تقدير آيونات الخارصين بإستخدام معايرة ايديتا – Determination of Zn(II) by EDTA titration






Complexometric Titration of Zn(II) with EDTA

Unknown Solution

Submit a clean 250-mL volumetric flask to the instructor so that an unknown zinc solution may be issued. Your name, section number, and your locker number should be written legibly on this flask. Note that the flask must be turned in at least 1 lab period before you plan to do the experiment, so that the Teaching Assistants will have time to prepare the unknown

Preparation of Solutions

USE ONLY DEIONIZED WATER (NOT DISTILLED WATER!) IN THIS EXPERIMENT

1- EDTA 0.01 M

   Dissolve about 3.8 g of the dihydrate
of the disodium salt (Na
2H2Y•2H2O) and 0.1 g MgCl2 in 1 liter of water. Store
in a plastic bottle. A small amount of sodium hydroxide may be added if there
is any difficulty in dissolving the EDTA. Try not to exceed 3.8 g of
Na
2H2Y•2H2O because much more than this dissolves only with difficulty. The
EDTA solution should be filtered using suction filtration. See TA for the apparatus.

NOTE: Break the suction before you turn off the water flow on the vacuum
aspirator.

2. Buffer pH 10

  Each titration will require the addition of pH
10 ammonia buffer. The stock buffer solution has been prepared for you, and the
appropriate quantity is dispensed directly into your titration flask from the
“Repipet” repetitive dispenser located in hood #7. The buffer should
be added immediately before 
you titrate a sample.

3. Calcium Standard Solution 

  A Ca2+ solution is prepared as the primary
standard. Obtain from the TA approximately 0.7 g of predried
analytical-reagent-grade CaCO
3. Accurately weigh a 0.25-g sample by difference
into a 100-mL beaker. Add about 25 mL water and then add dilute HCl dropwise
until the sample dissolves, then add 2 drops more. Mild heating will speed
dissolution if necessary. Transfer quantitatively to a 250-mL volumetric flask
(rinse well with water) and carefully dilute to the mark. (eye dropper!) Mix
thoroughly. Because this 
Ca2+ standard solution is used to standardize the EDTA
titrant, it must be prepared very carefully so you know its exact molarity, and
therefore the exact (to ± 0.1 mg) mass of Ca
CO3 weighed out.


Standardization of EDTA Solution

1. Pipet 25-mL aliquots of the standard Ca2+ solution into three or four 250-mL Erlenmeyer flasks. Remember that each
aliquot contains one-tenth of the CaCO
3 weighed out to prepare the standard
solution.

2. Take each sample to completion before
starting next sample. Add 7-8 mL of pH 10 buffer, 15 mL of water, and 3 drops
of Eriochrome Black T and titrate immediately with EDTA until the light red
solution turns a LIGHT SKY BLUE. 

  Titrations must be performed swiftly (but
carefully) because ammonia will evaporate and thus the pH of the solution will
change. In general, the faster the titrations are performed the better the
results will be (as long as the endpoint is not overshot due to the speed). It
is advantageous to perform a trial titration to locate the approximate endpoint
and to observe the color change. In succeeding titrations, titrate very rapidly
to within about 1 or 2 mL of the endpoint, then titrate very carefully
(dropwise) to the endpoint. (See Note 1 at the end of the report.)

3. Calculate the molarity of the EDTA from the
volume of EDTA used in the titration of each aliquot. 




  The values (MEDTA and titration volumes) should
all agree very closely. If not, titrate additional aliquots until
agreement is reached, and any spurious values can be rejected with confidence

Determination of Zinc

1. Dilute your unknown sample in the 250-mL
volumetric flask to the mark with water. Mix thoroughly. 
                                      
2. Pipet 25.00-mL aliquots into 250-mL
erlenmeyer flasks. Add 15 mL of water, 9-10 mL of pH 10 buffer, and 2 drops of
Eriochrome Black T immediately prior to titrating a sample. 

3. Titrate with standardized EDTA until the red
solution turns blue. (See note at the end of the report.) 


4. Calculate the number of milligrams of zinc in
the total sample. Remember that each aliquot represents one-tenth of the total
sample volume.

= MEDTA x (V, mL EDTA) x (molar mass Zn) x 10

Notes 

1. Eriochrome Black T Indicator 

The color change of Eriochrome black T at the endpoint is rather subtle. It is not an abrupt chang from bright red to a dark blue; but rather it is from a light red (or pink) to a pale blue. At least one trial titration is recommended. (You can always discard a “bad” value when you know there is a definite reason for its being bad.) 

If you have trouble distinguishing the endpoint, a “before” and an “after” flask are recommended. Prepare two 250-mL flasks exactly the same as for the samples, except use distilled water instead of a sample; add additional distilled water to approximately equal the volume of EDTA titrant that would be titrated into the flask for the sample. Add the indicator. To one flask (the “after” the endpoint flask) add a small amount of EDTA to just past the color change at the endpoint. Stopper the flasks and keep them nearby for comparison of the colors. Titrate against a white background for better discrimination of colors (Extra flasks can be checked out from the stockroom) 

Sometimes the Eriochrome black T solution goes bad because of air oxidation. If the endpoints seem very indistinct to you, try a fresh bottle of indicator. Alternatively, try adding a small amount of solid Eriochrome black T mixture (1 g indicator ground with 100 g NaCl); a small amount on the end of a spatula is sufficient 


2. pH 10 Ammonia Buffer 

  Dissolve 64.0 g of ammonium chloride in 600 mL
of concentrated ammonia. Slowly and carefully add 400 mL deionized water. This
should be sufficient for over 120 titrations


Reference: D. A. Skoog, D. M. West, F. J. Holler, and S. R. Crouch, Analytical Chemistry: An Introduction, 7th ed. Chapter 15, pp. 345-381. 

شاهد أيضاً

إيجاد قيم ثوابت معادلة فاندرفالز Van der waal equation Constants

معادلة الحالة لفان در فالزvan der Waals Equation of State   – معادلة فاندرفالز هي معادلة …

اترك تعليقاً

لن يتم نشر عنوان بريدك الإلكتروني. الحقول الإلزامية مشار إليها بـ *