Experiment 3 - Determination of Mixed Alkali

Experiment 3: Determination of Mixed Alkali dela Cruz, Marie Giecel V. Pabilane, Alma Group 6, Chem 27.1, SEJ1, Ms. Noime Walican January 22, 2011 I. Abstract Alkalimetry is uses two indicators to determine the total amount of real alkali in solutions with mixed alkaline salts. The results can be expressed as %(w/v), and can be calculated from the molarity of the standard solution titrant. In this experiment, the mixed alkali solution was titrated with HCl to determine the amount of Na2CO3 and NaHCO3 in the solution. It was found that the unknown solution contained 0.5036% (w/v) Na2CO3 and 0.2172% (w/v) NaHCO3. II. Keywords: alkalimetry, double indicator titration, mixed alkali, %(w/v) Na2CO3, %(w/v)NaHCO3 III. Objectives In this experiment, the students must be able to: (1) Prepare a standard hydrochloric acid solution; and, (2) Determine the carbonate and bicarbonate content in a given sample. IV. Introduction and Principles Alkalimetry is the determination of the quantity of real alkaline in alkali solutions or salts, either through gravimetric or volumetric analysis. Most alkalimetric procedures are based on Gay-Lussac’s method, wherein a solution of carbonate is titrated with a solution of strong acid (HCl or HNO 3), following a series of steps until a color change is achieved. In this experiment, two indicators were used to measure the amount of carbonates and bicarbonates present in a solution of mixed alkali. In Figure 1, the reaction of carbonate with HCl produces bicarbonate and water. Further reaction of the bicarbonate with HCl will produce a chloride salt, CO2 and water.

Figure 2. A sample titration curve showing the probable results of using two indicators in titration. From Figure 2, it could be noted that after the first equivalence point, only HCO3- are present in the solution. This shows that, ideally, all CO32- have reacted with HCl and were transformed to NaHCO3. However, if NaOH is present in the solution, the titration curve will be slightly different because of the presence of the hydroxide.

Figure 1. The reaction of Na2CO3 with HCl and its further reactions.

V. Methodology and Materials A. Preparation of 0.1M HCl

Figure 2 shows the probable titration curve that could result from titration using two indicators, phenolphthalein and methyl orange. However, in this experiment, the other indicator was bromocresol green instead of methyl orange.

The volume of concentrated hydrochloric acid (HCl) needed to prepare 1L of 0.1M concentration was computed. A volume equal to the calculated amount was transferred into a 500-mL beaker with 300 mL CO2-free distilled water. The solution was mixed and transferred to a 1-L volumetric flask and diluted to the mark. The final solution was transferred to a 1-L amber bottle. B. Standardization of 0.1M HCl Five grams of primary-standard grade Na2CO3 was dried at 110°C for 2 hours and was cooled in a dessicator. After drying, three 0.20 to 0.21 g of the

Chemistry 156.1, Experiment 4: Surface Tension Page 1 of 2

dried sodium carbonate was weighed by difference and transferred to three labeled Erlenmeyer flasks. The weights of the samples were noted and recorded. Each sample was dissolved in 25 mL distilled water and 2 drops of bromocresol green was added as indicator. Each sample was titrated with 0.1M HCl until the color changed gradually from blue to green. After the color change, the solution was boiled gently for 2 minutes, until the blue color returned. The sample was allowed to cool while covered with a watch glass. The titration was continued until a sharp blue to green or yellow color change. The molarity of the titrant was calculated from the recorded volumes. C. Analysis of the Unknown The unknown was dried at 110°C for 2 hours and cooled in a dessicator. After drying, 0.20-0.21 g unknown was weighed and transferred to three numbered Erlenmeyer flasks. Twenty five milliliters of CO2-free distilled water was transferred to the flasks to dissolve the sample. Two drops of phenolphthalein was added as indicator. The solution was immersed in ice bath and then titrated with the standardized HCl until the pink color disappears. The solution was boiled gently for 2 minutes, until blue color returns. After boiling, the solution was cooled while covered with watch glass and then titrated with the standardized HCl until color changes from blue to green. The %(w/v) Na2CO3 and NaHCO3 as calculated for the sample. VI. Results VII. Discussion VIII. Conclusion and Recommendations IX. References Spon, Ernest. “Acidimetry and Alkalimetry”. American Library Edition of Workshop Receipts. Vol 2. Spon and Chamberlain, 1903. ChestofBooks.com: Read Books Online for Free. 22 October 2010. Web. Accessed 06 January 2011 http://chestofbooks.com/crafts/mechanics/Works hop-Receipts-2/Acidimetry-And-Alkalimetry.html

I hereby certify that I have given substantial contribution to this report,

Chemistry 156.1, Experiment 4: Surface Tension Page 2 of 2

ALCARAZ, ANNA LYNN C. DELA CRUZ, MARIE GIECEL V. DIANALAN, PUTRI ZAINAB UMEESALAM NUR-NASRIN R.