University of Santo Tomas Faculty of PharmacyPhysical Pharmacy LaboratoryDETERMINATION OF BENZOIC ACIDS SOLUBILITY IN WATER

Lagarteja, M.C.B.1; Lim, H.G.N.1; Lizo, K.J.R.1; Macalino, M.D.L.1, Macapala, C.M.113D-Pharmacy, Faculty of Pharmacy, University of Santo Tomas

AbstractSolubility is one of the most fundamental physicochemical properties in the Pharmacy field particularly in discovering new drugs. In relation to this, experiment was conducted in order to determine the solubility of benzoic acid, a chemical present in some drug products, in water. In an Erlenmeyer flask a corn grain amount of benzoic acid was placed. The solution was agitated for 15 minutes, and then filtered. Five milliliters of the filtrate was placed in the two tared evaporating dishes, and allowed to dry in the oven. The evaporating dishes were weighed again and the solubility of the sample was computed. Based on the computation, 20 mL of water is needed in order to dissolve one gram of benzoic acid, and therefore the description is soluble. The incomplete drying of the filtrate contributed to error in the experiment. The principle behind is as the nonpolar portion of hydrocarbon increases, the solubility of carboxylic acids in water decreases. Benzoic acid is an unsaturated six membered ring with carboxylic acid attached to it.

Background of the Study Solubility is one of the most fundamental physicochemical properties. It is remarkably useful for a wide variety of applications important in the different industries especially in the field of Pharmacy. Accurate solubility data are needed for process and product design, including production and purification of chemical compounds, separation, precipitation processes, crystallization, chemical reaction systems and other processes involved [2]. Solubility of a substance is the maximum amount of solute that will dissolve in a given amount of solvent. If water is going to be used as the solvent, the temperature must at least be 25C but if the solvent is volatile, the temperature must be 15.56C. Other temperatures used for solubility may range depending on the solvent used. The traditional concentration terms such as molarity, molality, mole fraction, and percentage provide and accurate, quantitative account of the drugs property. For semiquantitative assessment of drug solubility, the United States Pharmacopoeia, USP describes the solubility of drugs as the parts of a solvent needed to dissolve one part of the solute with units gram and/ or milliliters, and also the USP adopts a number of descriptive terms [1].Table 1. United States Pharmacopeial Designation of Solubility Ranges [1].

DescriptionPart of solvent that dissolves 1 part of solute

Very soluble10,000

There are several factors that affect a substances solubility. The first is the forces between particles. The nature of the intermolecular forces or interionic forces in both the solute and the solvent affects the solubility. When one substance dissolves in another, the attractive forces in both must be overcome. The dissolving solute must be able to break up the aggregation of molecules in the solvent, meaning the hydrogen bonds between molecules or the dispersion forces between molecules in a nonpolar solvent, and the molecules of the solvent must have sufficient attraction for the solute particles to remove them one by one from their neighbors in the undissolved solute. A general rule is that like dissolves like [5].Another factor is temperature. Usually, the solubility of both liquid and solid increase as the temperature increases, but solubility of gases decreases with increasing temperature [5].Other factors are pressure and particle size [1], although pressure may have very little effect on the solubility of liquids and solids, but has an enormous effect on the solubility of gases. As partial pressure of a gas in the atmosphere above the surface of a solution increases, the solubility of that gas increases which explains carbonated beverages and how carbon dioxide is dissolved [5].The objective of this experiment was to determine the solubility of a chemical compound, which is benzoic acid, in the sample.

Methods

The laboratory apparatuses used in this experiment were Erlenmeyer flask, funnel, 10 mL-serological pipette, stirring rod, small evaporating dish, and Memmert oven. The sample used was benzoic acid. A corn grain amount of benzoic acid was placed in an Erlenmeyer flask containing 20 mL of distilled water. The solution was shaken continuously for 15 minutes; since the benzoic acid did not dissolve, addition of the sample was not needed. After filtering the solution two portions of 5 mL of the filtrate were pipetted and placed in separate tared evaporating dish. The two evaporating dishes containing the filtrate were placed in the Memmert oven set at 105 C for almost two hours. The cooled oven dried evaporating dishes were weighed again in order to get the weight of the benzoic acid present after drying.

Results and DiscussionsAfter getting the two evaporating dishes inside the oven, very small amount of residue was observed. Each evaporating dish was weighed again in order to get the weight of benzoic acid present on it. The first evaporating dish weighs 114.15g; subtracting it from its original weight, which is 114.11 g, the weight of benzoic acid is 0.4 g. The second evaporating dish weighs 108.16 g; subtracting it from its weighed when empty which is 108.15g, the weight of the benzoic acid is 0.1 g. The average weight of the two samples was 0.25 g. The average weight of the sample was used in the computation of benzoic acids solubility in water. Solubility was computed using ratio and proportion.

= 20 mLBased on the computation, 20 mL of water could dissolve 1 g of the sample. Since 20 is in the range of 10-30 in the United States Pharmacopeial designation of solubility ranges, the description is soluble, but the solubility of benzoic acid and derivatives are very low [3], so this means that there were factors contributed to the result of the experiment and this might be the incomplete drying of the filtrate. Furthermore, carboxylic acids are soluble in water because its molecules interact with water molecules by hydrogen bonding through both the C=O and O-H groups. Carboxylic acids of four carbon atoms or fewer are soluble in water in all proportions [4].ConclusionCarboxylic acids are soluble in water because its molecules and water molecules interact with each other by hydrogen bonding, but this rule is applicable to carboxylic acids with four carbons or fewer. As the nonpolar portion of hydrocarbon increases, the solubility of carboxylic acids in water decreases [4], like in the case of benzoic acid wherein the six membered ring with unsaturated hydrocarbon is attached to carboxylic acidReferences[1] Ma, J., & Hadzija, B. (2013). Basic physical pharmacy (Philippine Edition ed., pp. 80-82). Burlington, MA: Jones & Bartlett Learning.[2] Niti S., et al. (2013). Determination and modeling of aqueous solubility of 4-position substituted benzoic acid compounds in a high-temperature solution, Fluid Phase Equilibria, Volume 338, 25 January 2013, Pages 217-223, retrieved from http://dx.doi.org/10.1016/j.fluid.2012.11.023. (http://www.sciencedirect.com/science/article/pii/S037838121200547X)[3] Qing-Zhu, J., et al. (2006). The effect of temperature on the solubility of benzoic acid derivatives in water, Fluid Phase Equilibria. Volume 250(Issues 12), Pages 165-172. Retrieved form http://www.sciencedirect.com/science/article/pii/S0378381206004420[4] Rodriguez, E. (1997). Basic Principles of Organic Chemistry (p. 217, 386). Quezon city: Open University.[5] Rogers, E., et al. (2000). Solubility. Retrieved from http://www.chem.wisc.edu/deptfiles/genchem/sstutorial/Text11/Tx112/tx112.html