EXPERIMENT 2
TITLE : COMPLEXOMETRIC TITRATION OBJECTIVES OF THE EXPERIMENT : After completing the experiment, we are able to: 1.
Define
a complexometric titration.
2.
Describe
the reaction between a metal cation and EDTA.
3. Calculate the concentration the unknown solution given the titration data. INTRODUCTION: Many metal ions react with electron pa ir donors to form coordination compounds or complex ions. The formation of a particular part icular class of coordination compounds, called chelates, are especially well suited for quantitative methods. A chelate is formed when a metal ion coordinates with two (or more) donor groups of a single ligand. Tertiary amine compounds such as ethylenadiaminetetraacetic acid (EDTA) are widely used for the formation formation of chelates. EDTA solution is not only stable - it can be stored st ored for months - but it can be also prepared without a need o f standardization. First, First, EDTA can be obtained in the form pure enough. Second, after thorough thoro ugh drying its crystallic from has pretty well defined amount of water of crystallization. Most commonly used solutions are 0.01M (that is 0.01N - regardless of the fact that EDTA has four protons it always reacts with metal cat ions on a 1:1 base). However, depending on the needs (concentration of metal to be determined) it is possible possible to prepare and use EDTA solutions of concentrations ranging from 0.1M to 0.001M. Crystallic EDTA - in the form of either disodium EDTA dihydrate or anhydrous anhydro us disodium EDTA salt - has to be thoroughly dried out before solutions preparation. Also not e, that solution preparation is time consuming - EDTA dissolves in water very slowly.
Complexometric titrations with EDTA have been reported for the analysis of nearly all metal ions. Because EDTA has four acidic protons, the formation of metal-ion/EDTA complexes is 2+
dependent upon the pH. For the titration of Zn , one must buffer the solution to a pH of 10 so that complex formation will be quantitative. The reaction of Zn2+ with EDTA may be expressed 2+
as:
Zn
4-
2-
+ (EDTA) [Zn(EDTA)]
This reaction can be used to determine the concentration of zinc ion by using an EDTA 2+
standard solution and an indicator which can form a colored complex with Zn . The following two factors are important for determination of the co ncentration: i) The indicator used must form a co mplex with zinc (much more slowly) compared to the 2+
EDTA. Zn
2+
indicator complex should only form after all the EDTA has reacted with the Zn
cation. 2+
ii) Zn
indicator complex must have a different colour from the indicator. ³Eriochrome Black
T´ (EBT) indicator, fulfill all the requirements mentioned earlier and ca n be used for this purpose. This method of determination is the simplest way to determine the concentration of the metal because 1 mol of EDTA usually reacts with 1 mol of metal. METHODOLOGY: (A) Preparation of EDTA 0.1M solution. 1. 9.5g of disodium EDTA (dehydrate) salt was weighed approximately and it was transferred into a 250cm3 beaker. 2. It was dissolved with distilled water and diluted until the mark. It was stirred until homogenous.
(B) Standardization of EDTA solution. 1. The burette was rinsed with small amount o f the dilute EDTA solution it was drained, then the burette was filled to the mark. 2. 25.00cm3 of the calcium carbonate solution given was pipette into 250cm3 Erlenmeyer flask. 3
3. It was diluted to about 100cm with distilled water. 3
3
4. 2cm of magnesium complex solution, 5cm of buffer solution and 1-2 drops of (EBT) indicator was added. 5. It was titrated with EDTA solution until the dark colour turned blue. 6. The titration was repeated three times and the average concentration of the EDTA solution was determined. (C) The determination of Zn solution concentration. 3
2+
1. 25cm of Zn
3
solution given was pipette into a 250cm Erlenmeyer flask.
2. It was diluted with distilled water to approximately 100c m3. 3
3. 10cm of buffer solution and 1 drop of indicator was added. 4. It was titrated slowly and carefully with standard EDTA until the end point (dark red colour turned blue). 2+
3
5. The titration was repeated two or t hree times. The concentration of Zn in g/dm was calculated.
RESULT: (A)Preparation of EDTA solution Weight of EDTA salt: 9.5002g
(B) Standardization of EDTA solution Experiment
1
2
3
Initial burette 3 readings (cm )
0.00
0.00
0.00
Final burette 3 readings (cm )
25.30
25.30
25.30
Volume of EDTA 3 used (cm )
25.30
25.30
25.30
1
2
3
Initial burette 3 readings (cm )
0.00
0.00
0.00
Final burette 3 readings (cm )
27.40
26.30
26.30
Volume of EDTA 3 used (cm )
27.40
26.30
26.30
3
3
Average Volume of EDTA used: 25.30cm = 0.0253dm
(C) The determination of Zn solution concentration Experiment
3
3
Average volume of EDTA used: 26.67cm = 0.0267dm
CALCULATION: (A)Preparation of EDTA solution 1. Formula of EDTA: C10H16 N2O8
Molar mass = (12.0107×10) + (1.00794×16) + (14.0067×2) + ( 15.9994×8) = 292.24g/mol 2. No. of mole of EDTA: 3. Molarity of EDTA:
=
=
= 0.0325 mol
= 0.130 M
(B) Standardization of EDTA solution 2+
1. Concentration of Ca
2+
solution: Let a=EDTA, b=Ca
MaVa=M bV b 3
3
(0.130M)(0.0253dm ) = M b(0.10dm ) M b
= 0.0329 M
2. Amount of Ca2+ ion present in the Erlenmeyer flask: n = MV/1000 =
= 0.00329mol
3. Concentration of the EDTA solution: MEDTA =
=
= 1.285 M
(C) The determination of Zn solution concentration: 2+
4-
2-
1. Equation of the reaction: Zn + (EDTA) [Zn(EDTA)] 2+
1 mol Zn
reacts with 1 mol of E DTA
2. Concentration of Zn2+: 2+
Let a=EDTA b=Zn , MaVa=M bV b 3
3
M b=(1.285M)(0.0267dm ) / 0.1dm = 0.343M 2+
Concentration of Zn
3
3
in g/dm = 0.343 M × 65.409 g/mol = 22.44 g/dm
3. No. of mole of Zn2+ = 0.343M × 0.1 dm3= 0.0343mol 2+
1 mol of Zn
reacted with 1 mol of EDTA 2+
0.0343mol of Zn
reacted with 0.0343 mol of EDTA
4. Concentration of EDTA that used:
= 1.285M
DISCUSSION:
This complexometric titration is an example of classic titrimetry analysis. Classical methods of analysis such as titrimetric and gravimetric analyses are usually capable of very high precision and accuracy ± typically on the order of +0.1% or even better if done properly. However, there is always a tradeoff. Usually classical methods are slower and much less sensitive than modern instrumental methods of analysis such as ato mic absorption spectroscopy, gas and liquid chromatography, and mass spectrometry. In this experiment, we first have to find the unknown concentration of the calcium solution from the standardization by EDTA. We used at first 9.5002g of EDTA salt to give
0.13M of EDTA solution before diluted. After Part B is conducted, the concentration of calcium solution obtained is 0.0329 M and its amount calculated is 0.00329mol. After the addition of magnesium complex solution, buffer solution and 1-2 drops of (EBT)indicator into the standard solution, the colour changed to pink. A little of magnesium complex solution added to the calcium solution before the titration to initiate the rate of react ion between EDTA and the metal solution. After all, the average EDTA solution used to turn the dark colour to blue (end-point) in 3
Part B is 25.30cm with 0.000 deviation between its three consecutive repetition of the titration. The concentration of EDTA that determined in Part B is 1.285M. Meanwhile, in Part C, we want to determine the concentration of zinc so lution using the concentration of EDTA obtained previously. In this part, the average volume of EDTA used is 3
26.67cm with its deviation less than 3 parts per thousand which meant it is high accuracy 3
3
3
titration. The concentration of zinc solution is 0.34 3M in mol/dm and 22.44 g/dm in g/dm
unit. After that, we used the concentration of zinc to calculate again the concentration of EDTA used in this experiment, and we got the same value as before, which is 1.285M.
PRECAUTIONARY STEPS: 1.
Usually
an air bubble is present in the nozzle of the burette,it must be removed before
taking the initial reading. 2. Always read lower meniscus in case of colourless solution and upper meniscus in case o f coloured solutions. 3. Immediately add the buffer solution before titrating a sample to prevent the pH from changing.
4. Rinse all the apparatus that want to be used with the d istilled water followed by respective chemicals that to be filled in.
CONCLUSION: 1. Complexometric titration is the titration that involved it t itrant, a ligand, to reacts with the analyte, a metal ion, to form a complex, more specifically a chelate in this case. A chelate is a ligand that has two or more sites that bind to the central ion. 2. EDTA react with almost all metal cation. For t his experiment, it shows that EDTA react with calcium and zinc metal. 3. From the calculation, the concentration of the unknown calcium solution is 0.0329M and the concentration of the EDTA used is 1.285 M.
QUESTIONS: 1. Buffer solution must be used in titrations involving EDTA because we need to constant the pH value of the metal solution around pH10 so that the co mplex formation will be quantitative. 2. Other indicator that can be used in EDTA titration are EGTA and CDTA. REFERENCES: 1. D. A. Skoog, D. M. West, F. J. Holler, and S. R. Crouch, Analytical Chemistry: An Introduction, 7th ed. Chapter 15, pp. 345-381. 2. Vogel's Textbook of Qualitative Inorganic Analysis by Vogel, A.I., 3rd, Ed., Longman (1961) 444, 445. 3.Gary D.Christian: Analytical Chemistry Sixth Edition, by John Wiley and So ns Inc.
