Experiment 3 – Determination of Mixed AlkaliLOGRONIO, Alvin J. PAYNAGANAN, Renlyn Ivy DG. Group 1, Chem 27.1 TEJ, Ms. Jireh Joy E. Sacramento February 21, 2012 I. Abstract This determination of mixed alkali focused on the application of the acid-base titration in the determination of the composition of alkali mixtures of bases, such as carbonate or bicarbonate, either alone or in combination in a solution. This method is used for the determination of the safeness of water for irrigation and the determination of the buffer capacity of natural waters. In the process, the unknown is titrated with a hydrochloric acid solution, which was standardized by titration with a sodium carbonate primary standard. Then, the two-indicator method was used, with phenolphthalein and bromocresol green as indicators. The said method aimed to determine the two endpoints typically shown by the polyfunctional carbonate base, if applicable. The data gathered predicted the presence of NaHCO3, Na2CO3 or NaOH and their percent composition in the unknown. For a double-indicator method, the amount of carbonate and hydroxide present is taken from the amount of hydrochloric acid required to reach the phenolphthalein end point. While the amount of bicarbonate present is found by the amount needed to reach the bromocresol green end point. II. Keywords acid-base titration, mixed alkali, standardization, two-indicator method, carbonate, bicarbonate III. Introduction given sample using a two-indicator neutralization Alkalimetry is a specialized analytic use of titration with a standardized HCl solution as the titrant. acid-base titration to determine the concentration of a base. It also involves the determination of the quantity IV. Experimental of real alkaline in alkali solutions or salts, either The standard HCl solution required in the through gravimetric or volumetric analysis. experiment was prepared by diluting 8.23 mL of the Carbonates and bicarbonates of alkali concentrated HCl solution in sufficient amounts of CO 2 mixtures are important industrial chemicals since they free distilled water to make a 1 liter solution. Three are the salts of a strong base and a weak acid and are 0.20-0.21 g samples of the dried standard Na 2CO3 only readily available salts of these anions that are was transferred to three numbered 250 mL flasks soluble in water. Presence of these compounds may where 250 mL of CO2 free distilled water and 2 drops occur in different combinations that can be identified of bromocresol green indicator was added. These using titrations that involve neutralization reactions three samples was then titrated with the 0.1 M HCl with strong acids. solution until the color changed from yellow to light The carbonates and bicarbonates in the blue. When the color changed, the solution was then solution can be present in the forms CO 32-, HCO3- and boiled for 2 minutes and the flask was then covered OH-. Neutralization titrations are employed in the with a watch glass or inverted beaker to retard the quantitative determination of the constituents of a absorption of CO2 and allow the solution to cool. After mixed alkali solution. Two methods can be used to cooling, the solution was supposed to return into its determine a mixture of bases by titration: the two- yellow color. The titration was then continued until the titration method and the two-indicator titration method. yellow color was changed into a distinct blue color. In our experiment, we used the two-indicator method The molarity was computed and noted for future to determine the carbonate and bicarbonate mixtures computations. in the unknown. The two volumes of the endpoints of Three 0.20-0.21g samples was weighed by the two indicators used will help in determining the difference into three 250mL Erlenmeyer flasks. 25 mL carbonate and bicarbonate contents of the given of CO2 free water and 2 drops of phenolphthalein sample. indicator was then added to the sample. The flask was Indicators used in titrating your samples must immersed in an ice bath while it was being titrated with be carefully chosen to be able to detect the endpoints the 0.1 M standardized HCl solution. The solution was of the neutralization reactions employed in the slowly titrated with constant vigorous swirling until the experiment. In this two-indicator titration, pink color just disappeared. The volume was then phenolphthalein and bromocresol green was used. recorded on the table provided. Phenolphthalein was used in titrating Na2CO3, while The titrated solution was removed from the ice bath bromocresol green was used in titrating NaHCO3. and 2 drops of bromocresol green indicator was The aim of this experiment is to determine added. It was then titrated once more with the 0.1 M carbonate and bicarbonate mixtures present in the standardized HCl solution until the yellow color just Chemistry 27.1 Determination of Mixed Alkali Page 1 of 4 Discussion constituents cannot coexist together because of the Determination of carbonates (CO32-). which contains desiccants that 2 0.and OH. it is necessary to dry it in an Weight of Volume of Molarity of oven before weighing in order to achieve a uniform Trial Na2CO3 (g) HCl (mL) HCl (M) and reproducible composition. Chemistry 27. It can be cooled and 1 0.8). which results in erroneous measurements.036 0.1 9. pH break for HCO 3- stable. Trial After weighing. % (w/w) NaHCO3 in the sample - (H2CO3). an endpoint is detected using detects the endpoint for the second titration of HCO 3- its corresponding indicator.1 9. In the two-indicator method. - bicarbonates (HCO3 ) and hydroxides (OH ) in a basic between HCO3. OH- two-titration method. Since this Table 1. the primary standard Na 2CO3 can (ph) (bcg) M %(w/v) M %(w/v) 1 9. they must be Sample Computations: cooled to room temperature. 0.0995 reagent. V.1mL HCl mL 1mmol HCl In the determination of carbonate. another indicator is added this experiment are phenolphthalein (pKa = 9. two titration set-ups are HCO3-. its dilute solutions are break for CO32. The five mixture involves the titration of the mixture with a possible combinations.to HCO3. . it has very lttle oxidizing or reducing power.38% . to prevent any absorption by the 3 0.0982 mmol HCl 1mmol CO32 can be used to determine unknown mixtures. Standardization of HCl Solution reagent is hygroscopic.378% L 1000 mL 1mol Na2CO3 From this.5 0. which used. forming carbonic acid Ave. Average Molarity of HCl (M): 0.and the excess . For each set up. Analysis of Unknown closed interior of the analytical balance prevents any mL HCl mL HCl Na2CO3 NaHCO3 moisture absorption on the reagent.103M HCl Also. it coincides well with the pH break they detect. reducing the relative error in measurement.1 Determination of Mixed Alkali Page 2 of 4 .0382 L solution gives more accurate measurements.7 0. 0. previous reaction. which to the analyte in order to detect the second endpoint. hydrochloric acid (HCl) was their indicator range (ph: 8. resulting in CO32.to In the two-titration method. it must be noted that HCO 3- 2 0.to be in solution.3-10 bcg: 3. inside the analytical balance. Results which is recommended for higher accuracy.4) used as the titrant. and a Sample Computations: larger volume of titrant. detects the endpoint of the first titration of CO 32.0357 M Na2CO3 and OH.38% .3).2085 38. reducing indicator error.092 absorb moisture. since. After standardization.103 stored in a desiccator.+ OH. . like other strong acids. any heat transfer from the 1mol Na2CO3 2 mol HCl reagent to the air can cause the production of air 0.2085gNa2CO3 105. CO32-/ HCO3-. weighing must be done by difference since it 0.38 % bromocresol green (bcg). to H2CO3 ≈ 2-6) There is a third pH break for titration plus the chloride ion does not precipitate with most of OH. bicarbonate 1mmol Na2CO3 1 and hydroxide mixtures. and bromocresol green (pK a = 4. we can also say that the three VI. The drawback for HCl is that it is not a be properly detected by any indicator.≈ 7-10.-> CO32.036 0. then be titrated by the HCl titrant by either 2 9.2 0. forming HCO 3.cannot coexist together since they react to 1mmol CO3 25mL solution form CO32-. one and HCO3-.8-5. (pH gives a large Ka in titration.to H2O (around pH 11. % (w/w) Na2CO3 in the sample 0. CO32-/OH-. 9.0982 It must be noted that before weighing any reagents that are previously heated.turned into a distinct light blue color. The indicators used for detected using an indicator.or by Ave.4 0.2082 42.2057 39. Also. and also the Table 2. The second Sodium carbonate (Na2CO3) is the most common volume was also recorded in the table.0357mol Na2CO3 1L 105. This is because.99 g 1mol Na2CO3 currents. The latter is preferred due to the formation of a larger pH break. titration set-up is used for the determination of two This determination involves the titration of the equivalence points. considering the above strong acid using either a two-indicator method or a conditions are: CO32-.0 0.34) but it is too small to cations. reagent used for standardization of acid solutions and is readily available in the primary standard grade. to H2CO3.99 g HCO3. phenolphthalein (ph).+ H2O 100 0. Both of these indicators are chosen since For this experiment. After the first endpoint has been unknown with the HCl titrant. the HCl titrant 0. primary standard and must be standardized to determine its exact concentration. This would not only push Constituents in Sample an Equal Volume of the equilibrium forward. From these statements. Also. H2O(l) + CO2(g) -> H2CO3(aq) bcg This would give a positive error to the final amount of CO32. giving a positive error. Temperature changes the equilibrium constant. along with the indicator error which is also the titration reaction for the produced by the indicators.1 Determination of Mixed Alkali Page 3 of 4 . Titration curve of CO32-. The constituents present in the solution can be In the first titration reaction: determined based on the relative volumes of acid used in the titration of the mixture.as one of its products. Titration curve of CO32-. When the endpoint is reached. and we The second titration reaction: cannot fully prevent the loss of dissolved CO 2 resulting in a negative error. which dissociates. CO2-free distilled water prevents that problem. the solution is cooled. with HCl. carbonate. Not doing this leads to an error. only CO2-free distilled water must be used since dissolved CO2 in water produces carbonic acid. titration inside Relationship between Vph and Vbcg in the Titration of an ice bath is recommended. ph enhancing the sharpness of the pH break.is higher at low temperature. which both indicator standardization reaction is expected to change from a ranges border the pH breaks they detect. showing the effect of boiling the solution before second equivalence point gain or CO2 loss. with HCl. If the solution remained blue.will predominate in the solution. (CO2) gas formed by CO32. volume relationships in the analysis of mixtures This reaction is a neutralization reaction and therefore. then the endpoint has been detected. bcg but if the solution becomes yellow.determined. carbonate or hydrogen carbonate Ions for a double- indicator method It is important that for the whole experiment. producing CO32. the solubility of the carbon dioxide carbonate ions. but Sample also prevent a decrease in CO32.concentration. Even though we employ methods to prevent CO 2 Figure 2. After cooling. containing hydroxide. boiling the solution would remove produced and dissolved CO2. making a higher pH break. we cannot fully protect the solution from the atmosphere. preventing the slow change in pH. Figure 3 summarizes the pink color and HCO3. Also. and therefore the equivalence point so it is important to cool the solution first before continuing titration in order to obtain the proper equivalence Figure 1. HCl(aq) + NaHCO3(aq) -> NaCl(aq) + CO2(aq) + H2O(l) Another error would be the misreading of the color change at endpoint. Volume relationships in the analysis of mixtures containing hydroxide. Chemistry 27. Na2CO3 Vph > Vbcg Na2CO3. showing indicator ranges point. This also removes the buffer produced by H2CO3 and HCO3-. yellow to a blue color in solution and H 2CO3 will predominate and will decompose into CO2 and H2O. and hydrogen is exothermic. After confirming the HCl(aq) + Na2CO3(aq) -> NaHCO3(aq) + H2O(l) presence of the species. of phenolphthalein and bromocresol green the analyte is titrated further until endpoint. Also. Vbcg > 0 NaOH. to prevent any CO 2 to be dissolved in the solution all containers must be sealed at all times. the concentration of the respective components can be known from the the analyte is expected to become colorless from a amounts of acid consumed. NaHCO3 Vph < Vbcg Figure 3. NaOH Vph > 0 Vbcg = 0 Na2CO3 Vph = Vbcg H2CO3(aq) + ∆ à H2O(l) + CO2(g) NaHCO3 Vph = 0. 47) is nearer to the pH at equivalence point (4. twice as large as in the case of simple titration with one end point. Warder titration - simultaneous determination of sodium hydroxide and carbonate.1 Determination of Mixed Alkali Page 4 of 4 . NJ. September 6).titrations. such as sodium carbonate with the acid titrant in order to determine its concentration. This standardization is done through the titration of a primary standard analyte. or produced during titrations is also a source of error. The error that is produced from the standardization of HCl is carried over to the determination of unknown. with a standardized acid titrant such as hydrochloric acid. The final value for this experiment has a relative error of around 3% compared to the theoretical data. Many more sources of error can be obtained. G. solutions since they are also acidic or basic. Renlyn Ivy D.titrations. even in a PAYNAGANAN. USA: Prentice Hall. so it is very important to be careful when doing the experiment. Conclusions and Recommendations Determination of mixed alkali is done through acid-base titration of an unknown alkali mixture.info/acid-base-titration- sodium-hydroxide-and-carbonate ChemBuddy. such as a carbonate and bicarbonate mixture. stacking to the final error LOGRONIO. small amount can cause errors in the final values obtained. which means final error can be contribution to this report. (2011. References ChemBuddy.info/titration-errors Chemistry 27.8) and would give a lower indicator error. VII. (2009.a wrong amount of indicator can be a cause for error Hargis. Alvin J. Retrieved from: http://www.sources of errors. Titration and titrimetric methods . This error can be minimized by better controlling the environment to prevent CO2 loss or addition or by changing bromocresol green to methyl orange since its pKa (3. L. obtained. These volumes would then determine what the components of the mixture are and how much of each component is in the mixture.03) compared to bromocresol green (4.G. Analytical chemistry: principles because indicators also contribute to the pH of the and techniques. Air locks that are hidden. May 25). Error that is obtained from not fully transferring 100% of the solids or liquids to other containers is very common. VIII. Retrieved from: http://www. in order to prevent them. Another important point is that both calculations of amount of carbonate and hydroxide are based on We hereby certify that we have given substantial two measurements. Accidental spills and leaks from burettes. (1988). An unknown is then titrated against the acid titrant with two indicators either on one setup or on different ones to obtain the volume needed in order to reach each of the equivalence points for the base.