Mannitol for Cerebral Oedema: A Study on Osmosis and Buffer Action - Lab Report Example

Mannitol for Cerebral Oedema — A Study On Osmosis And Buffer Action Introduction The increase in brain water content is termed as cerebral oedema. This excess water causes not only swelling and morbidity, but also compression of brain tissue against natural structure, which could lead to death. Cerebral oedema is attributed to the excessive accumulation of sodium ions in the brain cells, which creates an osmotic gradient in the cell, in turn causing an increase in intracellular water content. (Jha p. 326) Osmotherapy is used to treat this condition. In this therapy, an osmotic gradient is created to remove excess water from the brain. Mannitol (hexan-1,2,3,4,5,6-hexol, C6H8(OH)6), a sugar alcohol is often used. An optimum amount of mannitol, when administered to patients with cerebral oedema, causes an osmotic gradient and decreases blood viscosity (Jha p. 328). As with the injection of any drug into the bloodstream, the intravenous feed of mannitol must be isotonic with blood (Atkins 2006 p. 156). In aqueous solution, mannitol acts as a Brønsted acid — it loses a proton and turns the solution acidic (Wishart 2006). In blood plasma and intracellular fluids, the protein buffer system resists this change. Methodology To learn how Mannitol reduces cerebral oedema, we conducted three experiments to study osmosis, buffer action, and osmosis in red blood cells. Experiment details are available in the laboratory schedule. Results Experiment 1: Osmosis Using Visking Tubing Experiment To study the principle of osmosis, we placed a sealed visking tubing sac filled with sugar solution and dye vertically in a beaker of water. We observed the increase in dyed solution level in a capillary tube connected at the other end of the tubing. The level of coloured solution in the capillary tube increased. The tubing sac turned turgid. Experiment 2A: pH Prediction and Measurement in Acetic Acid–Sodium Acetate Mixtures Five conical flasks with different volumes of acetic acid (0.1mol/dm3) and sodium acetate (0.1mol/dm3) solutions showed different pH levels when measured with a pH meter. The pH calculated using Henderson–Hasselbalch equation (Equation 1) varied for each flask (Table 1). Table 1 Equilibrium table of acetic acid–sodium acetate solutions Flask Volume of Acetic Acid (aq)(mL) Volume of Sodium Acetate (aq) (mL) pH Calculated pH Measured with pH meter 1 40 10 4.09 4.35 2 30 20 4.52 4.76 3 25 25 4.7 4.84 4 20 30 4.87 5.03 5 10 40 5.3 5.42 Experiment 2B: Buffer Action: Effect of Addition of Strong Base and Strong Acid Strong base (sodium hydroxide) and strong acid (hydrochloric acid) showed varied reactions in distilled water and acetic acid-sodium acetate buffer solutions (Table 2). Table 2 Reaction to strong acid/strong base in aqueous1 and buffer2 solutions Beaker No. Beaker Contents Indicator Added Reactant Added (0.1 mol/dm3) Colour Change Expected Drops of Reactant Added 1 Distilled water Methyl orange HCl Pink 10 2 Distilled water Phenolphthalein NaOH Purple 2 3 Buffer Methyl orange HCl Pink 26 4 Buffer Phenolphthalein NaOH Purple 40 Experiment 3: Tonicity and Red Blood Cells Red blood cells showed varied reactions to different concentrations of saline solution when observed under a microscope (Table 3). Table 3 Reaction of Red Blood Cells to saline solutions Slide Label or % Saline Solution Reaction 0.45% Haemolysis 0.9% No reaction observed 1.8% Crenation Discussion Osmosis Osmosis is the spontaneous movement of a pure solvent into a solution separated by a semipermeable membrane — permeable for the solvent but not for the solute. The concentration gradient between the solvent and the solution causes solvent to move into the solution across the semipermeable membrane. The solute molecules cannot cross the semipermeable membrane because of their higher mass and their association with water molecules. Osmotic pressure is the pressure applied by the solution to stop influx of solvent. (Atkins 2006 p.154) Mannitol is considered to create an egress for intracellular and extracellular water molecules in the brain through osmosis. (Jha 328) When mannitol is injected into the blood stream, the osmotic pressure is likely to cause water molecules in the brain cells to move out. In Experiment 1, water molecules move into sugar solution through the visking tubing, which acts as a semipermeable membrane. The sugar molecules, which are larger, cannot cross the semipermeable membrane. The movement of water molecules across the tubing causes an increase in solution level in the capillary. At equilibrium, the surface tension in the capillary equals the osmotic pressure between the solvent and the solution. This stops further increase in solution level in the capillary. Osmosis in Red Blood Cells Cells have membranes that allow smaller molecules, hydrated ions and water molecules to pass, but stop the flow of larger molecules. The concentration gradient of solutes inside and outside the cells causes osmotic pressure and water molecules enter into the cell. This water content helps cells maintain the structure — dehydrated cells tend to shrink while cells with excess water tend to burst. (Atkins 2006 p. 156) When a drug is injected into the blood stream: The injected solution should be isotonic with blood, i.e., it should have the same osmotic pressure as blood. A dilute injection solution is hypotonic — water molecules flow into the cells, which burst and die. This phenomenon is known as haemolysis. A concentrated injection solution is hypertonic — the exodus of water molecules dehydrates cells and they shrink or crenate. (Atkins 2006 p. 156) A low dose solution of mannitol is given to patients with cerebral oedema to remove excess water from the brain cells. Here, the exodus of intracellular water molecules prevents haemolysis of brain cells. Experiment 3 demonstrates this effect of osmosis on red blood cells. A 0.9% saline solution is isotonic with blood, 0.45% solution is hypotonic, and 1.8% solution is hypertonic. Our findings were consistent with this (Table 3). Weak Acid–Weak Base Mixtures Acetic acid is a weak acid. In aqueous solution, it does not ionise completely: MERGEFORMAT Equation 1 (Atkins 2004) Addition of sodium acetate causes an increase in CH3COO- ions. This shifts the equilibrium to the right, and more hydronium ions are produced to compensate. This causes an increase in pH of the solution. (Atkins 2004) The pH of the solution is predicted qualitatively using Henderson–Hasselbalch equation: Equation 2 (Atkins 2004) In Experiment 2A, the equilibrium table of acetic acid–sodium acetate mixtures shows an increase in pH calculated and measured with increase in sodium acetate concentration. Buffer Action and Protein Buffer System Buffer action is the ability of a solution to resist pH changes when a strong base or strong acid is added. In a buffer solution, the molar concentrations of weak acid and its salt are equal. The logarithmic dependence of Henderson–Hasselbalch equation (Equation 2) forms the mathematical basis for buffer action. (Atkins 2004) When the molar concentrations of acid and base are equal, Equation 3 (Atkins 2004) In Experiment 2, acid/base indicators reacted quickly to strong acid/strong base in aqueous solution. In buffer solution, where molar concentrations of acetic acid and sodium acetate were equal, a large amount of strong base or strong acid was added to bring a colour change. This is because: The large number of CH3COO- ions in the buffer solution can accept protons from hydronium ions when a small amount of the strong acid is added. The large number of CH3COOH molecules that are not ionised can supply hydronium ions to neutralise a small amount of the strong base. (Atkins 2004) In intracellular fluid and plasma, proteins act as buffers. Proteins are made of amino acids, which have carboxyl (COOH) and amino (NH2) groups. At neutral pH (pH=7), these groups exist as COO­- ions and NH3+ ions: In acid environment, the COO- groups gain a proton and change to COOH, thus decreasing pH of the blood to 7. In basic environment, NH3+ groups lose a proton and convert to NH2, thus increasing pH of the blood to 7. This protein buffering system maintains blood plasma and intracellular fluid pH at 7 when drugs such as mannitol, which loses hydronium ion in aqueous solution, are administered. References Atkins, P., & De Paula, J. (2006). Atkins Physical Chemistry, 8th ed. Oxford: Oxford. Atkins, P., & Jones, L. (2004). Chemical Principles: the Quest for Insight, 3rd ed. W. H. Freeman. Jha, S.K. (Lt. Col., Retd.). (2003). Cerebral edema and its management. MJAFI, 59, 326-331. Retrieved November 18, 2008, from http://medind.nic.in/maa/t03/i4/maat03i4p326.pdf. Wishart, D.S. et al. (2006) DrugBank: Showing Mannitol (DB00742). Retrieved November 20, 2008, from http://www.drugbank.ca/drugs/DB00742. Read More