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Material Type: Lab; Class: Quantitative Analysis; Subject: Chemistry; University: Georgia Southwestern State University; Term: Unknown 1989;
Typology: Lab Reports
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Chem 3250 Experiment #
I. Introduction Reference Method 27C-3, 27C-6, 27C-10, in Skoog et. al. 7th^ Ed. (Note- typically the method you are to use follows and the method in the book should be similar and might help add some insight into the procedure. If you deviate from the method given here be sure to mention this in your lab book).
The classic titration of soda ash (sodium carbonate) involves the titration of the sodium carbonate as a base with standardized HCl. The HCl can be standardized against previously standardized NaOH or more appropriately, against primary standard sodium carbonate. This offers the analyst an opportunity of analyzing the sample under exactly the same conditions as in the standardization and realistically all the procedure can be run sequentially before any calculations performed affording the opportunity of completing the experiment with only one equipment preparation. That is, sample of standards and unknowns can be weighed out into individual flasks and titrated one after the other with calculations of HCl and analyte concentration left for post experimental analysis. Until you are fairly secure in your titration abilities it may prove advantageous to do the calculations for the standardization first to confirm the precision of your technique.
The reaction of HCl with sodium carbonate is two fold as the neutralization of the Na 2 CO 3 involves two steps:
H+^ + CO 3 2-^ Ù HCO 3 -^ ( HCl + Na 2 CO 3 Ù NaHCO 3 ) H+^ + HCO 3 -^ Ù H 2 CO 3 ( HCl + NaHCO 3 Ù H 2 CO 3 )
Since the carbonic acid produced in the reaction is a weak acid it also contributes to the pH around the second equivalence point. In order to minimize this and produce a more
distinct endpoint the solution near the second equivalence point is boiled to break the carbonic acid down to CO 2 and water. At this point the pH increases and the subsequent titration of the remaining bicarbonate results in a faster decrease in pH. (see Figure 16-1, page 430, Skoog et. al 8th^ Ed.) and a sharper endpoint color change.
II. Preparation Obtain a sample of unknown sodium carbonate from your instructor as well as a sample of primary standard sodium carbonate of about 1 to 1.5 g. Typically the unknown requires a clean weighing bottle with your initials in pencil in the space provided. The weighing bottle should be inside a beaker, which is covered by a watch glass to help prevent contamination. If you have cleaned the weighing bottle be sure it is dry before submitting it as any added moisture will increase the drying time for your sample. Place a piece of paper inside the beaker along with your weighing bottle with your name and the unknown required written on it. This will help insure you getting the correct unknown and make it easier for you to find your sample among many others. You can use a second weighing bottle for your standard sodium carbonate and place it in the oven in the same beaker as your unknown if both are clearly marked. Dry your samples in the oven at approximately 115 for one to two hours. DO NOT dry longer than 2 hours. Be sure the top is open on the weighing bottle so moisture can escape. After the sample has dried you can close the weighing bottle and store it in your desiccator. While your sample is heating you can clean your buret and at least three 250ml flasks. Prepare a solution of approximately 0.10M HCl by diluting concentrated HCl to about 1000ml with water in a clean bottle and mix well. (If concentrated HCl is about 12M, how many ml of it will you use?) Note this solution will be very approximate. You will determine the concentration of this solution against the standard sodium carbonate.
III. Procedure
1. Standardization of HCl with primary standard sodium carbonate Weigh out three samples of the primary standard sodium carbonate into each of the three flasks (note % purity if specified on bottle for correction of sample mass). Be sure you have labeled the flasks so you can keep track of which sample is which. For this