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A specific conductance method for quality control in water analysis
14ater Research V o l 11, pp. 91 1o 94. Pergamon Press 1977 Printed in Great Britain.
A SPECIFIC CONDUCTANCE METHOD FOR
QUALITY CONTROL IN WATER ANALYSIS D. P. H. LxXE'N Department of Environmental Sciences, University of Lancaster, Lancaster LA1 4YA, England
(Received 30 April 1976; in revised form 15 July 1976) Al~raet--A method to aid quality control in water analysis is described. It can be used both to help maintain a high quality of analytical data and to highlight inconsistencies in existing data. The procedure, either computational or graphical, relies on a comparison of measured specific conduetance with a specific conductance calculated from the analytical data using an extended version of the Onsager limiting equation. The method is discussed in relation to water analyses typical of limestone catchments. A number of limitations are set forth and the method is shown to be suitable for most waters with a specific conductance in the range 80-350 #s cm -1, although it can probably be extended to waters with a specific conductance in the range 50-1000 #s cm-1.
THEORY
INTRODUCTION
A knowledge of both the precision and accuracy of water quality data is essential before any meaningful interpretation can be made. The data precision in partieular limits the interpretation of temporal and spatial patterns Other within or between sample sets, whilst the accuracy more specifically affects ionic ratios, equilibrium indices and inter-comparisons with other studies. Several methods are available to assess the precision and accuracy of either an analytical technique or a specific water analysis. The reliability of standard analytical techniques has generally been well established, based on reproducibility and recovery tests, and the information is available scattered through the literature (Edmunds, 1971; l.twin et at., 1974; Philbert and Traversy, 1973) and also as a result of inter-laboratory evaluation schemes (Kingsford et al., 1973; Ekedahl and Rondell, 1973; APHA, 1971). This information, however, provides only a generalized indication of the reliability that can be attached to a particular set of results. A more specific assessment can be made from an examination of the balance between summed cations and anions and from the measurement of the specific conductance of a diluted sample (APHA, 1971). The former method does not necessarily reveal an error and the latter requires an accurate dilution to a narrow range of specific conductance, 90-120 gs crn-1. The data quality analysis method described in this paper is a refinement of the specific conductance method. The dilution step is dispensed with and field determined conductance is used. The method relies on the theoretically determined and empirically verified relationships between specific conductance and ionic strength, both measures of the total ionized species (Lind. 19701.
A generalized expression of the relationship between specific conductance and concentration is given by L= 1000AN, (1) where L is the specific conductance in gs cm-~, A the equivalent conductance and N the normality of the solution. (Normality is identical to mequiv 1-~ x 10-3 and can be calculated from the concentration expressed in m g l - 1 using the factors in Table 1.) Specific conductance can also be related to the ionic strength (I) by the normality of the solution using the definition of ionic strength i
! = ½ ~ (~,z2),
(2)
II=1
where m, is the molality of ion n and z, the charge. For dilute solutions molality is approximately equal to molarity (M) which is related to normality (N) by N = M ~ :, and equation (2) becomes
(3)
i
I = ½ ~ (N,z,).
(4)
n=l
Table 1. Factors to convert concentration in mg 1- ~ to normality Ion Na ÷ K÷ Ca 2÷ Mg 2+ CINO~ HCO~ SO~91
Conversion factor 0.0435 0.02557 0.0499 0.08226 0.02821 0.01613 0.01639 0.02082
× × x x x x x x
10 -s 10 -s 10 -3 10-3 10 -3 10 -3 10 -3 10 -3
92
D. P, H. LAXEN
The second variable in equation (lj is the equivalent conductance term (,l). which can be considered the sum of the equivalent conductance of both the cation and anion component of the ionized salt ,I=2.
+2_.
(5)
The equivalent conductance of each ionized species depends on the ionic strength of the solution and several other factors which are incorporated in the expression of the equivalent conductance of an ion at infinite dilution (2°). Onsager's limiting equation expresses this relationship (Robinson and Stokes, 1965) .o _ r0"7852[z,* z,_ Iq2.°+ ] ,;,,. = a,., L. ]-T~ + 30.321z,+1 1 ':2. t6)
Table 3. Values of [x] for use with equation (9) Ion Na * K" Ca'* Mg"" CI NO~ HC03 SO~-
Value of Ix] 41.8 47.1 115.I 109.2 47.8 46.7 40.5 133.9
and values of x for the different ions are given in Table 3. The specific conductance of a mixed salt solution can now be calculated as the sum of the contributions of the individual ions.
where q is a term incorporating the ionic charge and i the values of equivalent conductance at infinite diluL = 1000 Z (2.N,). II0) tion. When Iz,+l = Iz,-{, q has the value 0.5, reducing n=l slightly for mono-divalent salts. Selecting the value with the values of 2. determined by equation 19). of q as 0.5 for all salts introduces a negligible error (~1%). To allow equation (61 to be used for single PROCEDURE ions in a mixed salt solution, the simplification . (a) Computational [z,-I = [z,-I is made. This will introduce a slight A theoretical value of specific conductance is calcunegative error for single salt solutions of the m o n o divalent type, e.g. Na,SO,~. Equation (6) can be lated from the analytical results for the major dissolved ionized species using equations (9) and (10). extended for concentrations of up to 0.1 N using the This value is then compared to the measured specific modification suggested by Robinson and Stokes conductance, a discrepancy indicating an error in the (1965), where by the term 11 -" is replaced by l t ' : / ( l + Ba P ' ) . The parameter B is a constant, whilst a analysis. represents the mean distance of closest approach of Ibt Graphical the ions in solution. The value of Ba can be approxiControl lines are prepared to represent the ionic mated as 1 without sacrificing accuracy. strength-specific conductance relationship for the Equation (6) therefore takes the form waters under analysis. A proportional composition (in -ol _ [0.02292ol + 30.32] I~ -',/(I + I ~2) (7) terms of normalities or mequiv l - t ) similar to that A, n l -~- Arl of the analytical data is used in the calculation. For for monovalent ions and for divalent ions example: a water analysed as 0.0005N in Ca 2. ,o _ [0.9162oa + 6 0 . 6 4 ] i t 2/(1 + it :). (8) HCO3, Na*, C | - (i.e. 25~o, respectively) will from A•n 2 ~ /'n2 equation (4) have an ionic strength of ~ t + Values of equivalent conductance at infinite dilution 0 . 5 + 0 . 5 + 0 . 5 ) x 10 - 3 = 1.25 x l0 -3. and from (2 °) are summarized in Table 2 for the major disequations (9) and (10) a specific conductance of 113.1 solved ionized species found in natural waters. Equaps c m - L. Similarly, for the same proportional compotions (7) and (8) can both be expressed in more gensition but double the concentration, the ionic strength eral terms as will be 2,5 x l0 -3 and the freshly calculated specific ,;. = 2 o - I x ] 1 l 2'(1 + I12), (9) conductance 218.7 #s c m - l A series of values derived in this fashion allow control lines to be drawn, against Table 2. Equivalent conductance at infinite diluwhich measured values of specific conductance and tion if.o) From Robinson and Stokes 11965) ionic strength are compared. A departure of the apart from HCO3 taken from Lind ([970) measured value from the control will indicate an error Equivalent conductance ,;.') in the analysis• It is important not to extend the conIon cm2 lequiv-~V trol lines beyond the range for which a constant proportional composition has been established, and indiNa 50.1 vidual control lines should be developed for each sigK 73,5 Ca-" 59.5 nificant change in proportionality. Mg: 53.0 C1 76.35 DISCUSSION NO~ "~1.46 HCO 3 44.5 Table 4 presents values of conductance calculated SO~.s0.0 by the above procedure and measured tliterature~
Specific conductance method for quality control in water analys.is
93
Table 4. Calculated specific conductance for single salt solutions and measured values Salt NaC1 Na2SOa MgSO4
Normality 0.001 0.01 0.001 0.01 0.001 0.01
Ionic strength x 10-3 1 10 1.5 15 2 20
Lc
Lml
Lm2
Lm3
123.7 1180 123 1111 122.6 1030
125 1180 126 1130 107 800
124 1180 124 1120 118 889
123.7 1185 124 1124 --
-
Lc calculated specific conductance gs era-~. Lml Lind (1970). Lm2 Tanji (1969). Lm3 Milazzo (1963).
values. A close agreement exists for the mono-monoThe method is subject to a number of limitations valent salt, whilst the mono-divalent salt reveals the which should be borne in mind either before applislight negative error introduced by the simplifying cation or whilst evaluating the significance of any assumption Iz..I--Iz.-[ in equation (6). The di--di- departures. valent salt shows a significant positive error, particu1. The method is most appropriate for waters with larly at higher concentrations (>0.00IN). Tanji one or two predominant salts. (1969) has shown that this can be accounted for by 2. It provides most information about the accuracy a reduced ion activity as a result of ion pairing. of the major ionized species. The procedure has been applied to data selected 3. The sensitivity is reduqed as the waters are randomly from the literature (Fig. l). The samples diluted. all represent waters dominated by the Ca 2÷ and 4. Inaccuracies in separate deternainants can cancel HCO~ ions with a major ion proportional composi- out providing a false indication of accuracy. This sittion within the range of the two control lines. The uation should be apparent in a biased cation/anion close agreement between the sample points and the balance. control lines both confirm the validity of the method 5. There is an interdependence of accuracy evaluand suggest a high degree of accuracy for the reported ation (choice of control line) and accuracy itself. This data, a feature to be expected of published results. reduces the sensitivity slightly; however, it does not The additional data points in Fig. 1 represent apply to the use of the computational procedure• samples collected and analysed by the author for a 6. The calculated conductance fails to account for natural limestone catchment in the north west of Eng- ion pairing. This is only significant for divalent ions land. These data should lie within the control lines; i.e. SO~-, particularly at higher concentrations however, the considerable departures that are evident, >0.001 N i.e. > 4 8 m g 1 - 1 SO~-. particularly at the higher concentrations, suggest a 7. Complexation with organic ligands could also poor data accuracy. The variable magnitude of the reduce the ionic activity, hence conductance, but not departures also suggests a poor precision. necessarily the measured concentration. There is little The departures can be further interpreted to help identify the source of the error by making use of the relatively high degree of accuracy with which specific conductance can be measured, generally 1-5Yo ~O'7// , ~ ~ ¢~ (APHA, 1971; Kingsford et al., 1973). This accuracy can be both maintained and confirmed by regular standardization of the conductance bridge with potas-~ ~ + + J ".,-" sium chloride solutions of known conductance o ,~, / I (APHA, 1971). The measured conductance can therefore, as a first approximation, be taken to be a true value and hence from the control lines a 'true' ionic strength can be obtained. For the data under consideration the difference between 'true' and measured /,¢ ,, s ~ ' ' S ionic strength indicates a deficiency of determined 9 o~ ions. of up to -0/o, This deficiency was subsequently attributed, after examining the cation/anion balance and the results of an inter-laboratory evaluation Specific conduc'Ponce, / ~ e c m -~ scheme, to a poorly performed calcium and magnesium determination. It is also possible to suggest that Fig. 1. Ionic strength vs specific conductance. Control lines Ca 2* 42.5~o, Mg -'~ 5°'0, Na ~ 2.5°0. HCO~ 40°.o. SO~the error was due to sample deterioration prior to (A) 7.5°,o, CI- 2.5~:o.(B) Ca 2+ 40%o, Mg -'÷ 5°o, Na t 5°0, HCO~ analysis, as the conductance was measured in the 40°i. SO~- 5%. CI- 5~g. Data points: + Author. O' Lind field. (19701. 0 : Jacobson and Langrnuir (1974).
94
D . P . H . LAXEN
information available at present, with which to judge the significance of this effect. Extra care is therefore required when dealing with organic-rich waters and the validity of the method should be established before it is applied to such waters. 8. In view of the above, the method is probably best limited to waters with a conductance in the range 50-1000 #S cm-1. CONCLUSION The procedure described is particularly suitable as a control on data quality during routine water analysis. It can also be used to retrospectively assess the quality of existing data. An obvious application for the method is to the quality control of analyses of limestone waters, for which analytical accuracy is of particular significance in the calculation of saturation indices, the degree of non-carbonate hardness and other parameters of value to the study of limestone hydrogeochemistry and the chemistry of potable water supplies. REFERENCES
APHA (1971) Standard Methods Jor the Examination of Water and Waste Water. Am. Publ. Health. Assoc. Inc., New York.
Edmunds W. M. (1971) Hydrooeochemistry oJ Groundwaters in the Derbyshire Dome. with Special Reference to Trace Constituents. Inst. Geol. Sci. Rep. 7~;7. H.M.S.O., London. Ekedahl G, & Rondell B. (1973) lnterlaboratory study of methods of chemical analysis of water. Vatten 29(4). 341-356. Jacobson R. L. & Langmuir D. (1974) Controls on the water quality of some carbonate spring water. J. Hydrol. 23, 247-265. Kingsford M.. Stevenson C. D. & Edgerley W. H. L. (1973) Collaborative tests of water analysis (the Chemaqua Programme); part l--sodium, potassium, calcium, magnesium, chloride, sulphate, bicarbonate, carbonate and conductivity. N.Z. Dept Sci. Ind. Res., Chem. Div. Rep. CD 2159. 58 pp. Lewin J., Cryer R. & Harrison D. I. (1974) Sources of sediments and solutes in Mid Wales. In Fluvial Processes in Instrumental Watersheds (Edited by Gregory K. J. & Walling D. E.). Inst. Br. Geog. Spec. Publ. 6, p. 73. Lind C. J. (19701 Specific conductance as a means of estimating ionic strength. U.S. Geol. Surv. Prof. Paper 700D, D272-D280. Milazzo G. (1963) Electrochemistry. Elsevier, Amsterdam. Philbert F. J. & Traversy W. J. 11974) Methods of sample treatment and analysis of Great Lakes water and precipitation samples. Proc. 16th Conf. Great Lakes Res., Part 16, pp. 294-308. Robinson R.A. & Stokes R. H. (1965) Electrolyte Solutions. 2nd edn. Butterworth, London. Tanji K. K. (19691 Predicting specific conductance from electrolytic properties and ion association in some aqueous solutions. Proc. Soil Sci. Soc. Am. 33, 887-889.
A SPECIFIC CONDUCTANCE METHOD FOR
QUALITY CONTROL IN WATER ANALYSIS D. P. H. LxXE'N Department of Environmental Sciences, University of Lancaster, Lancaster LA1 4YA, England
(Received 30 April 1976; in revised form 15 July 1976) Al~raet--A method to aid quality control in water analysis is described. It can be used both to help maintain a high quality of analytical data and to highlight inconsistencies in existing data. The procedure, either computational or graphical, relies on a comparison of measured specific conduetance with a specific conductance calculated from the analytical data using an extended version of the Onsager limiting equation. The method is discussed in relation to water analyses typical of limestone catchments. A number of limitations are set forth and the method is shown to be suitable for most waters with a specific conductance in the range 80-350 #s cm -1, although it can probably be extended to waters with a specific conductance in the range 50-1000 #s cm-1.
THEORY
INTRODUCTION
A knowledge of both the precision and accuracy of water quality data is essential before any meaningful interpretation can be made. The data precision in partieular limits the interpretation of temporal and spatial patterns Other within or between sample sets, whilst the accuracy more specifically affects ionic ratios, equilibrium indices and inter-comparisons with other studies. Several methods are available to assess the precision and accuracy of either an analytical technique or a specific water analysis. The reliability of standard analytical techniques has generally been well established, based on reproducibility and recovery tests, and the information is available scattered through the literature (Edmunds, 1971; l.twin et at., 1974; Philbert and Traversy, 1973) and also as a result of inter-laboratory evaluation schemes (Kingsford et al., 1973; Ekedahl and Rondell, 1973; APHA, 1971). This information, however, provides only a generalized indication of the reliability that can be attached to a particular set of results. A more specific assessment can be made from an examination of the balance between summed cations and anions and from the measurement of the specific conductance of a diluted sample (APHA, 1971). The former method does not necessarily reveal an error and the latter requires an accurate dilution to a narrow range of specific conductance, 90-120 gs crn-1. The data quality analysis method described in this paper is a refinement of the specific conductance method. The dilution step is dispensed with and field determined conductance is used. The method relies on the theoretically determined and empirically verified relationships between specific conductance and ionic strength, both measures of the total ionized species (Lind. 19701.
A generalized expression of the relationship between specific conductance and concentration is given by L= 1000AN, (1) where L is the specific conductance in gs cm-~, A the equivalent conductance and N the normality of the solution. (Normality is identical to mequiv 1-~ x 10-3 and can be calculated from the concentration expressed in m g l - 1 using the factors in Table 1.) Specific conductance can also be related to the ionic strength (I) by the normality of the solution using the definition of ionic strength i
! = ½ ~ (~,z2),
(2)
II=1
where m, is the molality of ion n and z, the charge. For dilute solutions molality is approximately equal to molarity (M) which is related to normality (N) by N = M ~ :, and equation (2) becomes
(3)
i
I = ½ ~ (N,z,).
(4)
n=l
Table 1. Factors to convert concentration in mg 1- ~ to normality Ion Na ÷ K÷ Ca 2÷ Mg 2+ CINO~ HCO~ SO~91
Conversion factor 0.0435 0.02557 0.0499 0.08226 0.02821 0.01613 0.01639 0.02082
× × x x x x x x
10 -s 10 -s 10 -3 10-3 10 -3 10 -3 10 -3 10 -3
92
D. P, H. LAXEN
The second variable in equation (lj is the equivalent conductance term (,l). which can be considered the sum of the equivalent conductance of both the cation and anion component of the ionized salt ,I=2.
+2_.
(5)
The equivalent conductance of each ionized species depends on the ionic strength of the solution and several other factors which are incorporated in the expression of the equivalent conductance of an ion at infinite dilution (2°). Onsager's limiting equation expresses this relationship (Robinson and Stokes, 1965) .o _ r0"7852[z,* z,_ Iq2.°+ ] ,;,,. = a,., L. ]-T~ + 30.321z,+1 1 ':2. t6)
Table 3. Values of [x] for use with equation (9) Ion Na * K" Ca'* Mg"" CI NO~ HC03 SO~-
Value of Ix] 41.8 47.1 115.I 109.2 47.8 46.7 40.5 133.9
and values of x for the different ions are given in Table 3. The specific conductance of a mixed salt solution can now be calculated as the sum of the contributions of the individual ions.
where q is a term incorporating the ionic charge and i the values of equivalent conductance at infinite diluL = 1000 Z (2.N,). II0) tion. When Iz,+l = Iz,-{, q has the value 0.5, reducing n=l slightly for mono-divalent salts. Selecting the value with the values of 2. determined by equation 19). of q as 0.5 for all salts introduces a negligible error (~1%). To allow equation (61 to be used for single PROCEDURE ions in a mixed salt solution, the simplification . (a) Computational [z,-I = [z,-I is made. This will introduce a slight A theoretical value of specific conductance is calcunegative error for single salt solutions of the m o n o divalent type, e.g. Na,SO,~. Equation (6) can be lated from the analytical results for the major dissolved ionized species using equations (9) and (10). extended for concentrations of up to 0.1 N using the This value is then compared to the measured specific modification suggested by Robinson and Stokes conductance, a discrepancy indicating an error in the (1965), where by the term 11 -" is replaced by l t ' : / ( l + Ba P ' ) . The parameter B is a constant, whilst a analysis. represents the mean distance of closest approach of Ibt Graphical the ions in solution. The value of Ba can be approxiControl lines are prepared to represent the ionic mated as 1 without sacrificing accuracy. strength-specific conductance relationship for the Equation (6) therefore takes the form waters under analysis. A proportional composition (in -ol _ [0.02292ol + 30.32] I~ -',/(I + I ~2) (7) terms of normalities or mequiv l - t ) similar to that A, n l -~- Arl of the analytical data is used in the calculation. For for monovalent ions and for divalent ions example: a water analysed as 0.0005N in Ca 2. ,o _ [0.9162oa + 6 0 . 6 4 ] i t 2/(1 + it :). (8) HCO3, Na*, C | - (i.e. 25~o, respectively) will from A•n 2 ~ /'n2 equation (4) have an ionic strength of ~ t + Values of equivalent conductance at infinite dilution 0 . 5 + 0 . 5 + 0 . 5 ) x 10 - 3 = 1.25 x l0 -3. and from (2 °) are summarized in Table 2 for the major disequations (9) and (10) a specific conductance of 113.1 solved ionized species found in natural waters. Equaps c m - L. Similarly, for the same proportional compotions (7) and (8) can both be expressed in more gensition but double the concentration, the ionic strength eral terms as will be 2,5 x l0 -3 and the freshly calculated specific ,;. = 2 o - I x ] 1 l 2'(1 + I12), (9) conductance 218.7 #s c m - l A series of values derived in this fashion allow control lines to be drawn, against Table 2. Equivalent conductance at infinite diluwhich measured values of specific conductance and tion if.o) From Robinson and Stokes 11965) ionic strength are compared. A departure of the apart from HCO3 taken from Lind ([970) measured value from the control will indicate an error Equivalent conductance ,;.') in the analysis• It is important not to extend the conIon cm2 lequiv-~V trol lines beyond the range for which a constant proportional composition has been established, and indiNa 50.1 vidual control lines should be developed for each sigK 73,5 Ca-" 59.5 nificant change in proportionality. Mg: 53.0 C1 76.35 DISCUSSION NO~ "~1.46 HCO 3 44.5 Table 4 presents values of conductance calculated SO~.s0.0 by the above procedure and measured tliterature~
Specific conductance method for quality control in water analys.is
93
Table 4. Calculated specific conductance for single salt solutions and measured values Salt NaC1 Na2SOa MgSO4
Normality 0.001 0.01 0.001 0.01 0.001 0.01
Ionic strength x 10-3 1 10 1.5 15 2 20
Lc
Lml
Lm2
Lm3
123.7 1180 123 1111 122.6 1030
125 1180 126 1130 107 800
124 1180 124 1120 118 889
123.7 1185 124 1124 --
-
Lc calculated specific conductance gs era-~. Lml Lind (1970). Lm2 Tanji (1969). Lm3 Milazzo (1963).
values. A close agreement exists for the mono-monoThe method is subject to a number of limitations valent salt, whilst the mono-divalent salt reveals the which should be borne in mind either before applislight negative error introduced by the simplifying cation or whilst evaluating the significance of any assumption Iz..I--Iz.-[ in equation (6). The di--di- departures. valent salt shows a significant positive error, particu1. The method is most appropriate for waters with larly at higher concentrations (>0.00IN). Tanji one or two predominant salts. (1969) has shown that this can be accounted for by 2. It provides most information about the accuracy a reduced ion activity as a result of ion pairing. of the major ionized species. The procedure has been applied to data selected 3. The sensitivity is reduqed as the waters are randomly from the literature (Fig. l). The samples diluted. all represent waters dominated by the Ca 2÷ and 4. Inaccuracies in separate deternainants can cancel HCO~ ions with a major ion proportional composi- out providing a false indication of accuracy. This sittion within the range of the two control lines. The uation should be apparent in a biased cation/anion close agreement between the sample points and the balance. control lines both confirm the validity of the method 5. There is an interdependence of accuracy evaluand suggest a high degree of accuracy for the reported ation (choice of control line) and accuracy itself. This data, a feature to be expected of published results. reduces the sensitivity slightly; however, it does not The additional data points in Fig. 1 represent apply to the use of the computational procedure• samples collected and analysed by the author for a 6. The calculated conductance fails to account for natural limestone catchment in the north west of Eng- ion pairing. This is only significant for divalent ions land. These data should lie within the control lines; i.e. SO~-, particularly at higher concentrations however, the considerable departures that are evident, >0.001 N i.e. > 4 8 m g 1 - 1 SO~-. particularly at the higher concentrations, suggest a 7. Complexation with organic ligands could also poor data accuracy. The variable magnitude of the reduce the ionic activity, hence conductance, but not departures also suggests a poor precision. necessarily the measured concentration. There is little The departures can be further interpreted to help identify the source of the error by making use of the relatively high degree of accuracy with which specific conductance can be measured, generally 1-5Yo ~O'7// , ~ ~ ¢~ (APHA, 1971; Kingsford et al., 1973). This accuracy can be both maintained and confirmed by regular standardization of the conductance bridge with potas-~ ~ + + J ".,-" sium chloride solutions of known conductance o ,~, / I (APHA, 1971). The measured conductance can therefore, as a first approximation, be taken to be a true value and hence from the control lines a 'true' ionic strength can be obtained. For the data under consideration the difference between 'true' and measured /,¢ ,, s ~ ' ' S ionic strength indicates a deficiency of determined 9 o~ ions. of up to -0/o, This deficiency was subsequently attributed, after examining the cation/anion balance and the results of an inter-laboratory evaluation Specific conduc'Ponce, / ~ e c m -~ scheme, to a poorly performed calcium and magnesium determination. It is also possible to suggest that Fig. 1. Ionic strength vs specific conductance. Control lines Ca 2* 42.5~o, Mg -'~ 5°'0, Na ~ 2.5°0. HCO~ 40°.o. SO~the error was due to sample deterioration prior to (A) 7.5°,o, CI- 2.5~:o.(B) Ca 2+ 40%o, Mg -'÷ 5°o, Na t 5°0, HCO~ analysis, as the conductance was measured in the 40°i. SO~- 5%. CI- 5~g. Data points: + Author. O' Lind field. (19701. 0 : Jacobson and Langrnuir (1974).
94
D . P . H . LAXEN
information available at present, with which to judge the significance of this effect. Extra care is therefore required when dealing with organic-rich waters and the validity of the method should be established before it is applied to such waters. 8. In view of the above, the method is probably best limited to waters with a conductance in the range 50-1000 #S cm-1. CONCLUSION The procedure described is particularly suitable as a control on data quality during routine water analysis. It can also be used to retrospectively assess the quality of existing data. An obvious application for the method is to the quality control of analyses of limestone waters, for which analytical accuracy is of particular significance in the calculation of saturation indices, the degree of non-carbonate hardness and other parameters of value to the study of limestone hydrogeochemistry and the chemistry of potable water supplies. REFERENCES
APHA (1971) Standard Methods Jor the Examination of Water and Waste Water. Am. Publ. Health. Assoc. Inc., New York.
Edmunds W. M. (1971) Hydrooeochemistry oJ Groundwaters in the Derbyshire Dome. with Special Reference to Trace Constituents. Inst. Geol. Sci. Rep. 7~;7. H.M.S.O., London. Ekedahl G, & Rondell B. (1973) lnterlaboratory study of methods of chemical analysis of water. Vatten 29(4). 341-356. Jacobson R. L. & Langmuir D. (1974) Controls on the water quality of some carbonate spring water. J. Hydrol. 23, 247-265. Kingsford M.. Stevenson C. D. & Edgerley W. H. L. (1973) Collaborative tests of water analysis (the Chemaqua Programme); part l--sodium, potassium, calcium, magnesium, chloride, sulphate, bicarbonate, carbonate and conductivity. N.Z. Dept Sci. Ind. Res., Chem. Div. Rep. CD 2159. 58 pp. Lewin J., Cryer R. & Harrison D. I. (1974) Sources of sediments and solutes in Mid Wales. In Fluvial Processes in Instrumental Watersheds (Edited by Gregory K. J. & Walling D. E.). Inst. Br. Geog. Spec. Publ. 6, p. 73. Lind C. J. (19701 Specific conductance as a means of estimating ionic strength. U.S. Geol. Surv. Prof. Paper 700D, D272-D280. Milazzo G. (1963) Electrochemistry. Elsevier, Amsterdam. Philbert F. J. & Traversy W. J. 11974) Methods of sample treatment and analysis of Great Lakes water and precipitation samples. Proc. 16th Conf. Great Lakes Res., Part 16, pp. 294-308. Robinson R.A. & Stokes R. H. (1965) Electrolyte Solutions. 2nd edn. Butterworth, London. Tanji K. K. (19691 Predicting specific conductance from electrolytic properties and ion association in some aqueous solutions. Proc. Soil Sci. Soc. Am. 33, 887-889.