Investigation of the carbonate - bicarbonate system Essay Example for Free

Investigation of the carbonate bicarbonate system Essay Alkalinity in this experiment can be defined as the measure of the pH buffering capacity in water (e. g. river water, sea water, lakes etc. ). This experiment was carried out using a prepared carbonate bicarbonate solution against hydrochloric acid (0. 1M HCl). Titration and pH indicator method was used to determine the equivalence point. The experiment also compares the titration and pH indicator method that is, looking at the option that will give a better approach in determining the equivalence point. Compared the digital and analogue pH meters that were used during the experiment by different groups. The experiment also aims at showing the importance of carbonate bicarbonate system in environmental chemistry as it affects the natural water system. INTRODUCTION In Nigeria, water pollution is a serious environmental issue as it relates to the oil industries in the Niger Delta region, this can be attributed but not limited to the continuous flaring of fossil fuel. This creates acid rain deposition thus reducing the buffering capacity of rivers and lakes. The major source of living and earning in this area is fishing, but the continuous flaring of gas has resulted in acid precipitation, which has affected fishing. When there is acid deposition, the pH of the water increases thereby making the water partially acidic, the drop in pH can harm and even kill some of the organisms inhabiting the water (Tope Akintola Jan 2004). The Nigerian government set up the Federal Environmental Protection Agency (FEPA), which issued, in 1988, a specific decree to protect, to restore and preserve the ecosystem. The decree also sets out water quality standards to protect public health and to enhance the quality of water (Enderlein et al). Although, the Federal Environmental Protection Agency did not have specific guidelines, the agency adopted the guideline for alkalinity by World Health Organisation in 1971. Parameter Permissible standard Issued by Alkalinity (mg/l) 200 600 WHO, 1971; FEPA, 1991 Alkalinity can be defined as that level that provides stable pH in rivers and lakes or simply put as withstanding a fall in pH. Stability is achieved through the neutralization of acids in an aqueous system. It should be noted that alkalinity in this experiment deals strictly with the acid base neutralization, whereas, pH is the concentration of the solution. When stability is achieved, alkalinity is said to buffer the solution. Buffering of an aqueous system is the equilibrium between the acid and base reaction, this is done when the hydrogen [H+] ion is either used up or donated so that the pH of the solution remains normal, that is, remains as the original pH of the system. In rivers, lakes and ocean, buffering involves the presence of carbon dioxide (CO2), carbonic acid (H2CO3), bicarbonate (HCO3-), carbonate (CO3-2) and hydrogen [H+]. The reaction involved is thus; CO2 (aq) + H2O H2CO3 (aq) 1 H2CO3- (aq) H+ (aq) + HCO3-(aq) . 2 HCO3-(aq) H+ (aq) + CO3-2(aq).. 3 Carbon dioxide from the atmosphere dissolves into the solution and also achieves equilibrium as shown below; CO2 (g) CO2 (aq) 4 The carbonic acid formed (H2CO3 (aq) ) from equation (1) is neutralized by the addition of base to for equation (2). Continued addition of base (alkalinity) further removes the proton (H+) present to form the carbonate in equation (3). Buffering is achieved at this point, and equivalence points are noted as shown in the figures below (figures 1 2). Further addition of base now drives the equation back and carbon dioxide (CO2 (g)) is given off. Figure (1) points E and F indicate where the gas is given off. The experiment was carried out using the titration and pH indicator method to determine the equivalence points of equations (2) and (3), phenolphthalein and methyl orange were used as indicators. Phenolphthalein was added to determine the first equivalence point. The initial pH of the water sample was 9. 75; the addition of phenolphthalein thus induced a colour change. Phenolphthalein produces a colour change in water in the pH range 8. 3 10. The alkalinity measured at this point is the half carbonate that has been neutralized in the reaction. Half carbonate reaction; from equation (3) only one proton H+ is consumed during the neutralization, therefore, half carbonate reaction is simply the amount of hydrogen ion (which is one) consumed during the phenolphthalein indicator method. Methyl orange indicator method measures the buffering capacity of the neutralized solution. The bicarbonate (HCO3-) ion initially present with those produced during the half reaction is completely neutralized. Methyl orange produces a colour change at pH range 3. 5 4. 5. HCO3- + H+ H2O + CO2 Both methods is used to determine the total alkalinity of the reaction, thus, total alkalinity is the addition of the equivalence points of the reaction. Methodology The experiment was carried out in two parts, titration and pH indicator methods. Titration method (phenolphthalein indicator) 25ml of carbonate/bicarbonate mixture was pipette into a conical flask and two drops of phenolphthalein was added, the solution changed from colourless to pink. The mixture was then titrated against 0. 1M HCl until the pink colour changed to colourless. Table (1) shows the value that was obtained. (Methyl orange indicator) 25ml of carbonate/bicarbonate mixture was pipette into a conical flask and two drops of methyl orange was added, the solution changed from colourless to yellow. The mixture was then titrated against 0. 1M HCl until the yellow colour changed to pink. Table (1) shows the value that was obtained.pH indicator method 25ml of carbonate/bicarbonate mixture was pipette into a beaker and distilled water was added, the mixture was then titrated against 0. 1M HCl. A pH digital meter was introduced to measure the initial and subsequent changes in pH of solution. Table (2) shows the value that was obtained. Results The investigation of carbonate and bicarbonate in the water sample can first be described by the following reactions: (a) H2CO3 (aq) H+ (aq) + HCO3-(aq) (b) HCO3-(aq) H+ (aq) + CO3-2(aq) Various results were obtained during the titration and pH indicator methods. Table (1) shows the values obtained when 0. 1MHCl was titrated against the carbonate/bicarbonate mixture. Table (2) shows the values for the pH indicator method, table (3) shows the values for the derivative curve and table (4) shows the class result (raw data). The value obtained was then plotted on a graph to obtain various readings; the graphs would be explained later in the discussion. The plot on figure (1) shows the relationship between the pH and volume of 0. 1M HCl used. Figure (2) is the derivative curve, figures (3 4) is used to determine the equivalence points of the reactions.