ANALYSIS OF CARBONATE MIXTURES

Transcription

1 ANALYSIS OF CARBONATE MIXTURES AIM The main objective of this experiment is to determine the constituents in a solution containing Na CO 3, NaHCO 3 and NaOH alone or in a mixture by using titrimetry. INTRODUCTION Qualitative and quantitative determinations of the constituents in a solution containing Na CO 3, NaHCO 3 and NaOH alone or in a mixture provide an interesting example for neutralization titrations. It is obvious that no more than two of these constituents can be present in a solution together since the reaction between some of them eliminates the third one. Mixing of NaOH with NaHCO 3 results in the formation of Na CO 3 until one or the other (or both) of the original reactants is used up. NaOH + NaHCO 3 Na CO 3 + H O Analysis of such mixtures requires the use of two indicators in their titrations with standard acid solutions. An indicator with a transition range in the ph range of 8 to 10, such as phenolphthalein, is used for detecting the first end point; an acid range indicator such as bromocresol green is used for the second end point. Titration can be done A) In one sample, using both indicators by successive addition of acid solution after each color change. B) Using two samples containing exactly the same amount of mixture solution and applying two indicators separately. The composition of the solution can be obtained using the volumes of acid used for reaching the each end point as shown in calculation part (Figure 1 and Table 1). Table 1. Volume relationship in the analysis of carbonate and carbonate mixtures in the titration of one sample. Composition Relationship between volume of acid needed to reach the phenolphthalein end point, (V ph) and bromocresol green end point, (V bcg) a) NaOH V phth>0, V bcg=0 b) Na CO 3 V phth = V bcg c) NaHCO 3 V phth=0 V bcg>0 d) NaOH and Na CO 3 V phth > V bcg e) Na CO 3 and NaHCO 3 V phth< V bcg 1

2 Figure 1. Titration curves for the analysis of mixtures containing sodium hydroxide, carbonate, and hydrogen carbonate mixtures. (a) NaOH, (b) Na CO 3, (c) NaHCO 3, (d) NaOH and Na CO 3, (e) Na CO 3 and NaHCO 3 Here titration using one sample will be discussed. REAGENTS AND APPARATUS Unknown NaOH, Na CO 3, NaHCO 3 solutions ( replicates for each student) Bromocresolgreen, phenolphthalein indicators (ready) Buret Erlenmeyer flasks of 50 ml 100 ml graduated cylinder Wash bottle PROCEDURE A. Titration of Solutions with Known Composition but Unknown Quantity In this experiment, 5 unknown solutions ( replicates from each one) will be given to each student. The list of unknown solutions is as follows:

3 Composition 1 NaOH alone Na CO 3 alone 3 NaHCO 3 alone 4 NaOH and Na CO 3 5 Na CO 3 and NaHCO 3 Obtain two replicate solution with known composition but unknown quantity from your assistant and to each unknown solution, the following procedure will be applied: 1) Add ml distilled water using a graduated cylinder and 5-6 drops of phenolphthalein. ) Titrate the solution with the standard HCl solution (standardized in the previous neutralization titration experiment) until the pink color of indicator disappears. This corresponds phenolphthalein end point (V phth). 3) Then, add 4-5 drops of bromocresol green (blue) and carry the titration with the standard HCl solution until a permanent green color appears. The volume of HCl used starting from the first end point (see Fig. 1) gives the amount of HCl consumed to reach the bromocresol green end point (V bcg). After the observation of permanent green color, the sample solution is heated. By this way buffering effect of HCO 3 - may be prevented and end point may be seen more easily. If there is CO in the solution, the indicator should change from green color to blue as CO is driven off during heating. When you see blue color, titrate this solution with 0.1 M HCl till green color is observed and this additional amount of HCl has to be added to V bcg to determine end point. B. Titration of a Solution with an Unknown Composition and Unknown Quantity 1) Obtain two replicate unknown solution from your assistant: ) Apply the same procedure that is given in Section A for unknown solutions. 3) Record the volumes of standard HCl solution which are used to obtain phenolphthalein, V phth, and bromocresol green end points, V bcg. 4) First determine the composition of the unknown solution using the relations given in Table 1 and then calculate the mass of the component(s) in your unknown solution for quantitative determinations. CALCULATIONS Calculations for Titrations of NaOH, Carbonate and Carbonate Mixtures Once the composition of the unknown solution is found then the mass of components in the unknown solutions can be calculated as follows; i. NaOH, NaHCO 3, Na CO 3 alone Mass of NaOH in mg = V phth x M HCl x fw NaOH Mass of NaHCO 3 in mg = V bcg x M HCl x fw NaHCO3 3

4 Mass of Na CO 3 in mg = ( V bcg + V phth ) x M HCl x fw Na CO 3 *For the analysis of Na CO 3 since V phth and V bcg are expected to be equal, the average of these values can be used in calculations. ii. For the mixture of NaOH and Na CO 3 Mass of NaOH in mg = (V phth V bcg ) x M HCl x fw NaOH Mass of Na CO 3 in mg = (V phth V bcg ) x M HCl x fw NaOH mg NaOH = (V phth-v bcg) x M HCl x fw NaOH mg Na CO 3 = V bcg x M HCl x fw Na CO 3 iii. For the mixture of Na CO 3 and NaHCO 3 mg Na CO 3 = V phth x M HCl x fw Na CO 3 mg NaHCO 3 = (V bcg V phth) x M HCl x fw NaHCO 3 where: fw: formula weight in mg/mmol PRE-LAB STUDIES Read Applications of Neutralization Titrations from the textbook. 1) Draw a titration curve for a polyprotic acid (any diprotic acid you can choose) with a strong base. Define all regions on the graph by writing related titration reactions. ) Explain why NaOH, NaHCO 3 and Na CO 3 cannot exist together and examine each case by writing related reactions. POST-LAB STUDIES 1) It is stated in Table 1 for the titration of NaHCO 3 V phth = 0; however, we consumed some small amount of HCl to observe the color change. State reason(s) for that event. ) Answer the following questions: a) Why do we heat the solution after observing the bromocresol green end point? Write the related reaction(s). b) What does the color change from green to blue indicate? c) If no color change is observed after heating, what does that indicate? d) If additional acid is not required after boiling, what does that indicate? In this case, what is the next step in the titration? 3) How did you decide the composition of your unknown? Sketch the titration curve corresponding to your unknown composition. 4

5 4) In the titration of NaOH and Na CO 3 mixture, three end points are expected; but there are two end points observed corresponding to phenolphthalein and bromocresol green indicator. Explain why? 5) The indicator Thymol Blue changes color (Blue to Yellow) over the same ph range as Phenolphthalein. Why is Phenolphthalein a better choice of indicator than Thymol Blue? 5

6 Name surname: Section: Date: Experimental Results: DATA SHEET ANALYSIS OF CARBONATE MIXTURES Concentration of HCl (standardized in neutralization titration experiment), M= Uknown composition Replicate Number NaOH 1 Na CO 3 1 Na HCO 3 1 NaOH+ Na CO 3 1 Na CO 3+ Na HCO 3 1 Unknown 1 Volume of HCl (ml), V phth phenolphthalein end point Volume of HCl (ml), V bcg bromocresol green end point Unknown Composition:.. True Values: NaOH Concentration, M Na CO 3 NaHCO 3 Uknown composition NaOH Volume, ml Na CO 3 Na HCO 3 NaOH+ Na CO 3 Na CO 3+ Na HCO 3 Unknown Unknown Composition:.. TA`s Name and Signature: 6