Preparation and Standardization of 0.010 M EDTA
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Lab #3 Preparation and Standardization of 0.010 M EDTA Jacob Lockhart Chem 212.02 Feb. 19th, 2009 Introduction Preparing standard EDTA solutions using disodium dehydrate EDTA is a common practice in analytical chemistry. Since it is usually in a high state of purity, its solution needs to be standardized by titration against a primary standard. For this experiment the primary standard is a known concentration of high purity zinc solution because it allows a faster titration and portrays a very sharp end point. Since zinc hydroxide (7.1 x 10-18 solubility product) begins to precipitate around a pH of 7, eriochrome black T indicator must have an alkaline buffer for the titration and to inhibit zinc hydroxide from precipitating out of solution. To solve this, ammonium will be added to create the zinc tetraammine complex (4.2 x 108 solubility product) which will not precipitate at pH of 10. The goal of this experiment is to create solutions of EDTA and primary standard zinc and titrate zinc with the EDTA in an ammonia-ammonium chloride buffer at a pH of ten, using eriochrome black T as the indicator, to standardize the EDTA solution. At the endpoint, the indicator forms a weaker chelate with the zinc ions than EDTA, which causes the color change to occur at the pH of 10 for these experimental conditions.
NaOH solution was added to the flask dropwise to raise the pH until a faint white color was observed. Then, 10 mL of 8.5M NH3/NH4Cl buffer, two drops of eriochrome black T indicator, and 50mL of deionized water were added to zinc solution, and the aliquot was ready for titration. A 50-mL buret was filled with the EDTA solution, and the zinc solution was titrated dropwise while swirling the flask until the color changed from purple to a pure rich blue color, which signified the endpoint. The color change was extremely sharp and was observed within a fraction of a drop. This procedure was repeated two more times with the remaining two aliquots. Results Table 1 describes the mass measurements for preparing the standard zinc and EDTA solutions. Table 2 includes results for each titration of the three aliquots. Table 3 holds the molarity of EDTA and zinc determined by the experiment. Table 1: Mass Measurements of High Purity Salts Disodium EDTA dehydrate Zinc metal Mass (±0.00001g) 3.6900g 0.5012g Table 2: Titration Results for Three Aliquots Run 1 2 3 Start volume in buret (±0.01mL): purple color in flask 0.00 15.19 31.00 End volume in buret (±0.01mL): blue color in flask 15.19 30.39 46.22 Amount EDTA required for to reach the Endpoint (±0.014 15.19 15.20 15.22 Sample
Other errors are inherent in the analytical balance because it can only measure out to its last significant digit. Also, errors could have occurred by using contaminated or dirty glassware, or leaving residual solid from the weigh boats and glassware after transferring the quantities. Simply making sure to use clean glassware can minimize the first, and the second can be solved by quantitatively transferring the analytes between containers and thoroughly washing the weigh boat or beaker with the solution it is entering. Another error in measurements could occur by human error in not estimating the volume on the buret accurately, which is inherent to the titration in this experiment. The best way to minimize human error is to be consistent with the estimations between hash marks on the glass buret. Conclusion The goals for this experiment were achieved by preparing solutions of zinc and EDTA, calculating the molarity of standardized zinc solution (0.01533(1) M), experimentally determining the standard molarity of EDTA solution (? = 0.010081 M EDTA) by titration, and calculating its standard deviation (s = 0.000011 M EDTA). In terms of precision, the relative standard deviation for the experiment was 1.04 ppt, which implies that the measurements were all quantitative, or precise. Literature Cited (1) Enke, Christie G.; The Art and Science of Chemical Analysis; John Wiley and Sons: 2001 p. 40.
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