.
Generally speaking, all molecules are capable of remaining in all the states, provided that an appropriate amount of energy can be given or taken away from them. The amount of energy transfer that is necessary to effect a change of state is different for different types of molecules. Further, the amount of energy that a molecule of an element or a compound possesses at room temperature is also different for different types of molecules. This is a property of compounds and elements. That is why different compounds remain in different states of matter at room temperature.
2.2.1 Solids
The molecules are close to one another in solid state; their intermolecular forces of attraction far exceed those of repulsion. Solid molecules have very little kinetic energy as their movement is restricted to vibration with respect to their mean position with respect to their neighbors. Molecules in solid state have a specific size and shape. The most important physical properties of the solids include their molecular arrangement, melting point, solubility and rate of dissolution, heat of melting (fusion) and sublimation.
Molecular Arrangement
Molecules in solid state can remain in many different arrangements. Sometimes solid molecules are arranged in a regular geometrical or special manner. This type of arrangement in solids is called crystals and the solids are themselves called crystalline solids. Generally, intermediate to small sized molecules, both inorganic and organic, remain in crystalline form.
Sometimes, solid molecules are capable of remaining in more than one crystalline form at room temperature. This phenomenon is called polymorphism and the involved crystalline forms are called polymorphs. Polymorphism is an extremely important concept for pharmacists because a number of the APIs are crystalline and remain in more than one polymorphic form. The stability, melting point, solubility, and the rate of dissolution of the different polymorphs of the same API differ drastically. In crystalline form, the molecules are arranged in a specific geometrical order; the intermolecular forces of attraction are high. The solubility and rate of dissolution of these types of molecules is low. The melting point of these types of molecules are high. Hence, the choice of an appropriate polymorph of an API for a specific dosage form and formulation is at least as important as the formulation. This is evident from the figure below (El-Zhry El-Yafi, A.K. and El-Zein, H.; 2015).
Larger molecules like waxes and also polymers remain in a random arrangement without any geometrical pattern. These types of solids are called amorphous solids. In the amorphous state, since the molecules are arranged randomly, the intermolecular forces of attraction between two neighboring molecules is low. As a result, the solubility and rate of dissolution of an API is high in an amorphous form compared to a crystalline polymorph. The amorphous solids and APIs have a lower melting point and lower chemical stability. As a result, if the APIs in amorphous form are chosen for formulations, they dissolve fast and are absorbed fast from the GIT, resulting in faster onset of action. Such APIs may also have lower shelf life in the formulation because of lower chemical stability. Some solid dosage forms like suppositories are designed to slowly melt to release the APIs in the site of action (vagina or rectum in this case). In these situations, an API in amorphous form may be the appropriate choice, especially for water insoluble (lipophilic/hydrophobic) APIs. This effect is enhanced in cases of co-amorphous solid dispersions, where two amorphous molecules or APIs are presented in the form of a solid mixture in the formulation (Karagianni, A., Kachrimanis, K. and Nikolakakis, I.2018).
Solid polymorphs can be classified in many different ways. The most important two are based on crystal shape and compositions respectively. When pure crystals are made up of one type of molecules, they are called homomeric. Most of the time though the crystals include some solvents with the molecules. These crystals are called solvates. Most often the solvent is water, in which case the
Figure 2.2:
Improved concentration
in plasma of the co-amorphous
hydrochlorothiazide
crystalline forms are called hydrates. Depending on the stoichiometric ratio (ratio of two or more different molecules in the crystal) of the molecule and water, we call them monohydrate, di-hydrate, tri-hydrate etc. (for one, two or three water molecules per API molecule respectively). For example, we can have anhydrous cortisone acetate (CA) in crystalline form (MW 402.2); we can also have cortisone acetate monohydrate (Formula Weight (FW) 420; CA: H2O =1:1) or cortisone acetate hemihydrate (FW 411; CA: H2O = 4:2). Sometimes the crystals contain a neutral molecule in addition to the molecule of interest (e.g. the API) . These crystals are called co-crystals. We can also have co-crystals containing a water molecule in the crystal structure (hydrated co-crystals). All the polymorphic forms discussed above remain in un-ionized form in the crystal. Sometimes the molecules are ionized in aqueous solutions and crystallize with their counter ions. These types of polymorphs are called salts. Similar molecules can be ionized in aqueous solution and form salt co-crystals. Just like non electrolytes (un-ionizable molecules), salts can also take the form of salt hydrates (ion + counterion + water) and salt hydrate co-crystals (ion1 + ion2 + counterion1 + counterion2 + water etc.). The polymorphic forms can also be classified by the shape of crystals. The geometric details of these crystal forms and classifications are given in figure 2.4b.
Figure 2.3:
2.2.2 Uncommon and complex ‘Change of State’ behavior of solids
Sublimation
Solid materials can change its state from solid state to gacious state directly without going through the liquid phase if the heat of transformation can be proveded by the surroundings. This phenomena is called sublimation.
In other words, sublimation happens when the vapour pressure of the solid phase exceed the atmospheric pressure at that temperature. A very common example of sublimation is dry ice which is carbon-dioxide in solid state in room temperature. The surroundings can supply enough energy for the vapour pressure of carbon-dioxide to rise over the atmospheric pressure at room temperature causing sublimation. Other common examples of sublimating solids include naphthalene (mothballs), iodine crystals, camphor, menthol etc., the last three solids are often used in compounding pharmacy (for example camphor in insect bite sticks that are often prepared in compounding pharmacies). Care should be taken not to provide too much energy when triturating the sublimating solids in morter and pestle. Sublimating solids are often mixed with other solid materials before trituration to minimize sublimation.
Figure 2.4a:
The physical state of a substance and
its phase-transition temperatures are represented
graphically in a phase diagram.
Figure 2.4b:
Dry ice sublimating
Eutectic mixture
Some solids with a low melting point often melt at room temperature when two or more of them are mixed together. The mixture of these solids is called eutectic mixture and the lowest melting point of the mixture is called eutectic point. The melting point of such a mixture varies with the changing composition of the components of the mixture. An example of such a mixture is shown below (Phaechamud, Tuntarawongsa and Charoensuksai, 2015). Camphor, menthol, testosterone and their combinations can produce eutectic mixtures. All of these compounds are used in compounding pharmacies. Care needs to be taken while triturating them. Usually other solid meterials like dry powder diluents like starch and modified celluloses are mixed with these eutectic combinations so that the liquidified mixture is adsobed in the solid.
Figure 2.5:
2.2.3 Liquids
While an overwhelming majority of the APIs are solids, one API nitroglycerin or glycerol tri-nitrate remains in liquid state in room temperature. Nitroglycerin is frequently used in the form of sublingual tablets. Since the API is liquid at room temperature, it can evaporate from tablet dosage form even through the plastic packaging, thereby decreasing the API strength. One solution to this problem is to decrease the vapour pressure of the liquid by admixing glycerol in the preparation. Addition of glycerol, a liquid molecule, decreases the mole fraction and vapour pressure of nitroglycerin thereby decreasing its propensity to evaporate.
A number of drug preparations, both oral and injectables, remain in liquid state. The properties of pure liquids are thus less important for the purposes of pharmaceutical sciences. On the other hand, as there are many liquid preparations, the properties of solutions are of immense importance to us. The most important of these properties will be discussed subsequently.
Most important topics involving the solute molecules in solution are the mass terms and concentration terms of the solute molecules and ions. The relationship between these terms and interconversion among these various units form an essential skill set for the pharmacy students.
Mass terms
Formula weight is a mass term that needs to be used for accurately measuring the amount of solute of interest. In cases where the solute is available in the pharmacy in a hydrate, solvate or co-crystal form, the actual molar concentration of the solute of interest is much lower than the weight of the form divided by its molecular weight. For example, the corticosteroid cortisone acetate is available as anhydrous cortisone acetate (CA; MW-402.5), cortisone acetate monohydrate (CA, H2O; formula weight (FW) = 420.5) and cortisone acetate dihydrate (CA, 2H2O; FW = 438.5). So, one has to weigh out one formula weight amount of the drug to get one mole of cortisone acetate. In terms of mass in gram, one has to measure out 402.5 g of CA; 420.5 g of CA, H2O and 438.5 g of CA, 2H2O to get one mole of cortisone acetate depending on the polymorphic form of cortisone acetate that is available in the pharmacy.
Formula weight (FW)= MW of the solute of interest+ MW of the associated molecule in the proportion in which they are present in the polymorph
Example:
Formula weight (FW) of cortisone acetate dihydrate = 402.5 (CA)+ 2×18 (MW or 2 moles of water)=438.5
Concentration terms
Molar is a concentration term generally used to express the concentration of a solute in a solvent. A solvent is a pure liquid and a solute is a solid that is dissolved in a solvent. Sometimes, more than one pure liquids are mixed together. In these situations, the pure liquids are said to be miscible and the liquid mixture is referred to as a solvent system. One mole of solute dissolved in 1 liter of total solution makes the solute concentration to be one molar (expressed as M).
When one mole of solute is dissolved in 1 kg of solvent, the concentration is said to be one molal (expressed as m).
Note: In dilute solution (< 5% solute) M and m concentrations are very very close numbers because the density of the solution remains practically the same. Thus, when we are working with dilute solutions, molar and molal solutions are often used as synonyms. However, for more concentrated solutions with solute content more than 5%, the density starts to differ appreciably and 1L of solution does not correspond to 1 Kg of solvent. In those situations, molar and molal concentrations are totally different. This is very evident in cases of concentrated mineral acids; 18.5 M H2SO4 is equal to 639.93 m H2SO4 which is equal to 98% (w/w) H2SO4.
Osmolar (OsM) refers to the concentration of particles of solute dissolved in one liter of solvent (typically water). The molar and osmolar concentration are same for non-electrolytes like glucose as one molecule produces one particle in aqueous solution. However, for the electrolytes (especially strong electrolytes like NaCl, KCl etc.) one molecule produces more than one particle following dissociation/ionization. For NaCl and KCl one molecule of these solutes produce two particles (one Na+ or K+ and one Cl- per molecule). Osmolar concentration for NaCl and KCl OsM = 2xMW of NaCl or KCl. In case of magnesium chloride (MgCl2), one molecule of MgCl2, one molecule produces three particles (one Mg++, and two Cl–). Osmolar concentration for MgCl2 OsM = 3 x MW of MGCl2. Two particles for NaCl and KCl or three particles from MgCl2 are the maximum possible numbers of particles respectively. In reality, the number of particles from one molecule is a little less, as defined by the ionization factor (i). The idea of the ionization factor will be discussed in greater detail in a later chapter.
OsM=M*number of particles=M* number of (sum of cations and anions) [not sum of charges]
Normality is a concentration term that refers to the strength of strong acids or strong bases.
Or N = molarity (M)*number of positive or negative charges in the solute molecule (not their sum)
Percent refers to parts in each 100 part. When two liquids are mixed, then each part is easier to measure in volume. So, for an alcoholic beverage like wine, 11% ethyl alcohol means in each 100 ml of wine, there is 11 ml of ethyl alcohol. This kind of % calculation is known as volume percent (v/v). However, in case of a solution where the solute is a solid and solvent is a liquid, the percent term refers to mass of solute in volume of total solution (m/v). So, 0.9% sodium chloride (saline) refers to 0.9 g of NaCl in 100 ml of total aqueous solution. If not specified, percent refers to g of solute/100ml of solution. The mg percent refers to the number of mg of solute in 100 ml of total solution.
2.2.4 Gases: properties of pharmaceutical importance
Universal gas law
There are only a few gases that are used in medicine; mostly in surgeries as anastasia, but sometimes in imaging too.
| Gas | Common Medical Use |
| Carbon dioxide | Insufflation during certain surgeries |
| Desflurane | General anesthesia |
| Isoflurane | General anesthesia |
| Nitrogen | Cryosurgery |
| Nitrous oxide | Anesthesia, analgesia |
| Oxygen | Oxygen supplementation (examples: for emphysema and pneumonia) |
| Sevoflurane | General anesthesia |
| Xenon | General anesthesia, neuroprotection, contrast imaging |
The most important property of the gases is the relationship among the temperature, pressure and volume of a specific amount of gas molecules known as the universal gas law.
PV = nRT
where, P=pressure (expressed in atmosphere, Pascal, kiloPascle etc)
V = volume (expressed in litre)
n = number of moles (measure of the mass of gas)
R = universal gas constant [R = 0.082 litre.atm/(degree Kelvin.mole) or R=1.986 Calorie/(mole.degree Kelvin)]
T = temperature in absolute scale (degrees Kelvin = 273 degrees Celsius)
This mathematical relationship means that for 1 mole of gas, the multiplication product of pressure P and volume V, when divided by the temperature T, is always a constant. This constant is called the universal gas constant R.
Gas mixture, mole fraction, partial pressure and solubility
Partial pressure
The letter ‘P’ refers to the total pressure of the gas mixture. So, it is obvious that the partial pressure of a component of a gas mixture is determined by the relative abundance of that gas component in the mixture in terms of mole fraction as opposed to mass fraction.
Solubility of gases in a liquid, Henry’s law
Henry’s law states that the amount of a gas dissolved in a liquid is proportional to the partial pressure of the gas that is in contact with the liquid and is in equilibrium with the liquid. The proportionality constant is called Henry’s constant (H). So, from the above example, the concentration of the dissolved gas component G1, C1 = H * P1 assuming the three-component gas mixture is in contact with the liquid and is in equilibrium with it.
For the gas components G2, C2 = H * P2
and for G3, C3 = H * P3
In essence, the relative abundance of a gas component in a gas mixture determines the partial pressure of that component in the mixture. Further, the partial pressure of the gas component in the mixture that is in contact with the liquid determines the dissolved concentration of that component in the liquid that is in contact.
Practical use
CO2+ H2O ↔ H2CO3 ↔ H+ + HCO3 –
Normal Range for Blood Gas Values (for adults) | |||
Arterial | Venous | ||
pH | 7.35 to 7.45 | pH | 7.33 to 7.43 |
PaO2 | 80 to 100 mm Hg | PvO2 | 35 to 45 mm Hg |
PaCO2 | 35 to 45 mm Hg | PvCO2 | 45 to 50 mm Hg |
HCO3 | 22 to 26 mm Hg | HCO3 | 24 to 28 mEq/l |
SaO2 | 92-100% | SvO2 | 70 to 75% |
BE | -2 to +2 mEq/L | BE | 0 to +4 mEq/L |
Partial pressures of oxygen and carbon dioxide
| Denver | Sea Level |
PO2 | 70 mm Hg | >80 mm Hg |
O2 saturation of hemoglobin | 93% | >95% |
PCO2 | 38 mm Hg | 40 mm Hg |
This condition is called metabolic acidosis. Metabolic acidosis can be treated with systemic alkalizers, while the overall situation can be well handled by improving ventilation through oxygen support.
Henry’s law comes into play in case of deep-sea diving. The pressure is high deep under the ocean surface due to increased hydrostatic pressure. The deep-sea divers, thus, dissolve a high concentration of gases in their blood stream and these gases remain in equilibrium with the high external pressure. The divers encounter decompression as they resurface, the dissolved gas come out from dissolved state to gaseous state. If the decompression process is too rapid, gas bubbles can form within the veins, a potentially lethal condition as the bubbles can block/interfere with blood circulation.
2.3 Forces between and within molecules
Forces of attraction exist between atoms. When the force of attraction between atoms is strong enough to bring them together to make a stable entity, the force of attraction is called bond and the stable entiry that is created is called a molecule. For example, the force of attraction that brings two hydrogen atoms and one oxygen atom together as a stable entity is called a bond and the stable entity that is created by the bond is called the water molecule. Forces of attraction also exist between molecules. These forces are generally not strong enough to created a stable single entity (that is no molecule is created). These forces of attraction however, are strong enough to bring the molecules closer to other neighboring molecules or orient the involved molecules in a certain geometry. These forces are called intermolecular forces.
2.3.1 Bonds
There are three major types of bonds; the electrovalent bond, the covalent bond and the coordinate covalent bond.
Electrovalent bonds, also called ionic bonds, are characterized by a transfer of one or more electron(s) from one atom to another. The transfer makes the donating atom a positively charged ion (cation) and the receiving atom a negatively charged ion (anion). These opposing charges create an electrical force of attraction that keeps the cations and anions together as a molecule. The group 1, 2 and 3 elements (metals) have the propencity to donate electrons and become cations while the group 5, 6, and 7 elements have the propencity to accept electrons to become anions. It is to be noted that ionic compounds often dissociate partially or completely in polar solutions like aqueous solutions, in which the cations and anions often exist independently. The electrovalent bonds have a high bond strength of 100-200 KCal/mole (Martin’s Physical Pharmacy and Pharmaceutical Sciences, ISBN:0-7817- 5022-x, page 24).
Covalent bonds are created when two atoms share a pair of electrons (one contributed by each atom) between them. There is no outright transfer of electrons that is involved in the formation of a covalent bond. Covalent bonds are found primarily in organic compounds i.e. compounds containing carbon, hydrogen, oxygen, nitrogen etc. atoms. Covalent bonds are relatively weaker and have a strength of 50-150 KCal / mole.
The elements of group 5 of the periodic table have five valence shell electrons. Typically, three of those five valence shell electrons are shared in covalent bond formation leaving a pair of valence shell electrons uninvolved in the bond formation process (lone pair electrons). Ammonia (NH3) molecule is one such example (nitrogen is a group 5 element). The lone pair of nitrogen atom in ammonia molecule has a propensity of sharing its lone pair of electrons with a hydrogen ion (proton) to produce ammonium ion. This type of bonds in which one participating atom/molecule contributes both the electrons in a pair to be shared by both the participating atoms is called coordinate covalent bond.
Figure 2.6:
2.3.2 Intermolecular forces
Hydrogen bond is a kind of intermolecular force of attraction that can exist both between two molecules and also between two atoms in the same molecule (usually a big molecule). Generally speaking, though there is no outright electron transfer between the participating atoms in a covalent bond, there is often partial charge separation. It means, that one of the atoms share the electron pair a little more than the other and the atom sharing the electron pair a little more, gets partially electronegative and the atom sharing the electron pair a little less becomes partially electropositive. The hydrogen atom in an alcoholic group (-OH) in an organic molecule becomes a little electropositive, while the oxygen atom in the same functional group becomes a little electronegative. These two atoms, existing in separate neighboring molecules or different regions of the same macromolecule, are then weakly attracted towards each other by electrostatic forces of attraction called hydrogen bond. Hydrogen bond is weak in itself, and cannot form a molecule. That is why hydrogen bond is not a true bond, it is a kind of intermolecular force. Hydrogen bond can only bring two molecules closer together or bring two atoms in the same macromolecules together. This phenomena (hydrogen bonding) can occur between two parts of the same (usually macromolecule) molecule or between two or more different molecules.
Figure 2.7:
Practical significance
Hydrogen bonding is actually extremely significant and is often underestimated. The secondary, tertiary and quaternary structures of the biologically important molecules are held together by numerous hydrogen bonds. For example, the double helical structure of the DNA molecule, the preserver of life and specific trait of living beings, are held together by hydrogen bonds. The folding of proteins in a single plain (secondary structure), and in two different plains (tertiary structure) or the holding together of two different protein units (for example heam and globulin in hemoglobin; quaternary structure) are all accomplished through hydrogen bonds. Finally, hydrogen bonds are very common in water molecules, the degree of hydrogen bonds depend on the temperature. Hydrogen bonding in water is highest at 4°C, above and below this temperature hydrogen bonding gradually decreases. As the extent of hydrogen bonding is highest at 4°C for water, the water molecules are closest together at 4°C and the density is also highest at that temperature. The extent of hydrogen bonding is lower in ice (below 4°C), thus density of ice is lower than that of water and as a result, ice floats in water. Ice is one of the very few examples in which a compound in solid state is less dense than its liquid state. Floating of ice over the water bodies ensures the existence of aquatic ecosystem in cold regions of the world.
2.3.3 Mass and Concentration Terms
Mass
Mass refers to the amount of matter. Mass of a matter is usually measured in gram (g), milligram (mg), kilogram (Kg) or pound (lb). All parameters in the universe can be carried out based on three fundamental parameters namely mass, length and time. There are three fundamental systems of measurement of these fundamental parameters.
CGS system-This system is named after the units of the fundamental parameters. Mass is measured in gram (g), length is measured in centimeter (cm) and time is measured in seconds (s).
MKS system-This system is named after the units of the fundamental parameters. Mass is measured in kilogram (K), length is measured in meter (m) and time is measured in seconds (s).
FPS system-This system is named after the units of the fundamental parameters. Mass is measured in pounds (P or lb), length is measured in foot (f) and time is measured in seconds (s). The time unit in all the above systems is the same, i.e. second. MKS system of measurement is generally preferred in the fields of science and technology because of the ease of conversion from one parameter to be measured to another.
Measurement of mass in any of the above units serve the purpose in common day-to-day applications. But in chemistry, biology and other related fields, measurement of mass in Kg is often problematic. In the fields of chemistry and/or biology, the interaction of one material with another material is quantitatively dependent on the molecular ratio (stoichiometry) of the two materials. Further, one material can break apart to make two or more different materials (for example: NaHCO3→Na2CO3+CO2+H2O etc.). Sometimes two or more compounds react to produce one or more compounds and/or elements. These changes occur quantitatively in the stoichiometric ratio of the reacting compounds or elements. That is why it is always better to express compound/element mass in terms of molecular counts.
Mole-One mole is the mass of an element or a compound that is numerically equal in g to its molecular weight. For example, 1 mole of hydrogen gas =2g (molecular weight of H2 =2). Similarly, 1 mole of water has a mass of 18.015 g (H2O molecular weight =18.015). This system of counting mass takes into account of the relative abundance of the atoms in the molecule and the atomic weight making it easier the mass of the products in molar terms in any chemical change/interaction based on the stoichiometric ratio. One more important point about the mole is that 1 mole of all elements or compounds contain the same number of atoms or molecules respectively. This number is called the Avogadro’s number A (A = 6.022*1023, extremely large). The most important point to remember is that mole is a unit of mass not concentration.
Equivalent – An equivalent is the amount of mass per charge (positive or negative, not both) Equivalent weight (E) can be mathematically defined as
Equivalent weight is especially useful in acid-base chemistry and titration. One equivalent weight amount (i.e. equivalent, E) of all strong acids represent same amount of acidity, one equivalent (E) of all strong bases represent the same amount of basicity and one equivalent of all strong acid will quantitatively neutralize one equivalent (E) of all strong bases.
Concentration
Concentration refers to how much mass is in a unit volume. Commonly used terms of concentration are given below.
Percentage– Percentage refers to the g of solute (minor component in a mixture) in 100 ml of solution. This concentration term is commonly referred to as g% (weight/volume; w/v). When both the solute and the solvent (major components of the mixture) are solids, a weight /weight measurement (w/w) is used for the concentration term.
Molar– For the same reasons that are elaborated above, it is often advantageous to express concentration term in biology and chemistry in molar term. One mole of solute dissolved in one liter of total solution makes the concentration 1 molar (expressed by the symbol M). One molar of any molecule contains Avogadro’s number of the same molecule in one liter.
Molal– Another term that is often used is molal. One mole of solute dissolved in one Kg of only solvent makes the concentration 1 molal (expressed by the symbol m). One molal of any molecule contains Avogadro’s number of the same molecule in one Kg of solvent.
Normality– Normality, expressed by the symbol (N), is a concentration term that is frequently used in acid-base chemistry. One equivalent amount of a solute in 1 liter of total solution has a concentration of one (N).
Osmolarity– Osmolarity refers to the concentration of particles (not molecules) in the solution. The particles in this context refer to any small molecule, macromolecule or ion that is capable of independent existence. Osmolarity is expressed by the symbol Osm. The osmolarity calculation in mathematical terms is given below.
The above calculation is an approximate calculation made with the assumption that all of the above solutes ionize completely. This assumption is not always true. Further refinement in the calculation of Osm can be carried out using appropriate ionization factor (i) for each of the solutes. This refinement in calculation of Osm will be discussed further in Chapter 3 (Isotonicity). The particle concentration (Osm) is very useful in calculating the extent of colligative properties like freezing point depression, boiling point elevation, osmotic pressure and ultimately tonicity of the body fluids. The tonicity of the body fluids and isotonicity of the large volume parenteral products is of utmost important to the pharmacists and pharmaceutical scientists. Preparation and/or evaluation of isotonic large volume parenteral products necessitate a thorough understanding of Osm and its calculation.
References
- Karagianni, A., Kachrimanis, K. and Nikolakakis, I. (2018). Co-Amorphous Solid Dispersions for Solubility and Absorption Improvement of Drugs: Composition, Preparation, Characterization and Formulations for Oral Delivery. Pharmaceutics, 10(3), p.98.
- El-Zhry El-Yafi, A.K. and El-Zein, H. (2015). Technical crystallization for application in pharmaceutical material engineering: Review article. Asian Journal of Pharmaceutical Sciences, 10(4), pp.283–291.
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