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AIMS
1. To master the principles,methods and calculation of complexometric titration.
2.To know how to use indicators and their color change.
What is hard water,and why does it have this effect?Hard water contains a higher than normal concentration of calcium and magnesium ions,since most hardness is caused by carbonate mineral deposits.These ions form precipitates with soap,causing the build up of soap scum.Additionally,since the soap molecules are being precipitated by the Ca 2＋ and Mg 2＋ ions,there is less soap available to form lather.Taking a shower in hard water can be very frustrating!These cations form insoluble salts with a reagent in soap,decreasing its cleaning effectiveness.
Another effect of hard water is “boiler scale”.When hard water comes into contact with dissolved carbonates,a precipitate of insoluble calcium carbonate can form.This“scale”can build up on the inside of water pipes to such a degree that the pipes become almost completely blocked.The following chart shows how hard water is classified.For reporting purposes,hardness is reported as parts per million(ppm) of CaCO 3 (by weight).A water supply with a hardness of 100 ppm contains the equivalent of 100 g of CaCO 3 in 1 million g of water or 0.1 g in 1 L of water(or 1 000 g of water since the density of water is about 1 g/mL).In other words,even though both Ca 2＋ and Mg 2＋ contribute to water hardness,it is reported as though all hardness ions are Ca 2＋ from CaCO 3 .Since Ca 2＋ and Mg 2＋ behave exactly the same,this convention is convenient shorthand.
The hardness of a sample of water can be measured by determining the concentration of the dissolved Ca 2＋ and Mg 2＋ ions.The procedure you will use is called a titration.To analyze for Ca 2＋ and Mg 2＋ ions you will add a substance,Na 2 EDTA the chelating agent,which will react with the metal ions and remove them from solution.
Na 2 EDTA is a complex molecule.Its name stands for ethylenediaminetetraacetic acid-disodium salt.The formula of disodium EDTA is Na 2 C 10 H 14 N 2 O 8 with the formula mass of 372.24 g·mol- 1 .
Figure 2-9 Structure of disodium EDTA dehydrate(372.24 g·mol -1 )
The four acid oxygen sites and the two nitrogen atoms have unshared electron pairs,which can form bonds to a metal ion forming a complex ion or coordination compound.The complex is quite stable,and the conditions of its formation can ordinarily be controlled so that it is selective for a particular metal ion.EDTA reacts with Ca 2＋ and Mg 2＋ in a one to one mole ratio.
Figure 2-10 Structure of CaEDTA
In a titration to determine the concentration of a metal ion,the added EDTA combines quantitatively with the cation to form the complex.The endpoint occurs when essentially all of the cation has reacted.
Since not only EDTA but also Ca 2＋ and Mg 2＋ are colorless,it is necessary to use a special indicator to detect the endpoint of the titration.The indicator most often used is called eriochrome black T,which forms a very stable wine-red complex,MgIn- ,with the magnesium ion.A tiny amount of this complex will be present in the solution during the titration.As EDTA is added,it will complex free Ca 2＋ and Mg 2＋ ions,leaving the MgIn- complex alone until essentially all of the calcium and magnesium have been converted to chelates.At this point EDTA concentration will increase sufficiently to displace Mg 2＋ from the indicator complex;the indicator reverts to its uncombined form,which is sky blue,establishing the endpoint of the titration.
The titration is carried out at a pH of 10,in a NH
3
/
buffer,which keeps the EDTA mainly in the form HY
3-
,where it complexes the Group 2 ions very well but does not tend to react as readily with other cations such as Fe
3＋
that might be present as impurities in the water.The equations for the reactions which occur during the titration are:
Titration reaction:
Endpoint reaction:
Since the indicator requires a trace of Mg 2＋ to operate properly,it is suitable for determination of the total content of Ca 2＋ and Mg 2＋ ions.If Ca 2＋ ions are measured alone,you need to change the indicator to calmagite due to the insensitive response of eriochrome black without Mg 2＋ .
At this case,the titration is carried out at a pH of 12,in a NaOH buffer,toprevent the interference of Mg 2＋ ions.
● 50 mL burettes,250 mL conical beakers,50 mLvolumetric pipettes,10 mL graduated cylinder,droppers,rubber bulbs.
● Water sample,0.02 mol· L
-1
disodium ethylenediamine tetraacetic(Na
2
EDTA)solution,0.1 mol· L
-1
buffer (pH ＝10),1 mol·L
-1
sodium hydroxide (NaOH)solution (pH ＝12),5 g·L
-1
Eriochrome black T solution,5 g·L
-1
calmagite solution,distilled and deionized water
Wash a 50.00 mL burette thoroughly with water,and then rinse it with distilled water and a few mLs of Na 2 EDTA solution sequently.Drain through the stopcock and then fill the burette with the Na 2 EDTA solution.
Pipet three 50.00 mL water sample into three clean conical beakers.To each beaker,add 5 mL
buffer to control the pH to about 10.Add 3—4 drops of eriochrome black T solution as indicator,and you will find the solution turns into wine red.Read the burette to 0.01 mL and add Na
2
EDTA to the solution until the last tinge of purple just disappears.The color change is rather slow,so titrate slowly near the endpoint.Read the burette to 0.01 mL again to determine the volume needed.
Refill the burette,read it,and titrate the second solution,then the third.
Refill the burette,Record to the nearest 0.01 mL the exact liquid level of Na 2 EDTA in the burette.
Pipet three 50.00 mL water sample into three clean conical beakers.To each beaker,add 2 mL NaOH solution to control the pH to about 12.Add 3—4 drops of calmagite solution as indicator,and you will find the solution turns into wine red.Slowly add the Na 2 EDTA solution from the burette until the endpoint is reached.The endpoint is the point at which the solution in the conical beakers turns a distinct blue.Read the burette to 0.01 mL again and calculate the volume used in the titration by subtracting the initial volume from the final volume of Na 2 EDTA.
Refill the burette,read it,and titrate the second solution,then the third.
You will use V 1 , V 2 ,and c (Na 2 EDTA)＝0.02 mol·L -1
Total permanent hardness＝German degrees°dH (1°dH＝10 mg CaO·L -1 )
1.Why do you need to select a pH range incomplexometric titrations?
2.What can be primary standard to standardize the Na 2 EDTA concentration?
3.Water is usually softened by using an ion exchange resin to replace each Ca 2＋ (and Mg 2＋ )ion with 2 Na ＋ ions.What must be true of the Na ＋ salt of soap?