4.8  Determination of strong and weak acids in precipitation

4.8.1  Coulometric titration method

4.8.1.1  Field of application

This method is applicable to determination of strong acids in precipitation samples within the concentration range 10-5 to 10-3 M. Higher concentrations of acidity are not expected in precipitation. The lower concentration limit is close to the concentrations at background sites without alkaline mineral dust.

4.8.1.2  Principle

In the coulometric titration method (Liberti et al., 1972), the acid is titrated at constant current with hydroxyl ions liberated at a platinum electrode, a silver-silver bromide electrode serving as the counter electrode. The overall reaction is:

          Br- + Ag + H2O ® AgBr + OH- + ½H2

The emf of a glass-calomel electrode pair is read at intervals and the results are used to construct a Gran’s plot (Gran, 1952; Rosotti and Rosotti, 1965), which gives the endpoint of the titration by extrapolation of the straight part of the curve.

The only necessary modification is the addition of a constant, known amount of acid to the sample before the titration, in order to facilitate the titration of weakly acidic or alkaline samples without interference from carbon dioxide.

4.8.1.3  Instrumentation

4.8.1.4  Chemicals and reagents

During analysis, use only reagents of recognized analytical grade. The water used for dilution and rinsing must be double-distilled or de-ionized and distilled.

Nitrogen gas (N2) 99.9%
Potassium bromide (KBr)
Sulphuric acid (H2SO4) 0.05M
Buffer solution pH = 4.00

Solution I: 1 M KBr and 2.5 · 10-3 M H2SO4
Transfer 120.0 g KBr and exactly 50 ml of 0.05M H2SO4 to a 1000 ml volumetric flask. Fill up to the mark with water.

4.8.1.5  Analytical procedure

Turn on all instruments, and allow heating for ½ hour. Adjust the pH-meter to pH = 4.00, using the buffer solution. Transfer 50 ml of the sample into the thermostated titration vessel, and add 1 ml of solution I. Start nitrogen purging and adjust flow to give continuous agitation of the solution. The bubbles should not disturb the solution between the sensing and the working electrodes. Measure the pH of the solution. If the pH of the sample is above 5.6 it may be necessary to add more than 1 ml of solution I.

Wait until pH reading is constant. Switch pH-meter to read millivolts (range 0-240 mV with glass electrode positive) and start the electrolysis current.

Read the glass electrode potential vs the calomel electrode every 20 seconds and continue until the potential changes sign (at pH ca. 8). Stop the electrolysis.

Plot Gran’s function, Y, at 25°C, see Table 4.8.1 against electrolysis time (in seconds). The plot intercepts the abscissa at the equivalence point, te (F = Faradays constant, R = the universal gas constant, T = absolute temperature).


Table 4.8.1: Gran’s function.

EmV

y

EmV

y

EmV

y

EmV

y

1

1.04

41

4.93

81

23.4

121

111

2

1.08

42

5.13

82

24.4

122

115

3

1.14

43

5.33

83

25.3

123

120

4

1.17

44

5.55

84

26.3

124

125

5

1.22

45

5.77

85

27.4

125

130

6

1.26

46

5.98

86

28.4

126

135

7

1.31

47

6.22

87

29.6

127

140

8

1.36

48

6.47

88

30.7

128

146

9

1.42

49

6.75

89

32.0

129

152

10

1.48

50

7.00

90

33.3

130

158

11

1.54

51

7.28

91

34.6

131

164

12

1.60

52

7.57

92

36.0

132

171

13

1.66

53

7.87

93

37.4

133

177

14

1.73

54

8.19

94

38.8

134

185

15

1.80

55

8.51

95

40.4

135

192

16

1.90

56

8.85

96

42.0

136

199

17

1.94

57

9.20

97

43.6

137

207

18

2.02

58

9.57

98

45.3

138

216

19

2.10

59

9.94

99

47.2

139

224

20

2.18

60

10.3

100

49.1

140

233

21

2.26

61

10.7

101

51.0

141

242

22

2.36

62

11.1

102

53.1

142

252

23

2.45

63

11.6

103

55.2

143

262

24

2.54

64

12.1

104

57.4

144

272

25

2.65

65

12.5

105

59.7

145

283

26

2.75

66

13.0

106

61.9

146

294

27

2.86

67

13.5

107

64.4

147

306

28

2.97

68

14.1

108

67.0

148

318

29

3.09

69

14.6

109

69.7

149

331

30

3.21

70

15.2

110

72.4

150

344

31

3.34

71

15.8

111

75.3

151

351

32

3.48

72

16.5

112

78.3

152

371

34

3.61

74

17.1

113

81.5

153

386

34

3.75

74

17.8

114

84.7

154

402

35

3.90

75

18.5

115

88.1

155

418

36

4.06

76

19.3

116

91.6

156

434

37

4.23

77

20.0

117

95.1

157

452

38

4.39

78

20.8

118

98.9

158

470

39

4.56

79

21.7

119

103

159

489

40

4.74

80

22.5

120

106

160

507

 

4.8.1.6  Expression of results

The concentration of strong acid in the sample is calculated from the formula:

         

or

         

where

          i          =  electrolysis current in ampéres
          te        =  electrolysis time at equivalence point (seconds)
          F         =  Faradays constant (coulombs/mol)
          Vo        =  initial sample volume (litres)
          NH2SO4 =  normality of added sulphuric acid
          VH2SO4 =  volume of added sulphuric acid (litres)

Notes:
Borosilicate glass can be used for storage of samples.
The glassware must be treated with hot dilute acid and thoroughly soaked in distilled water prior to use. 12 hours with 10% hydrochloric acid at 90°C followed by 24 hours soaking in distilled water is considered adequate. Otherwise, alkali metals from the glass will diffuse into the samples.

Equipment for the automatic plotting of Gran’s function is available. The equip­ment is described in Section 4.8.2.

4.8.1.7  References

Gran, G. (1952) Determination of the equivalence point in potentiometric titrations. Part II. Analyst, 77, 661-671.

Liberti, A., Possanzini, M. and Vicedomini, M. (1972) The determination of the non-volatile acidity of rain water by a coulometric procedure. Analyst, 97, 352-356.

Rosotti, F.J.C. and Rosotti, H.J. (1965) Potentiometric titrations using Gran’s plots. Chem. Educ., 42, 375-378.

4.8.2  Coulometric titration of strong acid by means of an instrument for automatic plotting of Gran’s function

4.8.2.1  Field of application

This automatic method can be used to determine the concentration of hydrogen ions in precipitation within the range 10-5 to 10-3 M.

4.8.2.2  Principle

The basic principle is the same as for the manual method described in Section 4.8.1.

In the present method, the pH is continuously monitored by feeding the output from the pH-meter into an instrument for automatic plotting of the Gran’s function (APGRAF), which gives a recorder output from 1.0 mV at pH 7 to 10 V at pH 3. This signal is thus proportional to the hydrogen ion concentration in the solution. Since the volume of the solution is not altered during the coulometric titration, and the hydroxyl ions are supplied at a constant rate, the recorder output gives the Gran’s function directly.

The APGRAF consists of two amplifiers and a current source capable of delivering a constant current for coulometric titration ranging from 3.5 to 7.5 mA. The range can be extended to maximum 20 mA. The APGRAF is designed to work with the pH-meter RADIOMETER PHM 26c.

4.8.2.3  Instrumentation

Construction of the APGRAF
Figure 4.8.1 shows a block diagram of the layout, and a complete wiring diagram is shown in Figure 4.8.2. The 10 mV/pH output from the pH-meter is used as input signal to the preamplifier Al. The latter is adjusted to a gain of 1000 mV pH-1 and balanced to +2.0 V at pH 7 and -1.0 V at pH 4. The log amplifier A2 has an output of 1 decade per volt input, connected in the antilog of voltage mode, i.e. +10V out for a -2V input and +10 mV for a +1V input.

The gain is adjusted by the 5 kW input resistor (potentiometer). The balance is adjusted by the 100 W potentiometer connected via 10 kW to pin 3 on Al. The ZF 5.6 zener is used to stabilize “Balance” versus temperature changes.

Amplifier A3 is a unity gain, non-inverting amplifier providing a low source resistance for amplifier A4 (current source). A4 supplies a current that is proportional to the input voltage, but the output polarity is reversed.

The desired current is set by selecting one of the outputs of the voltage divider which is fed into A3.


 

 

 

 

 

 

Figure 4.8.1: Instrumental layout for automatic plotting of Gran’s function.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4.8.2: Wiring diagrams of a) amplifier and b) constant current source of the APGRAF (7.5 mA selected).

4.8.2.4  Chemicals and reagents

Use only reagents of recognized analytical grade. The water must be double-distilled or deionized and distilled.

Solution I: 1M KBr and 2.5 · 10-3 M H2SO4:
Transfer 120.0 g KBr and exactly 50 ml 0.05 M H2SO4 to a 1000 ml volumetric flask. Fill up to the mark with water.

4.8.2.5  Calibration

(1)     Connect the APGRAF to the pH-meter.
(2)     Place the calomel electrode in a buffer solution with pH 4.
(3)     Let the output from the preamplifier (A1) stabilize.
(4)     Read the output voltage from the preamplifier.
(5)     Adjust the output to approximately -1.0V
(6)     Recheck pH and preamplifier output, note the values.
(9)     Check gain.

 Example:                        pH 4.0                              output –0.900 V
                                      pH 7.0                              output +2.230 V
                  Difference      pH 3.0         Difference       output   3.130 V

                               

This should be adjusted to 1000 mV/pH-unit.

Try a couple of clockwise turns on “gain” potentiometer.

Repeat the procedure 1 through 9.

“Gain” and “Balance” interact. If gain is correctly set and balance has to be changed, recheck gain and readjust if necessary (“tracking”).

4.8.2.6  Analytical procedure

Turn on all instruments, and allow 30 minutes warm-up. Adjust the pH-meter to pH = 4.00, using the buffer solution. Transfer 50 ml of the sample into the thermostated titration vessel, and add 1 ml of solution I. Start nitrogen purging and adjust flow to give continuous agitation of the solution. The bubbles should not disturb the solution between the sensing and the working electrodes. Measure the pH of the solution. If the pH of the sample is still above 4.0 add more of solution I until the pH is less than 4.0.

Switch the pH-meter to the mV position, and turn on the recorder (paper speed 1 mm/sec). Select the electrolysis current (3-5 mA) and start the titration. This will result in a “jump” on the recorder trace, because the electrode potential is affected by the potential between the working electrodes. Minimize this effect by proper positioning of the electrodes. Mark the starting point on the paper. Continue the electrolysis until the recorder signal is zero (baseline). Stop the electrolysis and the recorder. Note the sample identification, electrolysis current, and the amount of acid (solution I) added, on the recorder sheet.

4.8.2.7  Expression of results

Draw a vertical line from the starting point on the paper, to the baseline. Extra­polate the first straight part of the titration curve until it reaches the baseline, see Figure 4.8.3. (The curve is usually not a straight line at the end of the titration.) Measure the distance (mm) along the baseline between these two points (l).


 

 

 

 

 

 

 

 

 

 

 

 

Figure 4.8.3: Titration graph for an actual precipitation sample.

The concentration of strong acid (in moles/litre) is calculated from:

where

= electrolysis current (A)
te  duration of analysis (s), determined from the distance between the start of titration and the end point on the chart, and the paper chart speed
F = Faraday constant (96500 C/mol)
Vo = sample volume (ml), normally 50 ml
NH2SO4 = normality of added sulphuric acid
VH2SO4 = volume (ml) of added sulphuric acid


Last revision: November 2001