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Weak and strong acids in the surface waters of the tovdal region in S. Norway
I I . h ' r R('~,'t#',h Vol. I I. i)p. 71~1 h) 7S3 P~.'ig~imoll I'IL'~ I.ul I')79. |'rinl¢~,.| i11 {~rc;ll ~irihlin.
WEAK A N D STRONG ACIDS IN THE SURFACE WATERS OF THE TOVDAL REGION IN S. NORWAY G. M. GLOVER and A. H. WErm Central Electricity Research Laboratories, Kelvin Avenue, Leatherhead, Surrey, England
(Received 15 October 1978; receivedfor publication 29 January 1979) Abstract--The concentrations of weak and strong acids in surface waters of the Tovdal region of southern Norway were measured during a spring snow-melt period. The determinations were made by the pH titration method due to Gran. The strong acid concentrations in Tovdal river water varied between 3 and 11 ~ I - I while the weak acid concentrations were between 62 and 106/~eq I-i and the contribution of the weak acids to the hydrogen ion concentration ranged between 10 and 60~,. The pH of the river water varied from 4,9 to 5.0 and in the absence of excess strong acid, the weak acids would have produced a pH of 5.2-5.3. The concentration of weak acids and their contribution to the hydrogen ion concentration were least during the period of most rapid thaw. No direct evidence of the nature of the weak acids was obtained, but routine chemical analysis data suggested that inorganic species derived from aluminium and silicon accounted for 40-60 ~ I- s, while 20-50/~eq I-~ were attributable to humic and fulvic acids. The titration characteristics of the weak acids could be approximated closely by a polybasic acid with a first ionization constant in the range 10-6 to 5 x I0 -~ and less well defined weaker ionizations. Measurements on old snow containing coniferous tree litter and on melt water from a rocky barren contained weak acid concentrations comparable to the riverwater, indicating that only slight contact with vegetable matter or the ground is required to obtain significant concentrations of weak acids.
INTRODUCTION The acidity of some rivers and lakes in southern Norway has been reported to have increased over recent years {Gjessing et al., 1976; Henriksen, 1972) and this has given rise to concern over the possible ecological consequences (Leivestad & Muniz, 1976), especially to fish life. In general, the acid content of surface waters has both strong and weak acid components. The strong acid anions, sulphate and nitrate, are derived mainly from precipitation, whilst the weak acids consist mainly of humic and fulvic acids, produced by biodegradation of vegetable material (Gjessing, 1976), and the inorganic weak acids based on hydrated aluminium, iron and silica species. The strong acids content of Norwegian waters, particularly those of the Tovdal region has been determined before (Henriksen, 1977) but data have not been available on the weak acids content. A level of --40/~cI I - I can be inferred, however, from the organic carbon content, ~ 2 mg I- I (Henriksen, 1977), using the values for the acidity of humic and fulvic acids given by Schnitzer & Khan (1972). Gamble (1970) has reported that fulvic acids can be considered as a single dibasic weak acid with a first ionization constant in the range of 2.5 x I0-a--4.7 x I0 -s. From these values it can be calculated that the weak organic acids could dominate the hydrogen ion concentration down to a pH of about 4.4. To shed light on this possibility we have examined the relative contributions of weak and strong acids to the pH of the surface waters of the middle Tovdal
region during the spring snow-melt of 1977. The snow-melt period has been held to be critical (Leivestad & Muniz, 1976) because the acid accumulated in snow during the winter may be released preferentially into the water courses in the early stages of melting. METHOD
The weak and strong acid concentrations were determined by a pH titration following the method due to Gran (1952) and discussed by Johansson (1970) for the deterruination of mixtures of weak and strong acids. The samples were thermostatted at 27°C and purged continuonsly with nitrogen to remove atmospheric carbon dioxide. In addition to the weak and strong acid determinations, the major chemical constituents of the samples were determined by standard procedures. The sampling period extended from 20 March to 27 April 1977, this period being chosen to include the most probable date for the spring snow-melt. Samples were taken from the Tovdal river at Tveit Bridge near Owe Ramse, from tributary streams and, to a lesser extent, from the snow-pack and from ground drainings. RESULTS AND DISCUSSION The main results of our measurements are shown in Fig. I, which refers to the Tovdal River and Fig. 2, which relates to a tributary stream, the Ramse Brook. The figures show water depth, pH, weak and strong adds concentrations and also the contribution of the weak acids to the hydrogen ion concentration, obtained from the difference between the strong acid measurement and the total hydrogen ion concen-' tration.
781
782
G.M. GLOVERand A. H. WI!.a -
River depth
/~./P g
J60 3
4°I 3 5
t
~3rook d e p t h
30
170
2.5
180
t,
301
l 5
I
l
,o
,5
J
20
t
---
20
1 25
I ,,, 301
I 5
I IO
I 15
I 20
2~
Brook pH T
bO
za 4r
¢,1 49
46
48/
g ::L
I_
I
I
I
t
I
30[
5
IO
15
20
25
io
,,,I 25
I 301
I 5
I , I0
t'~° I
tOO 90 80 70 60 5o
I 15
t
20
I 25
Weak acid
IO0
I
I 5
30i
g
4.5
---o
H'loll
fromweok
, ,I ,0
I ~5
I 20
I 25
t5
20
25
80
,,
,
25
3o]
5
,o
m
2o
25
acid
t,
o
I 30J
5
IO
April
March
Fig. 1. In examining these results, it is necessary to consider the weather conditions which prevailed. Immediately prior to the start of work on the 20th March, there had been a partial thaw. This was followed from the 20th March to the 21st April by a period of colder weather (Period 1) during which the snow-melt proceeded more slowly and, as can be seen from Fig. 1, the river level fell. From the 22nd to the 27th April, the weather became warmer, the rate of thaw increased and the river level rose again. We shall refer to this as Period 2. During the whole period of study the pH of the Tovdat River (Fig. 1) showed only small changes, rising from 4.9 to 5.0 during the cold weather of Period 1 and falling slightly during the melt of Period 2. The weak acid concentrations ranged between 62-106 geq t - t , i.e. 6-20 times higher than the corresponding strong acid concentrations, and the contribution of the weak acids to the hydrogen ion concentration lay between 10 and 60°/~ the higher contribution being during Period 1. The Ramse Brook (Fig. 2) gave similar results, although it had a slightly lower pH (between 4.5 and 4.8) and contained higher con~attrations of weak acids (between 86-130#eq 1-~). The weak acids in the Ramse Brook contributed less to the hydrogen ion concentration, the proportion being about 40% in Period 1 and fallmg to nearly zero in Period 2. Evidence of the source of the weak acids is given by the difference between the measurements on fresh snow and those on water or snow which had been
-5
I
I,
I
I
I
25
30[
5
JO
~5
March
I ~ 20
25
April Fig. 2.
in contact with vegetation or the ground. Fresh snow contained only ~ 25/~eq 1- ] (due largely to the ammonium ion content), old snow from a forested area, which incorporated coniferous tree litter, contained -,- 80 #¢q 1- ~, and water draining from a nearby barren area of rock contained ~ 1I0 #cq I - ~. Evidently the weak acids derive from the ground or w*ip=table matter and even slight contact is sufficient for the melt-water to acquire concentrations of weak acids similar to those found in the river water. The variation of weak acids concentrations shown in Figs I and 2 is also consistent with their ground origin. The concentrations were fairly constant during the wintry conditions of Period 1 and fell during the flood of Period 2, This behaviour is consistent with the reduced influence of ground interaction during spate conditions. The titrations give the total concentration of weak acids present, but no direct evidence of their chemical nature. Of the inorganic species present, iron, aluminium, silica and ammonia can act as weak acids, but routine chemical analysis showed that only aluminium and silica were present in significant concentrations. AlumiabJm is tetrabasic and may account for a maximum of 20-30 #eq I- ~ of the titrated weak acid, while silica shows an effective basicity of 0,g over the pH range of the titration, and may account for a further 2O-30#eql -J of weak acid (the exact contribution of silica is uncertain since at least some may be present in an inactive form). The remaining 1 7 - 4 6 #eq 1-1 of weak acids are attributable to weak
Weak at~d strong acids in the surface waters of the Tovdal region organic acids such as humic and fulvic. We have fitted a theoretical weak acid/strong acid function to the experimental pH curves, and this indicates that the mixture of weak acids can be represented by a polybasic weak acid with a first ionization constant in the range 10-6-5 × 10 -7 and less well defined weaker ionizations. It was not possible to determine individual weak acid ionization constants and concentrations more precisely because of the long equilibrium times in the middle pH range, but no evidence for significant concentrations of weak acids of the strength reported by Gamble (1970) was detected. Using the polybasic approximation, it can be calculated that the pH of the river water was significantly buffered by the weak acids and that in the absence of any excess strong acid, the Tovdal river would have had a pH of about 5.2-5.3 during the period of study. Although the weak acids concentrations found in the present work relate to a brief period of study, it seems likely that they are reasonably typical, since other chemical species were near the average for this period of the year. For example, the pH of routine samples taken from the end of March to the end of April averaged 4.85 in 1977 compared with an average of 4.87 for the same period in 1974-1976. Similarly, the sulphate concentration averaged 87 #eq 1- i in 1977, compared with 72/zeq i - t over the three previous years. The presence of such concentrations of weak acids is significant, firstly, because they contribute to the hydrogen ion concentration--at least during some seasons of the year. Second and more important, they indicate the magnitude of ground interaction effects.
783
Even in a fairly barren region like the Tovdal, where the soil cover is sparse, the chemical composition of precipitation is rapidly modified by contact with the ground. The mechanisms of the interaction are complex, but an understanding is essential if we are to identify the reasons for temporal changes in river chemistry. Acknawledoements--We thank the Norwegian SNSF Project for hospitality and facilities during this work. We are also indebted to the Norsk institutt for vannforskning for routine chemical analysis of the samples. This paper is published by permission of the Central Electricity Generating Board. REFERENCES
Gamble D. S. 0970) Titration curves of fulvic acid: the analytical chemistry of a weak acid polyelectrolyte. Can. J. Chem. 48, 2662-2669. Gjessing E. T. (1976) Physical and Chemical Characteristics of Aquatic Humus. Ann Arbor Science Publishers. Gjessing E. T., Henriksen A., Johannessen M. and Wright R. F. (1976) Effects of acid precipitation on fresh water chemistry. SNSF Research Report 6/76, 65-85. Gran G. (1952) Determination of the equivalence point in potentiometric titrations: Part II. Analyst 77, 661-671. Henriksen A. (1972) Regresjonsanalyse av. pH-og hardhets-observasjoncr i Szrlandseiver. Vann i, 68-76. Henriksen A. (1977) Private Communication. Johansson A. (1970) Automatic titration by stepwise addition of equal volumes of titrant. Analyst 95, 535-540. Leivestad H. and Muniz 1. P. (1976) Fish kill at low pH in a Norwegian river. Nature 259, 391-392. Schnitzer M. and Khan S. U. {1972) Humic Substances in the Environment, pp. 37-39. Marcel Decker, New York.
WEAK A N D STRONG ACIDS IN THE SURFACE WATERS OF THE TOVDAL REGION IN S. NORWAY G. M. GLOVER and A. H. WErm Central Electricity Research Laboratories, Kelvin Avenue, Leatherhead, Surrey, England
(Received 15 October 1978; receivedfor publication 29 January 1979) Abstract--The concentrations of weak and strong acids in surface waters of the Tovdal region of southern Norway were measured during a spring snow-melt period. The determinations were made by the pH titration method due to Gran. The strong acid concentrations in Tovdal river water varied between 3 and 11 ~ I - I while the weak acid concentrations were between 62 and 106/~eq I-i and the contribution of the weak acids to the hydrogen ion concentration ranged between 10 and 60~,. The pH of the river water varied from 4,9 to 5.0 and in the absence of excess strong acid, the weak acids would have produced a pH of 5.2-5.3. The concentration of weak acids and their contribution to the hydrogen ion concentration were least during the period of most rapid thaw. No direct evidence of the nature of the weak acids was obtained, but routine chemical analysis data suggested that inorganic species derived from aluminium and silicon accounted for 40-60 ~ I- s, while 20-50/~eq I-~ were attributable to humic and fulvic acids. The titration characteristics of the weak acids could be approximated closely by a polybasic acid with a first ionization constant in the range 10-6 to 5 x I0 -~ and less well defined weaker ionizations. Measurements on old snow containing coniferous tree litter and on melt water from a rocky barren contained weak acid concentrations comparable to the riverwater, indicating that only slight contact with vegetable matter or the ground is required to obtain significant concentrations of weak acids.
INTRODUCTION The acidity of some rivers and lakes in southern Norway has been reported to have increased over recent years {Gjessing et al., 1976; Henriksen, 1972) and this has given rise to concern over the possible ecological consequences (Leivestad & Muniz, 1976), especially to fish life. In general, the acid content of surface waters has both strong and weak acid components. The strong acid anions, sulphate and nitrate, are derived mainly from precipitation, whilst the weak acids consist mainly of humic and fulvic acids, produced by biodegradation of vegetable material (Gjessing, 1976), and the inorganic weak acids based on hydrated aluminium, iron and silica species. The strong acids content of Norwegian waters, particularly those of the Tovdal region has been determined before (Henriksen, 1977) but data have not been available on the weak acids content. A level of --40/~cI I - I can be inferred, however, from the organic carbon content, ~ 2 mg I- I (Henriksen, 1977), using the values for the acidity of humic and fulvic acids given by Schnitzer & Khan (1972). Gamble (1970) has reported that fulvic acids can be considered as a single dibasic weak acid with a first ionization constant in the range of 2.5 x I0-a--4.7 x I0 -s. From these values it can be calculated that the weak organic acids could dominate the hydrogen ion concentration down to a pH of about 4.4. To shed light on this possibility we have examined the relative contributions of weak and strong acids to the pH of the surface waters of the middle Tovdal
region during the spring snow-melt of 1977. The snow-melt period has been held to be critical (Leivestad & Muniz, 1976) because the acid accumulated in snow during the winter may be released preferentially into the water courses in the early stages of melting. METHOD
The weak and strong acid concentrations were determined by a pH titration following the method due to Gran (1952) and discussed by Johansson (1970) for the deterruination of mixtures of weak and strong acids. The samples were thermostatted at 27°C and purged continuonsly with nitrogen to remove atmospheric carbon dioxide. In addition to the weak and strong acid determinations, the major chemical constituents of the samples were determined by standard procedures. The sampling period extended from 20 March to 27 April 1977, this period being chosen to include the most probable date for the spring snow-melt. Samples were taken from the Tovdal river at Tveit Bridge near Owe Ramse, from tributary streams and, to a lesser extent, from the snow-pack and from ground drainings. RESULTS AND DISCUSSION The main results of our measurements are shown in Fig. I, which refers to the Tovdal River and Fig. 2, which relates to a tributary stream, the Ramse Brook. The figures show water depth, pH, weak and strong adds concentrations and also the contribution of the weak acids to the hydrogen ion concentration, obtained from the difference between the strong acid measurement and the total hydrogen ion concen-' tration.
781
782
G.M. GLOVERand A. H. WI!.a -
River depth
/~./P g
J60 3
4°I 3 5
t
~3rook d e p t h
30
170
2.5
180
t,
301
l 5
I
l
,o
,5
J
20
t
---
20
1 25
I ,,, 301
I 5
I IO
I 15
I 20
2~
Brook pH T
bO
za 4r
¢,1 49
46
48/
g ::L
I_
I
I
I
t
I
30[
5
IO
15
20
25
io
,,,I 25
I 301
I 5
I , I0
t'~° I
tOO 90 80 70 60 5o
I 15
t
20
I 25
Weak acid
IO0
I
I 5
30i
g
4.5
---o
H'loll
fromweok
, ,I ,0
I ~5
I 20
I 25
t5
20
25
80
,,
,
25
3o]
5
,o
m
2o
25
acid
t,
o
I 30J
5
IO
April
March
Fig. 1. In examining these results, it is necessary to consider the weather conditions which prevailed. Immediately prior to the start of work on the 20th March, there had been a partial thaw. This was followed from the 20th March to the 21st April by a period of colder weather (Period 1) during which the snow-melt proceeded more slowly and, as can be seen from Fig. 1, the river level fell. From the 22nd to the 27th April, the weather became warmer, the rate of thaw increased and the river level rose again. We shall refer to this as Period 2. During the whole period of study the pH of the Tovdat River (Fig. 1) showed only small changes, rising from 4.9 to 5.0 during the cold weather of Period 1 and falling slightly during the melt of Period 2. The weak acid concentrations ranged between 62-106 geq t - t , i.e. 6-20 times higher than the corresponding strong acid concentrations, and the contribution of the weak acids to the hydrogen ion concentration lay between 10 and 60°/~ the higher contribution being during Period 1. The Ramse Brook (Fig. 2) gave similar results, although it had a slightly lower pH (between 4.5 and 4.8) and contained higher con~attrations of weak acids (between 86-130#eq 1-~). The weak acids in the Ramse Brook contributed less to the hydrogen ion concentration, the proportion being about 40% in Period 1 and fallmg to nearly zero in Period 2. Evidence of the source of the weak acids is given by the difference between the measurements on fresh snow and those on water or snow which had been
-5
I
I,
I
I
I
25
30[
5
JO
~5
March
I ~ 20
25
April Fig. 2.
in contact with vegetation or the ground. Fresh snow contained only ~ 25/~eq 1- ] (due largely to the ammonium ion content), old snow from a forested area, which incorporated coniferous tree litter, contained -,- 80 #¢q 1- ~, and water draining from a nearby barren area of rock contained ~ 1I0 #cq I - ~. Evidently the weak acids derive from the ground or w*ip=table matter and even slight contact is sufficient for the melt-water to acquire concentrations of weak acids similar to those found in the river water. The variation of weak acids concentrations shown in Figs I and 2 is also consistent with their ground origin. The concentrations were fairly constant during the wintry conditions of Period 1 and fell during the flood of Period 2, This behaviour is consistent with the reduced influence of ground interaction during spate conditions. The titrations give the total concentration of weak acids present, but no direct evidence of their chemical nature. Of the inorganic species present, iron, aluminium, silica and ammonia can act as weak acids, but routine chemical analysis showed that only aluminium and silica were present in significant concentrations. AlumiabJm is tetrabasic and may account for a maximum of 20-30 #eq I- ~ of the titrated weak acid, while silica shows an effective basicity of 0,g over the pH range of the titration, and may account for a further 2O-30#eql -J of weak acid (the exact contribution of silica is uncertain since at least some may be present in an inactive form). The remaining 1 7 - 4 6 #eq 1-1 of weak acids are attributable to weak
Weak at~d strong acids in the surface waters of the Tovdal region organic acids such as humic and fulvic. We have fitted a theoretical weak acid/strong acid function to the experimental pH curves, and this indicates that the mixture of weak acids can be represented by a polybasic weak acid with a first ionization constant in the range 10-6-5 × 10 -7 and less well defined weaker ionizations. It was not possible to determine individual weak acid ionization constants and concentrations more precisely because of the long equilibrium times in the middle pH range, but no evidence for significant concentrations of weak acids of the strength reported by Gamble (1970) was detected. Using the polybasic approximation, it can be calculated that the pH of the river water was significantly buffered by the weak acids and that in the absence of any excess strong acid, the Tovdal river would have had a pH of about 5.2-5.3 during the period of study. Although the weak acids concentrations found in the present work relate to a brief period of study, it seems likely that they are reasonably typical, since other chemical species were near the average for this period of the year. For example, the pH of routine samples taken from the end of March to the end of April averaged 4.85 in 1977 compared with an average of 4.87 for the same period in 1974-1976. Similarly, the sulphate concentration averaged 87 #eq 1- i in 1977, compared with 72/zeq i - t over the three previous years. The presence of such concentrations of weak acids is significant, firstly, because they contribute to the hydrogen ion concentration--at least during some seasons of the year. Second and more important, they indicate the magnitude of ground interaction effects.
783
Even in a fairly barren region like the Tovdal, where the soil cover is sparse, the chemical composition of precipitation is rapidly modified by contact with the ground. The mechanisms of the interaction are complex, but an understanding is essential if we are to identify the reasons for temporal changes in river chemistry. Acknawledoements--We thank the Norwegian SNSF Project for hospitality and facilities during this work. We are also indebted to the Norsk institutt for vannforskning for routine chemical analysis of the samples. This paper is published by permission of the Central Electricity Generating Board. REFERENCES
Gamble D. S. 0970) Titration curves of fulvic acid: the analytical chemistry of a weak acid polyelectrolyte. Can. J. Chem. 48, 2662-2669. Gjessing E. T. (1976) Physical and Chemical Characteristics of Aquatic Humus. Ann Arbor Science Publishers. Gjessing E. T., Henriksen A., Johannessen M. and Wright R. F. (1976) Effects of acid precipitation on fresh water chemistry. SNSF Research Report 6/76, 65-85. Gran G. (1952) Determination of the equivalence point in potentiometric titrations: Part II. Analyst 77, 661-671. Henriksen A. (1972) Regresjonsanalyse av. pH-og hardhets-observasjoncr i Szrlandseiver. Vann i, 68-76. Henriksen A. (1977) Private Communication. Johansson A. (1970) Automatic titration by stepwise addition of equal volumes of titrant. Analyst 95, 535-540. Leivestad H. and Muniz 1. P. (1976) Fish kill at low pH in a Norwegian river. Nature 259, 391-392. Schnitzer M. and Khan S. U. {1972) Humic Substances in the Environment, pp. 37-39. Marcel Decker, New York.