4.  CONDUCTOMETRIC TITRATIONS

 

Exp. 1.  Determination of sulfates in drinking water

4.1.  The chemistry of the system

            A large number of the analytical methods for determination of sulfates are based on precipitation of BaSO4 or PbSO4. Gravimetric, turbidimetric and various titrimetric techniques have been used.

            The determination of the sulfates in drinking water is based on a conductometric titration, where sulfates are precipitated as BaSO4

            The precipitation process has been the subject of comprehensive studies. In respect to a conductometric titration of sulfates in drinking water, the following interferences have to be considered:

(i)  coprecipitation of Ca2+ and Mg2+;

(ii)  precipitation of bicarbonates and carbonates as BaCO3;

(iii)  H+ interferes if the indifferent anion of the titrant (Ba2+) is a weak acid.

            To overcome the interferences, an efficient pretreatment performed in an ion-exchange flow system is used1.

The strategy of the experiment in view of the above points:

            1. The interference of Ca2+ and other cations is overcome by replacing them with another cationic species via a cation exchange resin. Desirable cations are Li+ or Na+, however an intermediate step - replacement to H+ - is taken in order to discard carbonates and bicarbonates.

            2. As a result of the above step, the carbonates and the bicarbonates are transformed to carbonic acid. Carbonic acid can be purged by i) boiling the solution, ii) saturating the solution with a gas different than CO2. A different, recently developed approach, based on thin-film gassing out, which is both efficient and well adapted to flow processes, has been used.

            3. After discarding the interfering cations and anions, the solution contains protons and the anions originally present in the sample, except for the carbonic species. The protons are neutralized in order to enable the usage of the titrant containing acetate as counter anion and also in order to reduce the background conductance. The neutralization is normally carried out with a pH-metric titration. In this experiment a different, more efficient and well-adapted approach for a flow process is used: the protons are exchanged via a cation exchanger to Li+ or Na+ ions.

 

4.2.  Factors of importance in a precipitation conductometric titration

        In order to obtain a sharp-angled conductometric titration curve it is advantageous to use a titrant whose indifferent (counter) ion has a relatively low ionic conductance (Fig.4-1). By consulting Table 1 the counter anion of the titrant is chosen to be acetate.

 

Table 1.  Equivalent Conductivity at Infinite Dilution at 250C

Cation

Anion

H+

350

OH-

198

Li+

38.7

Cl-

76.3

Na+

50.1

Br-

78.4

K+

73.5

NO3-

71.4

Rb+

76.4

CH3COO-

40.9

Cs+

76.8

ClO4-

68

 

 

SO42-

80

 

The use of acetate anion pauses a restriction on the composition of the titrated solution: pH neutrality and absence of salts of weak acids or base are required. This, however, does not complicate the chemical pretreatment of the sample: the only salts of weak acids in drinking water are the carbonates and they are purged in step (2). The pH neutrality is desirable for other reasons as well as explained below and is performed in step (3).

        The solubility product of BaSO4 in aqueous solutions is moderately low (~10-10) and determines the detection limit of the titration. In order to reduce the detection limit of the method, the titration is carried out in presence of high concentration of ethyl alcohol: 100-200% in volume. The solubility product is strongly reduced in alcohol.

        High conductance background of the tested solution has two deteriorating effects:

(i)  At low sulfate concentrations the background conductance may be 100 times larger or more, compared to the variation of conductance as a result of the titration. To solve this problem a device for offsetting the conductance electric signal was designed. It enables to offset the entire background conductance and subsequently carry out the titration at an optimal sensitivity. (The electronical offset does not change the actual conductance of the solution).

(ii)  The conductance is affected by temperature (2% per 0C). This variation of the conductance may be of the same order of magnitude or larger than those of the titration. In most cases the temperature variations are constant during a single titration. It is strongly advised to reduce the background conductance as much as possible. This is the reason that in step (3) of the experiment all cations are exchanged to Li+ with the relatively lowest ionic conductance (Table 1).

        The kinetics of precipitation of BaSO4 is slow and dictates a slow rate of titration. A new form of titrant addition is used - that of transferring the titrant with a low flow-rate pump. This allows performing an automatic titration with titration rate of about 0.1 to 0.2 ml/min, rates usually unachieved with commercial piston burettes.

 


 


Fig.4-1  Conductometric titration of sulfates with Ba2+ as titrant and Cl- and OAc- as counter anions.

Sample: 5.00 ml 0.50 mM Na2SO4. Medium: water + ethanol 1:1.

Titrant: 10 mM Ba2+, flow rate: 0.122 ml/min.

 

4.3.  Description of experimental setup

*   Flow system for pretreatment of drinking water

            The titration of sulfates is preceded by a chemical pretreatment of the samples, described below:

 

Pretreatment of Drinking Water

Min+, Xin-, HCO3-

(Mi: Ca, Mg, Na, K, etc.)

(Xi: Cl, NO3, SO4, etc.)

exchange of cations by ion-exchanger in H+ form

 


H+, Xi-, H2CO3

purging of CO2 by thin-layer degassing device

 


H+, Xi-

neutralization by cation-exchanger in Li+ form

 


Li+, Xi-

 

The pretreatment setup (Fig.4-2) consists of two cation exchange columns, a thin layer degassing device and conductometric probes for following the conductance during the elution step.

 


 


Fig.4-2  Ion-exchange flow system for pretreatment of drinking water.

Dimensions of columns: length - 100 mm, ID (H+ column) - 5 mm, ID (Li+ column) - 6 mm. Degassing device: length of spiral - ~20 cm, ID - 0.7 mm. Conductometric probes (tungsten wires): 0.5 mm diameter, 2 mm length.

 

            Typical curves of conductometric titration of sulfates in tap water without and with pretreatment are shown in Fig.4-3.

 


 


Fig.4-3  Determination of sulfates in tap water.

Conductometric titrations without and with pretreatment are shown. Water sample - 5.00 ml from Tel-Aviv University. Medium - 50% ethanol. Titrant - 5.00 mM Ba(OAc)2, flow rate 0.20 ml/min.

 

*   Thin-layer degassing device. Purging of CO2

            The purging of CO2 is carried out with the thin-layer degassing device2. The solution to be degassed and an indifferent gas are mixed along a capillary tube (ID = 1 mm and length 30 - 60 cm). The pressure of the indifferent gas is ~0.3 bar, and the flow rate is adjusted so that the solution is pushed uniformly to the walls of the capillary, where an invisible thin film is formed. The intimate solution-gas contact enables an efficient purging of the gas originally present in the solution, in that case CO2. At the end of the capillary tube the solution is CO2-free.

 

*   Conductometric follow up of ion-exchange-column effluent

            The in-situ measurement of the conductance of the effluent is an efficient tool for optimizing the working conditions at ion-exchange columns. Several examples are given:

(a)  The conductometric probes at the exit of the column can fulfill the simple task of indicating the end of the rinsing stage following regeneration.

(b)  In separation processes, it can be used as a detector as in GC or HPLC.

(c)  And finally, in our case, it enables to follow the gradual changes occurring during the pretreatment of the sample (cf., Fig.4-4). With the passage of the sample through the columns, the conductance at the exit of each column increases from zero (the level of distilled water) to a constant value. At the plateau the composition of the effluent corresponds to that of the sample, in which all cations are replaced by H+ or Li+, depending on the type of the column. The conductance at the exit of the H+ column is higher than that of the Li+ column, due to next factors: (i) the specific conductance of H+ is considerably higher than that of Li+, (ii) the concentration of carbonic species at the exit of the Li+ column is equal to zero, (iii) there may also be variation in the cell constants of the probes.

 


 


Fig.4-4  Conductance at column exits during pretreatment

 

            Let us consider the hypothetical situation, in which a very large volume of the sample is passed through the columns. The capacity of the column surpassed, the conductance at the exit of the H+ column would start to decrease and that of Li+ column - to increase, until equality of specific conductivity are obtained (explain!).

            Typical follow up of the effluent conductance is shown in Fig.4-4. During the elution the conductance at the end of each column increases and reaches a plateau. (Explain the differences in location and height of the two curves). The sample for the titration is collected at the exit of the Li+ column when a constant value of the conductance is reached.

 

*   Estimation of the maximum volume of sample passed through the exchange column

            When a quantitative ion-exchange process is required, the amount of sample to be treated should correspond to not more than ~70% of the column capacity. The type of ion-exchange process, the type of resin, the size of the resin particles, the geometry of the column and the flow rate may strongly affect that value. A rough estimation of the maximum volume of the sample can be made, if the total concentration of the ionic species is approximately known. Measurement of the specific conductance, k, of the sample provides an estimate of the total ionic concentration. Assuming an average equivalent conductivity, L, of 120 ohm-1cm2eq-1, the total ionic concentration in mol/l is

 

The recording of the conductance at the output of the columns is an useful indicator of the functioning of the resin.

 

*   Determination of the conductometric cell constants of the titration cell and the ion-exchange columns

            Although not imperative, the knowledge of the cell constants is helpful. If not provided by the instructor, the cell constants may be measured as follows.

            Rinse the cell with a small amount of 1.00 mM KCl. Fill the cell with the same solution. Measure the conductance. The cell constant is calculated from: kcell = L/k. The specific conductance, k, of 10 mM KCl at 200C is 1.278 mS/cm, with temperature coefficient 2.14%/K.

 

4.4.  Experimental procedure

Chemicals

            1.  1 M HCl

           2.  1 M LiCl

           3.  5.00 mM Ba(OAc)2

           4.  0.5 mM Na2SO4

           5.  0.5 mM Na2SO4 + 2 mM Ca(NO3)2

           6.  0.5 mM Na2SO4 + 2 mM NaHCO3

           7.  Ethyl alcohol

           8.  Strongly acidic cation exchanger Dowex 50W, 100-200 mesh, 3-5 meq/g dry material

           9.  1 mM KCl (for calibration of conductometric cell)

 

*   Titration setup

 


 


Fig.4-5  Setup for conductometric titration of sulfates

 

*   The regeneration of the ion-exchange columns

            1.  Disassemble the columns from the flow system. Connect to each column a 15 ml reservoir. Connect the columns to a conductometer. Set the sensitivity of the conductometer and the recorder, so that the full-scale reading of the recorder is 0.2 Scm-1. Set the time scale to 1 cm/min. Start the recording device a few seconds before the beginning of the regeneration process.

            2.  For the regeneration of the H+ column use ~10 ml of 1 M HCl and for the Li+ column use ~10 ml 1 M LiCl. Pass the electrolyte through the resin, applying a small gas pressure to obtain a flow rate of about 2 ml/min.

            3.  Rinsing of the columns: use small portions of water to wash the glass walls. If any water is present above the resin allow it to soak into before continuing the washing. Pass about 10 ml water until the conductometric reading drops to the level of the conductance of the distilled water. Compare the specific conductance and not the absolute value of conductance, due to the variety of conductivity cells you work with.

 

*   Practicing with the flow system

            In order to get acquainted with the flow pretreatment setup, you should operate it at first with distilled water. Starting conditions:

        The pressure of the gas (nitrogen) is adjusted to 0.4 bar. The needle valve is closed.

        The level of the liquid in the columns is equal to that of the resin.

        The reservoir stopper above the Li+ column is removed. Operating the flow system:

            1.  Fill the reservoir above the H+ column with distilled water. Place a 50 ml beaker beneath the Li+ column.

            2.  Slowly open the needle valve. Pressure is applied on the column and the liquid starts flowing through it. Observe what happens along the glass helix. At slow flow rates solution and gas are interlaced. Increase further the flow rate of the gas. The glass capillary seems to be empty. A thin, invisible film is formed along the capillary. These are the suitable conditions for the operation of the thin-layer degassing device.

            3.  At that stage solution accumulates above the resin of Li+ column. Place back the stopper of the reservoir from this column. The solution starts to flow through the column. Under steady-state conditions the level of the liquid above the Li+ resin should be equal to that of the resin. In order to reach this condition, close for a short time the gas exit of the thin plastic tube, connected to the stopper.

            4.  Estimate the flow rate of the solution at the exit. If it is lower than 1 ml/min, increase further the gas flow rate. If solution accumulates in the reservoir of the Li+ column, you need to increase the flow rate of the solution through that column. First discard the excess of liquid as described above. Then prolong the plastic tubes for the outlet of the gas. This increases the pressure applied on the column and so increases the flow rate. Pay attention that this change did not affect the flow through the helix capillary. If everything is O.K., you are ready for the real experiment. Release the pressure by closing the needle valve and open the stopper above the Li+ column.

 

*   Pretreatment of the sample with the flow system

            The pretreatment of the sample for the determination of sulfates is carried out in the above flow system. A relatively large amount (up to 30 ml) of a sample can be introduced through the columns. The front of the sample pushes down the distilled water, which originally fill the columns. The sample undergoes the changes described in this chapter, section 4.3. Conductometric probes, located at the exit of the flow system, are used to follow the progress of the elution. The conductance changes from that corresponding to distilled water to that of an effluent with uniform concentration (Fig.4-4). The effluent with uniform concentration is collected for further analysis. The concentration of the sulfate in this fraction is identical to that of the original sample.

            It is assumed at that stage that the gas pressure and flow rate of the gas are adjusted in the previous steps. Check that the level of the liquid in both columns is equal to that of the resin. Use the conductometric detectors to ensure that the columns have been thoroughly washed with deionized water.

            Place a 10 ml graduated flask under the outlet. Prepare a 50 ml dry plastic beaker for collecting the sample. Disconnect the reservoir of the H+ column and replace it with a dry one, or rinse the original reservoir with a small volume of the sample to be analyzed. Connect the conductometric probes to a recording device. Set the sensitivity of the recorder to 100 mS full scale and use a time scale of 500 s.

            Introducing the sample in the flow system. The sample should be introduced at first in small amounts into the column in order to replace the distilled water without diluting the sample, located above the resin. Transfer with a Petri pipette few drops of the sample directly above the H+ column. Apply a pressure above the column for a short time. Let the solution be absorbed into the resin. Repeat several times the sequence of addition small amounts of sample and applying pressure. At that stage there is no more danger of diluting the sample. Fill the container with about 20 ml of sample. Open the needle valve. If the setting of the pressure and the flow rate of the gas have been adjusted correctly in the previous section, the flow rate of the effluent should be about 1 ml/min, the glass column is free of visible motion of solution and there is no accumulation of solution above the Li+ column. If not, there is still time to make last adjustments.

            Observe the variation of the conductance at the exit of the flow system. After about 4 ml of solution has passed the column, the reading should be constant with time. At that point start collecting the effluent for further use. After the required volume of effluent is collected, close the gas valve. The flow system is ready for further use if needed.

            At the end of the experiment rinse the flow system with distilled water.

 

*   Conductometric titration of SO42- with Ba2+

            Transfer 2.00 to 5.00 ml of the pretreated sample into the conductometric cell. Add ethanol according to the concentration of sulfate in the sample:

for [SO42-] < 0.5 mM, Vethanol/Vsample = 1.5;

for [SO42-] < 0.5 mM, Vethanol/Vsample = 1.

The total height of the liquid in the cell should be such that the electrodes are covered, also when the solution is stirred. The time needed to reach the end point should be no shorter than 2 minutes. Faster rate of titration may result a delay of the end point, due to the slow kinetics of precipitation. The concentration of the titrant Ba(OAc)2 is about five to ten times larger than that of the sulfate in the original sample.

            Get the recording device ready. Dip the tube filled with the titrant into the cell. Start the titration by turning on the pump. The titration curve is displayed. At the end of titration turn off the recording device and the pump.

            Repeat the titration to check the reproducibility.

            Titrate the unpretreated drinking water sample under identical conditions. Refer to the differences.

            Determine the flow rate of the pump as described in Titrimetric Methods of Analysis, section 1.6. Report the concentration of sulfate in mmol/l and ppm SO42-.

 

*   Effect of interferences

            You are provided with three synthetic solutions:

(a)  0.500 mM Na2SO4

(b)  0.500 mM Na2SO4 and 2 mM Ca2+

(c)  0.500 mM Na2SO4 and 2 mM HCO3-

Titrate each solution in presence of ethanol. Discuss the results.

 

*   Effect of ethanol concentration

            Titrate 5.00 ml of 0.500 mM Na2SO4 using different volume ratios of sample:ethanol (1:1.5, 1:1 and 1:0.5). Discuss the results.

 

References

1.  E. Kirowa-Eisner, D. Tzur, M. Brand and Ch. Yarnitzky, Microchem. J., 61, 40(1999).

2.  Ch. Yarnitzky, Electroanalysis, 2, 581(1990).

 

Exp.2.  Conductometric titration of strong and weak acids

Chemicals       1.  HCl: 0.1, 1, 10 mM

                        2.  HAc: 1, 10, 100 mM

                        3.  NaOH: 1, 10, 100, 1000 mM

                        4.  1 mM KCl (for calibration of conductometric cell)

 

Electrodes

            For precision measurements of conductance platinized-platinum electrodes are used to reduce the polarizing effect of the passage of the current between the electrodes. For the purpose of conductometric titrations, where the end point is to be precisely determined, but the absolute conductance is of lesser importance, other electrodes may be used (bright platinum, tungsten and others).

 

Titration setup

            The titration is performed with continuous addition of titrant using an automatic burette or positive displacement pump or peristaltic pump (cf., Fig.4-5). Viton or tygon tubing (I.D. about 1 mm) are recommended for the peristaltic pump. Read Titrimetric Methods of Analysis, section 1.6 for details of using automatic titrators. Determine the flow rate if a pump is used.

 

Procedure

            1.  Determine the conductometric cell constant as described in Exp.1 of this chapter (Determination of sulfates in drinking water).

            2.  Introduce 5.00 ml of acid sample into the conductometric cell. Titrate with NaOH at a flow rate of 0.8 - 1.2 ml/min. Perform a series of titrations according to the order and the conditions summarized in the Table below. Data should be collected for up to 100% excess of titrant.

            3.  Plot the graphs and determine the end point. Refer to the shape of the titration curves. Compare with the potentiometric titration curves.

            4.  Calculate the concentrations of the acids in mol/l.

            5.  At the end of the working session rinse the pump tube, the cell and the beakers with distilled water.

 

Analyte

Titrant (NaOH), M

Maximal conductance*

10-1 M HAc

1

25 mS

10-2 M HCl

10-1

5 mS

10-2 M HAc

10-1

2.5 mS

10-3 M HCl

10-2

0.5 mS

10-3 M HAc

10-2

250

10-4 M HCl

10-3

50

*For conductivity cell with a cell constant of about 1 cm-1.

 

 

Recommended Literature

A. I. Vogel, Textbook of Quantitative Inorganic Analysis.

 

 

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