Purpose

   We shall now illustrate how a combination of analytical techniques can serve to simplify an otherwise difficult analysis. The unknown is a mixture of Cl^-,  H⁺, Na⁺, Mg⁺⁺, and  Zn⁺⁺, and an analysis for each constituent is desired.

From the above data the quantity of each ion present may be calculated (Na⁺ by difference)

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Apparatus

Reagents

Principle, Ion Exchange

   Ion exchange resins have found wide application in many scientific fields since their recent introduction. In analytical chemistry they provide a method of separation and of equivalent exchange by which it is possible to solve many difficult problems. This experiment utilizes the principle of equivalent exchange.

   An ion exchange resin is an organic high polymer which contains ionic functional groups. A typical resin for exchanging cations is illustrated by sulfonated polystyrene crosslinked with divinylbenzene.

 The resin is polymerized in the form of spherical beads of various sizes. The beads are placed in a long glass tube giving an ion exchange column in which the chemical separations  or equivalent exchange can be carried out. The high-molecular-weight polymeric material might be considered as a highly insoluble, immobile, polybasic anion. The percentage of crosslinking in the polymer is controlled (by the amount of divinylbenzne added) so that the resin molecule has a permeable network configuration through which water as well as ions are able to move.

   If a solution containing NaCl is poured through an ion-exchange column containing a sulfonated poly-stylene resin, the following reaction takes place:

where H Res represents the immobile polymer on the "hydrogen cycle". The sodium ions have been replaced by an equivalent amount of hydrogen ion. The important result is that although NaCl was poured into the column the effluent is HCl. The amount of NaCl that can be converted into HCl depends on the capacity of the resin and the amount of resin in the column. Once the resin has absorbed its maximum capacity of sodium ions, the equilibrium can be reversed and the resin regenerated. This is done by passing a rather concentrated solution of acid through the exchange column, resulting in the reaction.

Anion-exchange resins, in which basic amine groups instead of sulfonate groups are incorporated into the polymer, and also be prepared. If such a resin is on the hydroxyl cycle, various anions can be exchanged for the hydroxyl ions; e.g. ,

In this case the sodium chloride is converted to sodium hydroxide.

           Ion-exchange resins have many uses. They are widely used in water softening, in which the water is passed through a "mixed bed" consisting of both cation and anion exchangers. The cation resin is put on a sodium cycle, and the anion exchanger is on the chloride cycle.  In this fashion all cations are converted to sodium ion and all anions to chloride, thus completely removing the hardness of the water. When the resins have exchanged their maximum capacity of ions they can be reverted to the original condition (Regeneration) by passing a strong solution of NaCl through the "mixed bed".

          Ion exchange resins have been used in many aspects of analytical chemistry.

    In this experiment, a mixture of salts and acid are placed in the top of a column, and the acid eluted is titrated with base to give the sum of acid and salts present. This analysis allows the determination of one salt (NaCl), provided the analysis of the other constituents ( H⁺, Zn⁺⁺, Mg⁺⁺) is known. It is important to recognize that two moles of H⁺ are freed for each mole of divalent metal ions, such as Zn⁺⁺ and Mg⁺⁺. 

Chelometric Titrations :

Titrations with Ethylenediaminetetraacetic Acid (EDTA)

   The introduction of ethylenediaminetetraacetic acid,

as a titrant in analytical chemistry has done a great deal to simplify the analysis of metal ions.  This reagent and the other chelons are unique in that they form soluble but very slightly dissociated complex ions with many polyvalent cations and consequently can be used effectively as titrating agents.

    Procedures, for example, have been developed for the volumetric determination of alkaline earths, rare earths, Mn, Fe, Cu, Hg, Cr, Co, Ni, Zn, Ce, Pb, Al, Ga, In, Ti, Pd, Bi, Zr, Sc, and others; but the first and probably still the most practiced is the determination of hardness (I.e. , Ca⁺⁺ and Mg⁺⁺ ) in water.

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Titration of Zinc and Magnesium

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   Ethylenediaminetetraacetic acid, abbreviated as EDTA or H₄Y, forms a series of salts in neutralization with sodium hydroxide. One of these salts, Na₂H₂Y¤ý2H₂O, is commercially available in a high degree of purity and is used in the preparation of the titration solution.^*

^*By proper purification and drying, this compound can be prepared sufficiently pure to serve as a primary standard. See Blaedel and Knight, Anal. Chem., 26:741 (1954)

   When this solution reacts with zinc and magnesium ions, the reactions can be represented by the following equations:

Use of Metal Ion Indicator

  A very satisfactory indicator for the titration of the sum of Zn⁺⁺ and Mg⁺⁺ at pH 10 is Eriochrome Black T (also called CI203, its color index number).

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It is a tribasic acid and forms colored soluble complexes with both zinc and magnesium ions.  The magnesium complex with the indicator is more stable than the complex of zinc with the indicator but less stable than the magnesium EDTA complex. Thus during a titration, the EDTA reacts first with the free zinc ions, then with the free magnesium ions, and finally with the magnesium in the indicator complex.

       When the end point is approached, the solution turns from a pink to a purple to a pure  blue.  The final pure blue is the end point.  The position of the equilibrium in the above equation shifts to the left as the magnesium ions react with EDTA, thereby causing the solution to become pure blue when a quantity of EDTA equivalent to the magnesium content has been added.  In other words, titrate until all of the pink tinge (Mg ⁺⁺- indicator complex) is completely discharged and the addition of an extra drop of EDTA does not produce a color change. Since the magnesium indicator complex is wine red in color and th free indicator is blue between  pH's of 6.5 and 11.5,

the color of the solution changes from wine red to blue at the end point.  

   A pH of 10 is best  for  the  titration of the sum of zinc and magnesium.  In more alkaline solutions,  zinc and magnesium hydroxides may be precipitated, whereas  in more acid media, the magnesium is not bound strongly enough to the indicator to give the wine red compound.   The optimum pH can be readily obtained and maintained by the addition of a sufficient amount of an ammonia-ammonium chloride buffer.

Titration of Mgnesium Only

   For this titration the solution is buffered at a pH of 10 and an excess of KCN is added to complex the zinc so tightly that EDTA cannot titrate it.   In this way the magnesium is titrated alone.  The CN^- thus acts as a "masking agent" for Zn⁺⁺  ion.   By  the use of such masking agents, EDTA  can be used as a more selective titrant.  In a similar manner CN^- will mask  Ni⁺⁺, Co⁺⁺,  and  Cd⁺⁺,  and  triethanolamine will mask Fe⁺⁺⁺, Al⁺⁺⁺,  and  Mn⁺⁺,  Zn⁺⁺,  and Cd⁺⁺ can be selectively "demasked" by the action of formaldehyde.  

   Another method by which the EDTA titration can be made more selective is pH selection.  Since EDTA is an acid , the H⁺ concentration enters into the equilibrium expressions for the stability of the metal chelates.  Thus the alkaline earths which form relatively weak complexes with EDTA can only be titrated in alkaline solution where the H⁺ concentration is low

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and the above equilibrium is shifted to the right.   However, metals like Bi³⁺ which form very strong complexes with EDTA can be titrated in acid solution.    The common indicators are similarly affected, and thus it is possible to titrate many metal ions (rare earths, transition and heavy metal ions ) in acid solution without interference from the alkaline earths.

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Procedure

   Dilute the unknown aqueous sample to 100 mL.

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Determination of Acid and Salt Equivalents

    Place the 10 g of resin from the doggy bag in a 250 mL beaker.  To ensure that the ion exchange column is in the "hydrogen cycle",  add 25 mL of 1:1 HCl, swirl, and then let it set for five minutes.  Decant, then rinse the excess HCl from the resin by washing five or six times with 25-mL portions of distilled water. Discard washings. After the fifth wash, test a few drops of the wash solution with wide-range indicator paper. If the wash solution is more acidic than the distilled water, continue washing the resin with more distilled water.

   When you are satisfied that your resin is prepared properly, close the stopcock on your buret, insert a small glass wool plug, and add the resin to the buret. Pipet a 10-mL aliquot of your sample into your exchange column and collect the effluent in a clean 250-mL titration flask. Wash with five 25-mL portions of distilled water (1-2 mL/min). Do not permit the liquid level to fall below the tip of the resin bed. Add a few drops of mixed indicator to the collected effluent, and titrate with standard sodium hydroxide solution. This titration permits the determination of total acid and salt equivalent. (If you are not familiar with the color change, perform a test tube experiment to determine the colors of the acidic and basic forms of the indicator.)

   Repeat this process using a second 10-mL aliquot of your sample. When this portion of the experiment is comlete, place the spent ion-exchange resin in the waste reservoir designated for this purpose.

Determination of Acid Equivalents

   Add a 10-mL aliquot of your sample (that has not been run through the column) to a clean 250-mL titration flask, add a few drops of mixed indicators, about 75 mL of distilled water, and titrated with 0.100 M NaOH. Repeat for a duplicate check.

Determination of Magnesium plus Zinc

   Pipet a 10-mL aliquot of your sample into a 250-mL titration flask, add approximately the correct amount of 0.1 M NaOH to neutralize the acid present (this was determined in the above step), and finally add 25 mL of 0.02 M EDTA (from buret), approximately 25 mL of ammonia buffer, and a small amount of solid CI203 (EBT) indicator (Note 1). The solution will then have a pink color. Titrate the remaining divalent ions with 0.0200 M EDTA (not NaOH solvent) until the solution turns from pink to a pure blue.  The final pure blue is the end point. Repeat for duplicate checks.  (What volume of EDTA do you use in your calculations?) 

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Determination of Magnesium

    Pipet a 10-mL aliquot of your sample into 250-mL titration flask, neutralize with the required amount of 0.1 M NaOH, add 25 mL of ammonia  buffer, approximately 1g of KCN, a small amount (the smaller the amount, the sharper the end point) of solid CI203 (EBT) indicator, and 50 ml of distilled water.  Titrate with 0.0200 M EDTA until a pure-blue color is obtained.   Repeat for duplicate checks.

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Report

    Calculate and report the number of mmoles of H⁺, Na⁺,  Zn⁺⁺, and Mg⁺⁺ present in your 100-mL sample.