Complexometri c Determination of Water Hardness
Pierola,Michelle A DR. ASMITA BDR!
Introduction:
In today’s lab, the goal was to determine the concentration of metal ion impurities in hard water using complexometric titration. This was to be achieved using EDTA, a disodium salt and chelating agent, and Eriochrome lac! T, an indicator for color ch ange. EDTA is standardi"ed first and then the titration is performed to determine the calcium con centration. Chemical Principle:
#omplexometric, or chelometric, titrations can be used to discover the water hardness or to discover metal ions in a solution. Ethylenediaminetetraacetic acid, also !now n as EDTA, is commonly used in complexometric titrations. In a complexometric titration, an electron donating group, also called a ligand, forms a complex ion with a metal ion. The exact point at which all the new bonds have been formed is determined by a metal indicator. In this lab, Erichrome blac! T is the indicator that can be used to determine calcium content in a solution. In this experiment, $.$$%& of EDTA will initially be prepared and then standardi"ed using a given calcium chloride solution. In this reaction, EDTA will combine with the calcium carbonate to form calcium chloride, a colorless complex. This complex need s to be held together using a buffer. In this case ammonia'ammonium chloride was prepared to be used as the buffer. Eriochrome blac! T was also prepared to be used as the indicator for color change. efore the titration, Eriochrome lac! T, initially blue, forms bonds with the calcium carbonate, and turns the solution a pin! color. Then, during the titration, EDTA is added and causes the existing bonds to brea!, forming a new colorless complex and combining with the existing color to create a purple color change. ()In'*a+ - &)- *a+
&In'*a+ - )(-*a+
At the endpoint of the titration, when the indicator is gone and EDTA has completely chelated the metal impurities, the solution will turn completely blue. EDTA%'*a+ - &In'*a+ - )(-*a+
()In'*a+ - &EDTA)'*a+
This experiment was to be repeated three times using a stoc! calcium ion solution and then three more times using an un!nown water sample. The hardness of the water can be calculated using the measurement of volume delivered. Procedure: •
•
repare /$$ m0 of $.$$%& EDTA solution $.1/23g of 4a)EDTA and dissolve in /$$m0 o 5tandardi"e the 4a)EDTA using a stoc! calcium ion solution
o
o
m0 of DI water to a )/$m0 flas! Add a magnetic stir bar and place flas! on magnetic stirrer Add 2m0 of ammonium chloride buffer and stir for 2$ seconds After titrating the flas! 3/m0, add 2 drops of Eriochrome lac! T
o
indicator solution. 5tir and continue titrating 8hen color changes from pin! to violet to blue, stop and record the
o o
•
Add 3$m0 of provided calcium ion stoc! solution *#a#6270 sol, and 2$
volume delivered 9epeat ) more times o Titrate the prepared, provided un!nown water sample with the EDTA solution Transfer )/m0 of the sample, and )$m0 of DI water to a )/$m0 flas! o Add a magnetic stir bar and place flas! on magnetic stirrer. o Add 2m0 of the ammonium chloride buffer o efore titrating, add % drops of Eriochrome lac! T, and continue stirring o Titrate solution with standardi"ed EDTA solution o 9ecord volume delivered o 9epeat ) more times o
Results :
5odium'EDTA used: $.1/23 g
Part 1
Part 2
Trial 3
Trial )
Trial 2
Trial 3
Trial )
Trial 2
Initial Volume of Titrations
;.$
/.2/m0
3).%/m 0
3$.1$m 0
3/.%
3%.%=m0
Final Volume of Titrations
23.$m0
2%.=m0
%$.%m0
)=.%3m 0
2)./
2).%=m0
Difference in Volume of Titrations
)%.=)m 0
)=.//m 0
)1.=/m 0
3<.13m 0
31.3m0
3<.$m0
Molarity of EDT:
3.
Calculations for Trial 1: !"Repeat for other trials#
3$.$m0 #a#62
3.$g #a#62
3 mol #a#62
3 mol 4a)EDTA
)%.=)m0 4a)EDTA
30 #a#62
3$$.3 g #a#62
3 mol #a#62
$.$$%$ &
)
$.$$22 &
2
$.$$2/ &
>$.$%$$&
$era%e Molarity:
$.$$2; &
&tandard
.$$%&-.$$22&-.$$2/& 2
De$iation:
'a 2EDTmean(
(.$$2;&
1%.$ ppt .$$$% ' .$$2;
)solute De$iation >>.$$
%$*.$$$%-.$$$2-.$$$ 372 .$$2;
Estimated Precision !ppt# > x3$$$
> 1%.$ ppt
Calculations Part 2: !"Repeat for other trials#
3<.13 m0
.$$2; &
4a)EDTA 4a)EDTA $.$)/$$0 30 #a#62 Ca*2 calculated !ppm#:
2 )/=./ ppm $era%e ppm Ca*2:
3$$.3g
#a#62 3 mol
#a#62 3 mol
4a)EDTA 4a)EDTA
#a#62
+ater ,ardness !ppm#
3 );=.1 ppm ) )%;./ ppm
3 mol
>
)/<.; ppm
>);=.1 ppm
Discussion:
?or the first part of the experiment, the results appear +uite practical. The volumes measured out do deviate from each other. The average molarity was calculated out to be .$$2;& which is almost accurate seeing as the solution made was supposed to be .$$%&. (owever, the estimated precision was much higher than what was to be expected. In the second part of the experiment, water hardness was calculated. The calcium concentration was found to be )/<.; @ 33.;. In conclusion, the results from this test were reasonable. In the end, I feel as though the results could have been more accurate. The color change is almost too slight and it was difficult to tell when the indicator was completely solvated since the color went from purple to blue. erhaps adding more of the indicator would have made the color change more obvious. During the lab, I remember being unsure as to whether it was ready or not, and because of that, may have added slightly more EDTA than necessary. This would explain the higher value for estimated precision. References :
?or details, the lab manual was referenced.