Abstract
The concentration of an unknown can be easily found by a scientific method called titration. Ethylene-diaminetetraacetate commonly known as EDTA is a complexing agent used to measure metal ions by titration. The endpoint of a titration is determined by a metallochromic indicator, this is the process of coloring metals. The indicator used in this titration is eriochrome black T, which change color from blue to red when combined with a metal ion to form a complex ion.
Introduction
The total calcium concentration of an unknown water sample will be found by titrating with EDTA solution, EDTA reacts in a ratio of 1:1 with metal (Ca2+) ions. Calcium is measured by titration with EDTA, which complexes the Ca2+ of Mg2+ metal ion, M2+ + Y4- MY2-. The purpose of this titration is to determine accurately the molarity of EDTA standard. At pH10 EDTA has a greater attraction to Ca2+; this is the ionic form of EDTA at pH 10(Gordon, Harman, Weiss, Pettegrew, 2001). EDTA is used as a strong metal-binding agent in industrial processes and in products such as detergents, cleaning agents, and food additives that prevent metal-catalyzed oxidation of food (Harris, 2007). According to Harris (2007) EDTA is the most widely used chelator in analytical chemistry, virtually every element in the periodic table can be measured with EDTA. The equation that will be used to determine the concentration in units of molarity of Ca2+ in the unknown sample is: MCa VCa = MEDTA VEDTA. It is known that EDTA – Ca2+ titration is a 1:1 titration. To obtain the concentration of Ca2+ unknown samples in ppm, the equation used is: ppm 〖CaCO〗_3=((M_EDTA)(V_e in L) (100.09g 〖CaCO〗_3/mol))/((L of unknown)) (〖10〗^3 mg/g) Experimental Procedures
The standard solution was prepared by accurately weighing 0.5g of dried to constant weight, pure CaCO3 into a 20-mL beaker. The weight of the powder plus the beaker is recorded before heating. When the CaCO3 has been heated to constant weight, it is transferred to a 150-mL beaker and 25-mL of distilled are added, then 1.5-mL of concentrated HCl are carefully added. The standard solution is covered until dissolved completely. The standard solution is transferred as accurately as possible to a 500-mL volumetric flask, and added distilled water to make up to 500-mL. The EDTA solution is prepared by weighing out approximately 4g of reagent grade disodium EDTA into a 250-mL beaker. Add 0.1g of magnesium chloride, five pellets of NaOH, and about 200-mL of distilled water to dissolve. The EDTA is stored and transferred to a 1-L bottle, and distilled water is added to fill the bottle. The titration procedure was conducted by pipetting three 25-mL ±0.03mL aliquots of standard calcium solution into 250-mL Erlenmeyer flasks, added 4-mL of ammonium chloride buffer (pH 10), and added 6 drops of Eriochrome Black T indicator solution. All three samples of the standard calcium were titrated with the EDTA solution until the color changes from violet through wine-red to blue. A calcium unknown was obtained by the instructor, and three samples of 25-mL ±0.03mL were accurately measure and placed into 250-mL Erlenmeyer flasks. 4-mL of pH 10 buffer, and 4 drops of indicator solution were added to each flask.
Data
Weight of calcium carbonate standard 0.5079 g ± 0.0001g
Molarity of CaCO3 primary standard 0.01 M

Standardized Titration with EDTA Titration Trial 1 Titration Trial 2 Titration Trial 3
Buret reading at start of titration (mL) 0 mL 28.10mL 15 mL 0 mL
Buret reading at end of titration (mL) 28.10 mL 50.15mL 21 mL 28.1 mL
Volume of EDTA solution used (mL), Ve 28.10 mL ±0.05mL 28.05 mL ±0.05mL 28.1 mL ±0.05mL
Molarity of EDTA (MEDTA) 8.89 x 10-3 M 8.91 x 10-3 M 8.90 x 10-3 M
Average molarity of EDTA 8.9 x 10-3 M

Titration of Unknown Sample Titration Trial 1 Titration Trial 2 Titration Trial 3
Buret reading at start of titration (mL) 0 mL 28.10mL 15 mL 0 mL
Buret reading at end of titration (mL) 11