QUANTITATIVE DETERMINATION OF TOTAL HARDNESS IN DRINKING WATER BY COMPLEXOMETRIC EDTA TITRATION DARLENE M. ROXAS1 and JULLIE ANNE B. SALGADOS 2 1,2NATIONAL
INSTITUTE OF MOLECULAR BIOLOGY AND BIOTECHNOLOGY UNIVERSITY OF THE PHILIPPINES, DILIMAN, QUEZON CITY 1101, PHILIPPINES DATE SUBMITTED: 31 JANUARY 2014 DATE PERFORMED: 23 JANUARY 2014
ABSTRACT
The determination of water hardness is a useful test that provides a measure of quality of water for households and industrial uses. The hardness of water is defined in terms of its content of calcium and magnesium ions. The objective of this experiment is to apply the concept of complexometric titration in the determination of total hardness in drinking water. The working standard CaCO3 solution was added with buffer solution and EBT indicator for the standardization of the EDTA titrant solution. The analysis of water sample involved the titration of a measured volume of mineralized water of the brand Viva! (plus buffer solution and EBT indicator) with the EDTA solution.A color change from wine red to clear blue indicates the titration endpoint. The computed total hardness as CaCO 3 ppm was 12.23 ppm which is classified as hard water, while the total cation content is 192.57 ppm.. Relative standard deviation was computed and it resulted to 41.68 ppm and confidence interval at 95% confidence level is[112.27 ppm, 138.19 ppm]. The total hardness proves the feasibility of the information about the quality of Viva! as a reliable source of water The tap water was also analyzed and the obtained total hardness is 62.15 ppm which is classified as moderately hard water.Certain errors during the experiment may have caused by the intrinsic error of the method and typical random errors including human error.
INTRODUCTION
One of the factors that establish the quality of a water supply is its degree of hardness. The hardness of water is defined in terms of its content of calcium and magnesium ions. Hardness in water is caused by the presence of a variety of certain dissolved polyvalent metallic ions in solution in water― predominantly calcium and magnesium, although other ions for example, aluminum, barium, iron, manganese, strontium, and zinc, also contribute. The source of the metallic ions are typically sedimentary rocks, and the most common are limestone (CaCO3) and dolomite (CaMg(CO3)2).[1] The determination of water hardness is a useful test that provides a measure of quality of water for households and industrial uses. Hardness is of concern in domestic water consumption because hard
water increases soap consumption, leaves a soapy scum in the sink or tub, can cause water heater electrodes to burn out quickly, can cause discoloration of plumbing fixtures and utensils, and is perceived as less desirable water. In industrial water use, hardness is a concern because it can cause boiler scale and damage to industrial equipment.[2] The total amount of hardness in water is expressed as the sum of its calcium carbonate (CaCO 3) and its magnesium hardness. However, for practical purposes, total hardness is expressed as parts per million of calcium carbonate. This means that regardless of the amount of the various components that make up hardness, they can be related to a specific amount of calcium carbonate. That is, hardness is expressed in mg/L or ppm of CaCO3. The mg/L of Ca and Mg must be converted into mg/L as CaCO3 before they can be added. The hardness of water can be classified based on the table below.
Table 1. The water hardness scale. Water Hardness ppm CaCO3 Soft 0-20 Moderately soft 20-60 Moderately hard 61-120 Hard 121-180 Very hard >180
In this experiment, total hardness in drinking water is quantitatively determined by complexometric titration using EDTA as titrant. IUPAC has defined 'complexometric titration' as a titration based on reaction of a metal ion with a ligand to form a soluble complex and in which one of the two reactants is used a titrant.[3] Since this type of titration depends upon the combination of ions (other than H + and OH-) to form a soluble and involved treatment of complex ions such as magnesium, calcium, iron, lead and zinc, some of its application includes determination of ion content in a living cell and introduce these ions into cell in case of deficiencies. Currently, this type of titration is widely used in the medical industry because of the micro liter size sample involved [4]. Water hardness can also be determined using different instruments. A colorimeter is a device that measures the absorbance of particular wavelengths of light by a specific solution. The rate of absorbance is directly proportional to the concentration of the solution given by the Beer-Lambert Law. The concentration of specific ions can be determined using this. Another method is the use of test strips. This chemically impregnated pads change to different color when reacted to a specific ion. There is a color chart that represents color and their various concentrations. [5] The objective of the experiment is for the student to be able to apply the concept of complexometric titration in the determination of total hardness in drinking water. For the standardization of the 0.01 M EDTA solution, 10 mL of the 0.0050 M working standard CaCO3 solution was measured and placed into each of three 250-mL Erlenmeyer flasks, then added with 75 mL distilled water. Three mL of the NH3-NH4+ buffer solution was put into the solution, followed by 2-3 drops of the EBT indicator. indicator. Titration of the the solution
with the standard EDTA solution followed immediately after, which must be done until a color change from wine red to clear blue is observed. In the analysis of the Viva! Mineralized water sample and tap water sample, 50 mL of the both water sample was measured into each of three 250 mL Erlenmeyer flasks. The same procedure from the standardization process was followed. Note that the type of sample water used is mineralized or spring water, since natural minerals (e.g., calcium, magnesium, iron) are still present. Purified water and distilled water cannot be titrated since contaminants, including minerals all undergone different stages of filtration process, so using this kind of water would not form any complex ions when titrated with EDTA. RESULTS AND DISCUSSION
Upon preparation of stock EDTA solution, MgCl 26H2O crystals were added to the dissolved salt EDTA. The fact that Mg-EDTA has a higher formation constant hence a higher tendency to form complexes than CaEDTA, Mg2+ from EDTA solution can easily displace Ca2+ ions and form Mg-EDTA complex.
Ca 2 + EDTA4 CaEDTA2 K f 5.0 x1010 Mg2 + EDTA4 MgEDTA2 K f 4.9 x108 Since Ca-EDTA has a lower K f , formation constant, Ca-EDTA is less stable because there is a greater energy for the complex to form, so adding more Mg 2+ ions will make the endpoint sharper. On the other hand, EDTA is essentially insoluble in water, and will only dissolve when pH is neutralized to 8. Addition of base, in this experiment NaOH pellets, facilitates dissolution of acid form of EDTA. Carbonate error can cause discrepancy in pH reading so adding HCl while dissolving CaCO 3 during the solution preparation is important for all reactions between metal ions and EDTA are pH dependent, and for divalent ions, solutions must be kept basic (and buffered) for the reaction to go to completion [6]. Most ligands are basic and bind to H + ions throughout a wide range of pH. Some of these H + ions are frequently displaced from the ligands (chelating agents) by the metal during chelate formation,so buffer was used to hold the pH constant. In this experiment, NH3-NH4Cl buffer was used since EBT
indicator only works when the pH is at 8 to 10, whereas the buffer has a pH equal to 10 [7]. The concentration of the two most common hard water ions, Ca2+ and Mg2+ was determined by titrating the water sample with a known chelating agent, ethylenediaminetetraacetic acid (EDTA). It was standardized with CaCO 3 thrice and the resulting average concentration of EDTA is 0.0069 M. During standardization, purple color appeared upon addition of EBT indicator compared to the expected wine red color of the solution. This may happen since there are no Mg2+ ions in the solution, and the indicator only exists as wine-red complex due to the magnesium ions. Mg 2+ + HIn 2- MgIn +H Viva! Mineralized water was titrated with the standardized EDTA. The starting color sample upon addition of EBT indicator was wine red due to the existence of Mg 2+ ions (3), and upon titrating with EDTA, the color gradually changes from wine red to clear blue, indication that the endpoint is reached, which occurs when Ca 2+ ions complexes with EDTA (1), the same time the Mg 2+ complexes with it (2) sequentially and the Mg-EBT complex breaks as illustrated in the equation. 4MgIn - +Y4+H3O + MgY2- +HIn2- +H2 O
( red - wine)
( clear blue)
Table 2 below shows the titration data of Viva! Mineralized water with standardized EDTA. The computed average hardness from three trials is 125.23 ppm (see Appendix B for calculations) which can be classified as hard based from water hardness scale. Q-test was first conducted and all values are accepted. The standard deviation computed is 5.22 ppm while the relative standard deviation is 41. 68 ppt. The confidence limit was also computed with confidence level at 95% and the resulting interval is [112.27 ppm, 138.19 ppm]. Table 2. Viva! Mineralized Water Analysis Titration Data Trial 1 2 3 Water 50 mL 50 mL 50 mL sample Final vol of 8.9 mL 18.4 mL 27.2 mL EDTA Initial vol of 0 mL 8.9 mL 18.4 mL EDTA
Net vol EDTA Total Hardness
8.9 mL
9.5 mL
8.8 mL
122.93 ppm
131.21 ppm
121.55 ppm
The total cation content of the water sample based from label’s given Ca 2+ and Mg2+ content was also computed by adding the total CaCO3 ppm and total CaCO3 ppm from MgCO3 is 192.57 ppm, which can be classified as very hard water. Comparing this with respect to the data obtained from EDTA titration, it claims to have a higher Ca 2+ and Mg2+ content when the computed total hardness from complexometric titration appears to be less. The large difference of the total hardness of the two may due to different errors arose during the titration or preparation of the solution. The discrepancy may be caused by several factors. Overtitration of the sample analysis is the common source of error since the detection of blue endpoint is not distinguishable to purple color. The delayed change of color of the solution can also be the cause of overtitration. It is also important to note that the indicator used must be suitable for the type of titration, which in this experiment we used EBT since it selectively binds with the Mg2+ ions. Neglect to add buffer can also cause error in the concentration of the standardized EDTA since it is very sensitive to pH change, so it is also important to note that proper selection of kind of buffer based on each pH range, in which in this experiment, is at 10. Another titration was done to determine the hardness of tap water. Table 3 below shows the titration data of the tap water titrated with standardized EDTA and the computed average total hardness as CaCO 3 ppm is 62.15 ppm which is classified as moderately hard based from table 1. Table 3. Tap Water Analysis Titration Data Trial 1 2 Water sample 50 mL 50 mL Final vol of EDTA 4.5 mL 9.0 mL Initial vol of 0 mL 4.5 mL EDTA Net vol EDTA 4.5 mL 4.5 mL Total Hardness 62. 15 ppm 62.15 ppm
Compared to the commercial mineral water’s total hardness, it is as half as much of it. But daily consumption of this kind of water is not advisable because it may contain chloride (to kill germs) that can easily react with other organic materials, and other alkaline substance (to maintain pH and prevents corrosion of pipes).
REFERENCES
[1] Selinus, O. Essentials of Medical Geology: Revised Edition. Edition. 2013. New York City: Springer. [2] Spellman, F. The Science of Water: Concepts and Applications, Second Edition. 2000. Boca Raton, Florida: CRC Press. 302.
SUMMARY AND CONCLUSIONS
The standardized EDTA resulted to a 0.0069 M after titrating it thrice with primary standard CaCO 3. Viva! Mineralized water was used as sample and has an initial Ca2+ content of 54 ppm and Mg2+ content with 14 ppm. The water sample was titrated with standardized EDTA using EBT indicator. The selection of the indicator was based on the range of its pH which is 8 to 10. NH 3-NH4Cl buffer with pH at 10 was used to maintain the pH of the solution since EDTA is pH dependent. After three trials of titration, the computed hardness of water are 122.93 ppm, 131.21 ppm, 121.55 ppm and has an average of 125.23 ppm. The values are checked using Q-test to eliminate outliers, and in this set, suspected outliers are accepted. The computed relative standard deviation is 41.68 ppm. The calculated confidence interval at 95% confidence level is [112.27 ppm, 138.19 ppm]. The total cation content based from the label of the sample water was also computed for comparison, and it resulted to 192.57 ppm, much way larger than the data obtained from the titration. The large difference in the value may be caused by overtitration or inaccurate solution preparation that may have affected some parameters including the concentration of the standardized standardized EDTA and the total hardness based from the titration data. Moreover, the two calculated hardness have showed the level of hardness of Viva! Mineral water which ranges from hard to very hard. The hardness of water only shows that pertinent ions, like Ca 2+ and Mg2+, are present and are good for daily consumption. On the other hand, another titration was done to test the hardness of the tap water. It resulted to a total ion content of 62.15 ppm as CaCO 3 ppm. It was classified as moderately hard water, which means few minerals or ions are present in the water, which in only true since this running water are all treated to filter off contaminants, in which some of the minerals may have been filtered off during the process. pr ocess.
[3] Iqbal, S., Setii, M. An Introduction to Analytical Chemistry. 1994. Chemistry. 1994. Delhi: Discovery Publishing House. 143-144. [4] . N.p.. Web. 29 Jan 2014. .
[5] "Measuring Water Hardness." www.lamotte.com . LaMotte Company. Web. 29 Jan 2014. . [6] . N.p.. Web. 29 Jan 2014. . [7] Andersen, Robert Arthur. Algal Arthur. Algal Culturing Techniques. 2005. Burlington, USA:Elsevier Academic Press. Press. 53-54
APPENDIX A – Tables Table 1. The water hardness scale. Table 2. Viva! Mineralized Water Analysis Titration Data Table 3. Tap Water Analysis Titration Data Table 4. Standardization of EDTA solution Titration Data Trial 1 Working Ca(II) standard 10 mL Final volume of EDTA 7.5 mL Initial Vol of EDTA 0 mL Net Vol of EDTA 7.5 mL M of EDTA 0.0068 M
2 10 mL 14.6 mL 7.5 mL 7.1 mL 0.0071 M
APPENDIX B – CALCULATIONS Calculations
A. Standardization of EDTA Solution
1 m ol 1 m ol Ca 2 m ol Ca Ca 0.50 41g 99 .97 g 1 mol C aC O 3 0.005041 mol Ca 2 2
M of Ca
2
0.00504 1 mol C a 2 1000 m L 1 10 1 00 mL L = 0.005041 M Ca 2
M of EDTA (1) M o f E DT A (2 ) M o f E DT A (3)
0.005 04 1 M C a 2 10 mL 7.5 mL 0.005 041 M C a 2 10 m L 7.1 mL 0.005 041 M C a 2 10 m L 7.4mL
0.0068 M 0 .0 0 7 1 M 0.0068 M
Titer 0.0069 M EDTA 99.97 g /mol CaCO3
0.6906 g /L
3 10 mL 22.0 mL 14.6 mL 7.4 mL 0.0068 M
B. Analysis of Viva! Viva! Water Sample
ppm CaCO3 CaCO3(1) = =
T x mL EDTA V sample sample
g/L 8.9 mL 1000 mg 0.6906 g/ x
50 mL
1 g
= 122.93 ppm
ppm CaCO3 CaCO3(2) = =
T x mL EDTA V sample sample
g/L 9.5 mL 1000 mg 0.6906 g/ x
50 mL
1 g
= 131.21 ppm
ppm CaCO3 CaCO3(3) = =
T x mL EDTA V sample sample
g/L 8.8 mL 1000 mg 0.6906 g/ x
50 mL
1 g
= 121.55 ppm
1mol 1molCaCO3 99.97mg 39.99 g 1molCa mol
ppmCaCO3 54.00 ppm = 134.99 ppm
1mol 1molM olMgCO3 1mmolC olCaCO3 99.97mg CaCO3 24.305 g 1molMg 1mmolCO3 mmol
ppmCaCO3 14.00 ppm = 57.58 ppm
total ion content ppm CaCO3 +ppm CaCO3 from MgCO3 =134.99 ppm 57.58 ppm = 192.57 ppm
n
X X SD
2
i 1
n 1 2
2
2
122.93 125.23 131.21 125.23 121.55 125.23
= 5.22 ppm
3 1
RSD
SD
x1000 ppt X 5.22 x1000 ppt 125.23 41.68 ppt
Con Confide idence Limit imit X
tSD n
=125.23
4.30 5.22 3
125.23 12.96 125 Confidence Limit: 112.27 ppm 138.19 ppm
C. Tap Water Analysis
ppm CaCO3 CaCO3(1) = =
T x mL EDTA V sample sample
g/L 4.5 mL 1000 mg 0.6906 g/ 50 mL
= 62.15 ppm
x
1 g
