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Physical & Chemical properties

Water solubility

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Administrative data

Endpoint:
transformation / dissolution of metals and inorganic metal compounds
Type of information:
experimental study
Adequacy of study:
key study
Study period:
The acute transformation/dissolution tests were carried out between 03 Aug 2000 and 14 Aug 2000
Reliability:
2 (reliable with restrictions)

Data source

Reference
Reference Type:
study report
Title:
Unnamed
Year:
2000

Materials and methods

Test guidelineopen allclose all
Qualifier:
according to guideline
Guideline:
other: ECD Guideline for testing of chemicals - draft proposal - February 2000 : Transformation dissolution of metals and metal compounds in aqueous media.
Qualifier:
according to guideline
Guideline:
other: OECD Guideline for testing of chemicals - draft proposal - December 1998 : Transformation dissolution of metals and sparingly soluble inorganic metal compounds in aqueous media
Qualifier:
according to guideline
Guideline:
other: Recommendations on parameters for a transformation/dissolution protocol for metals and sparingly soluble metal compounds. ECB meeting, February, 1997, Ispra. Report ECB/TM/9 (97) rev 2.
Qualifier:
according to guideline
Guideline:
other: Draft report of the OECD workshop on aquatic toxicity testing of sparingly soluble metals, inorganic metal compounds and minerals. Sept. 5-8, 1995 - Ottawa.
Qualifier:
according to guideline
Guideline:
other: International standard ISO 6341 (1989 (E)). Determination of the inhibition of the mobility of Daphnia magna Straus (Cladocera, Crustacea).
Principles of method if other than guideline:
The transformation/dissolution kinetic of a metal or metal compound in an ecotox medium is evaluated from transformation/dissolution testing. After addition of the test substance to the ecotox medium, the test vessels are continuously mixed (laboratory shaker, 100 rpm), the dissolved metal concentrations are meas¬ured at regularly times. The transformation/ dissolution rates and extents are calcu¬lated by statistical analysis, if possible.
GLP compliance:
not specified
Type of method:
flask method

Test material

Constituent 1
Chemical structure
Reference substance name:
Dicopper oxide
EC Number:
215-270-7
EC Name:
Dicopper oxide
Cas Number:
1317-39-1
Molecular formula:
Cu2O
IUPAC Name:
copper (I) oxide
Test material form:
solid: particulate/powder
Details on test material:
Purity: 97.8%

Results and discussion

Solubility of metal ions in aqueous mediaopen allclose all
Type of test:
full transformation/dissolution test - metals and sparingly soluble metal compounds
Mean dissolved conc.:
0.101 mg/L
Element analysed:
Copper
Loading of aqueous phase:
1 mg/L
Incubation duration:
7 d
Test conditions:
Medium: Swedish water 2
Buffering system: air buffering
initial pH:6.21, pH after 7 days: 6.19
Type of test:
full transformation/dissolution test - metals and sparingly soluble metal compounds
Mean dissolved conc.:
0.122 mg/L
Element analysed:
Copper
Loading of aqueous phase:
1 mg/L
Incubation duration:
7 d
Test conditions:
Medium: Swedish water 2
Buffering system: HCl/NaOH buffering
Initial pH: 6.21, pH after 7 days 6.09
Type of test:
full transformation/dissolution test - metals and sparingly soluble metal compounds
Mean dissolved conc.:
0.131 mg/L
Element analysed:
Copper
Loading of aqueous phase:
1 mg/L
Incubation duration:
7 d
Test conditions:
Medium: Swedish water 2
Buffering system: MOP buffering
Initial pH: 6.21, pH after 7 days: 6.12
Type of test:
full transformation/dissolution test - metals and sparingly soluble metal compounds
Mean dissolved conc.:
0.171 mg/L
Element analysed:
Copper
Loading of aqueous phase:
1 mg/L
Incubation duration:
7 d
Test conditions:
Medium: modified ISO 6341
Buffering system: air buffering
Initial pH: 6.00, pH after 7 days: 6.54
Type of test:
full transformation/dissolution test - metals and sparingly soluble metal compounds
Mean dissolved conc.:
0.232 mg/L
Element analysed:
Copper
Loading of aqueous phase:
1 mg/L
Incubation duration:
7 d
Test conditions:
Medium: modified ISO 6341
Buffering system: HCl/NaOH buffering
Initial pH: 6.00, pH after 7 days: 6.16
Type of test:
full transformation/dissolution test - metals and sparingly soluble metal compounds
Mean dissolved conc.:
0.361 mg/L
Element analysed:
Copper
Loading of aqueous phase:
1 mg/L
Incubation duration:
7 d
Test conditions:
Medium: modified ISO 6341
Buffering system: MOP buffering
Initial pH: 6.00, pH afetr 7 days: 6.03
Details on results:
The mean dissolved Cu concentrations after 7 days in the natural and artificial media are : Swedish water 2 (Brattfors) : 0.118 +/- 0.020 mg Cu/l; modified ISO 6341 medium : 0.254 +/- 0.090 mg Cu/l.

Any other information on results incl. tables

Acute transformation/dissolution test at 1 mg.l-1 in Swedish surface water - Brattfors (7 days)

      The results of the dissolved Cu concentrations measured after 7 days after addi¬tion of Cu2O powder to Swedish water 2 and using different buffering systems are presented in table 1.  

      The pH, temperature and dissolved oxygen concentration in the different test solutions were measured at the start of the test and at each sampling time : see table 2.  

      The dissolved Cu concentrations increased in function of time.  The dissolved Cu concentration after 7 days were :       

      - air-buffering : 0.1013 +/- 0.0122 mg Cu.l-1       

- HCl/NaOH buffering : 0.1222 +/- 0.0115 mg Cu.l-1       

      - MOPS buffering : 0.1308 +/- 0.0222 mg Cu.l-1

The statistical analysis revealed that the data could be fitted by a linear model :       

      - air-buffering : mg Cu.l-1 = 0.0138 * time + 0.0039              R2 = 0.993       

      - HCl/NaOH buffering: mg Cu.l-1 = 0.0169 * time + 0.0012              R2 = 0.996       

      - MOPS buffering: mg Cu.l-1 = 0.0182 * time + 0.0030              R2 = 0.998       

Acute transformation/dissolution test at 1 mg.l-1 in modified ISO 6341 medium (7 days)

      The results of the dissolved Cu concentrations measured after 7 days after addition of Cu2O powder to modified ISO 6341 medium and using different buffering systems are presented in table 3.  

      The pH, temperature and dissolved oxygen concentration in the different test solutions were measured at the start of the test and at each sampling time : see table 4.  

      The dissolved Cu concentrations increased in function of time.  The dissolved Cu concentration after 7 days were :       

      - air-buffering : 0.1707 +/- 0.0532 mg Cu.l-1       

      - HCl/NaOH buffering : 0.2317 +/- 0.0331 mg Cu.l-1       

      - MOP's buffering : 0.3610 +/- 0.0311 mg Cu.l-1

The statistical analysis revealed that the data could be fitted by a linear model :       

      - air-buffering : mg Cu.l-1 = 0.0239 * time + 0.0041              R2 = 0.997       

      - HCl/NaOH buffering: mg Cu.l-1 = 0.0335 * time + 0.0014              R2 = 0.998       

      - MOP's buffering: mg Cu.l-1 = 0.0510 * time + 0.0050              R2 = 0.999       

Table 1       Acute transformation/dissolution test (7 days) : Cu2O, in Swedish surface water 2 – Brattfors - mass loading : 1 mg.l-1 - different buffering systems.  Mean (mg dissolved Cu.l-1); SD : standard deviation (mg dissolved Cu.l-1); C.V. : coefficient of variation (%).       

Blank Swedish water 2 (after pH adjustment) : < 0.0025 mg Cu.l-1.

Table 1.1 Air buffering

time   test jar dissolved Cu (mg.l-1) mean per test jar   mean per sampling time  total SD total CV 
 2 hours 1 

0.0044

0.0046 

 0.0045  0.0046  0.0001  3 
  2 

 0.0045

0.0047

 0.0046       
   3

 0.0045

0.0047

 0.0046       
 6 hours  1

0.0075

0.007 

 0.0073  0.0080  0.0010  13 
   2

 0.0073

0.0076

 0.0075       
   3

0.0094

0.0092 

 0.0093       
1 day  1 

0.0234

0.0217 

0.021

 0.0020  0.0233  0.0024   10
  2 

 0.0215

0.0215

0.0218

 0.0216       
  3 

 0.0271

0.026

0.0261

  0.0264      
 4 days  1

0.0497

0.0496 

 0.0497   0.0570 0.0083  15 
   2

 00542

0.0536

 0.0539       
  3 

 0.0677

0.067

 0.0674       
7 days   1

00941

0.0931 

 0.0936   0.1013 0.0122  12 
  2 

 0.0922

0.0943

 0.0933       
  3 

0.0118

0.0116 

 0.1170       

Table 1.2       HCl/NaOH buffering

time   test jar dissolved Cu (mg.l-1) mean per test jar   mean per sampling time  total SD total CV 
 2 hours  1

 0.0039

0.0037

 0.0038  0.0040  0.0002  5 
  2 

 0.004

0.004

 0.0040       
  3 

0.0042

0.0042 

 0.0042       
6 hours  1 

 0.0061

0.0061

  0.0061  0.0064 0.0005  8 
  2 

 0.0058

0.0069

 0.0064       
  3 

0.0064

0.007 

 0.0067       
1 day   1

0.018

0.0187

0.0181 

 0.0183   0.0192  0.0015 8 
  2 

 0.0186

0.018

0.0178

 0.0181       
  3 

 0.021

0.0212

0.0211

  0.0211      
4 days  1 

0.053

0.0538 

0.0534 

 0.0635  0.0080  13 
  2 

0.0693

0.071 

 0.0702       
  3 

 0.0671

00665

 0.0668       
7 days   1

 0.109

0.109

 0.1090   0.1222 0.0115  9 
   2

0.133

0.136 

  0.1345      
   3

 0.123

0.123

 0.1230       

Table 1.3       MOP buffering

time   test jar dissolved Cu (mg.l-1) mean per test jar   mean per sampling time  total SD total CV 
2 hours  1 

0.004

0.0043

 0.0042  0.0053  0.0010   19
  2 

0.0064

0.0064 

 0.0064       
  3 

0.0053

0.0054

 0.0054       
6 hours  1 

0.0072

0.0067

 0.0070  0.0087  0.0016  18 
  2 

0.0104

0.0105

 0.0105       
  3 

0.0089

0.0087

  0.0088      
1 day   1

 0.0176

0.0185

0.0178

 0.0180  0.0233  0.0055  23 
   2

0.0302

0.0303

0.0302 

 0.0302       
   3

0.0223

0.0213

0.0213 

 0.0216       
4 days   1

0.0691

0.0702 

  0.0697 0.0740   00134 18 
  2 

 0.0896

0.0916

 0.0906       
   3

0.0642

0.0594 

 0.0618       
 7 days 1 

0.128

0.13 

 0.1290  0.1308  0.022  17 
  2 

0.156

0.157 

 0.1565       
   3

0.107

0.107 

 0.1070       

Table 2       Results of the pH, temperature and dissolved oxygen measurements during the acute transformation/dissolution test (7 days) : Cu2O powder, in Swedish surface water 2 – Brattfors-  mass loading : 1 mg.l-1 - different buffering systems.  

Table 2.1       pH

buffering system   replicate  0 hours  2 hours  6 hours  1 day  4 days  7 days
air buffering  1  6.21  6.27   6.28  6.29  6.23  6.24
  2    6.29   6.30  6.32  6.21  6.12
  3    6.28   6.28  6.34 6.26  6.20
mean      6.28   6.29  6.32  6.23  6.19
sd       0.01  0.01  0.03  0.03  0.06

HCl/NaOH

buffering

1 

6.21 

6.29 

 6.28

 6.27

 6.21

 6.01

 

2 

6.27

6.28

6.34 

 6.18

 6.07

3 

 

 6.30

 6.29

 6.39

 6.18

 6.18

mean 

 

 

 6.29

 6.28

 6.33

 6.19

 6.09

sd 

 

 

 0.02

0.01 

0.06 

0.02 

0.09 

MOP

buffering

1 

6.21 

6.22 

 6.23

 6.17

 6.17

 6.13

 

2 

6.21

  6.22

6.23

 6.19

6.13 

 

3 

 6.19

6.23 

6.18 

 6.19

6.09 

 mean

 

 

 6.21

6.23 

6.19 

6.18 

6.12 

 sd

 

 

0.02 

0.01 

 0.03

0.01 

0.02 

Table 2.2       Temperature (°C)

 buffering system  replicate  0 hours  2 hours  6 hours  1 day  4 days  7 days

air

buffering

 1

22.0 

21.8 

 21.8

 21.7

 21.7

 21.8

 

 2

 

 21.8

 21.8

 21.7

 21.7

 21.8

 

 3

 

21.8 

 21.8

 21.7

 21.7

 21.8

 mean

 

 

 21.8

 21.8

 21.7

 21.7

 21.8

 sd

 

 

 0.0

 0.0

 0.0

 0.0

 0.0

 HCl/NaOH

buffering

 1

22.0 

 21.8

 21.8

 21.7

 21.7

 21.7

 

 2

 

 21.8

 21.8

 21.7

 21.7

 21.8

 

 3

 

 21.8

 21.8

 21.7

 21.7

 21.7

 mean

 

 

21.8

 21.8

 21.7

 21.7

 21.7

 sd

 

 

0.0 

 0.0

 0.0

 0.0

 0.0

 

1

22.0 

21.8 

 21.8

 21.7

 21.8

 21.8

 

 2

 

21.8 

 21.8

 21.7

 21.7

21.8 

 

 3

 

 21.8

 21.8

 21.7

 21.8

 21.8

 mean

 

 

21.8 

 21.8

 21.7

 21.8

 21.8

 sd

 

 

 0.0

 0.0

 0.0

 0.0

 0.0

Table 2.3       Dissolved oxygen concentration (mg/l)

 buffering system  replicate  0 hours  2 hours  6 hours  1 day  4 days  7 days

air

buffering

 1

 8.01

8.01

 8.00

 8.05

 7.83

7.91 

 

 2

 8.03

8.03

8.06 

7.82

7.96 

 

 3

 

8.00 

8.00 

7.92 

7.85 

7.92 

 mean

 

 

 8.01

8.01 

8.01 

7.83 

7.93 

 sd

 

 

0.02 

0.02 

 0.08

0.02 

0.03 

 HCl/NaOH

buffering

 1

8.01 

7.96 

7.96 

 8.00

7.83 

8.01 

 

 2

 

7.92 

7.98 

 8.01

 7.96

 7.79

 

 3

 

 7.98

 7.98

 7.96

 7.81

 8.04

 mean

 

 

 7.95

 7.97

 7.99

 7.87

 7.95

 sd

 

 

 0.03

 0.01

 0.03

 0.08

 0.14

 

1

 8.01

 7.00

 7.05

 7.06

 7.05

 7.86

 

 2

 

 8.01

 7.99

 7.95

 7.93

7.96

 

 3

 

 8.03

 8.02

 7.93

 7.92

 7.95

 mean

 

 

 8.01

 7.99

 7.95

 7.93

 7.92
 sd     0.02

 0.04

 0.02

 0.02

 0.06

Table 3       Acute transformation/dissolution test (7 days) : Cu2O, in modified ISO 6341 medium - mass loading : 1 mg.l-1 - different buffering systems.  Mean (mg dissolved Cu.l-1); SD : standard deviation (mg dissolved Cu.l-1); C.V. : coefficient of variation (%).       

Blank modified ISO 6341 medium : < 0.0025 mg Cu.l-1.

Table 3.1       Air buffering

 time  test jar  dissolved Cu (mg.l-1)  mean per test jar  mean per sampling time  total SD  total CV
 2 hours  1

0.0071

0.0079 

 0.0075  0.0064  0.0020  31 
   2

0.008 

0.0077

 0.0079       
   3

0.004

0.0038 

 0.0039       
 6 hours  1

 0.0095

0.0099

 0.0097   0.0081  0.0021 26 
   2

0.0093

0.0093 

 0.0093       
   3

0.0055

0.0053 

 0.0054       
 1 day  1

0.0291

0.0281

0.0303 

 0.0292  0.0345  0.0099  29 
   2

0.0478

0.0473

0.0476

 0.0476       
   3

 0.0269

0.0267

0.0266

 0.0267       
 4 days  1

0.084

0.0864

 0.0852   0.100 0.0314  31 
   2

0.138

0.142

0.1400 

 

 

 

 

 3

0.0754

0.0741 

00748 

 

 

 

 7 days

 1

0.154 

0.1545 

0.1707

0.0532 

31 

 

 2

0.155 

0.2365 

 

 

 

 

 3

0.234 

 0.1210

 

 

 

Table 3.2       HCl/NaOH buffering

 time

 test jar

 dissolved Cu (mg.l-1)

 mean per test jar

 mean per sampling time

 total SD

 total CV

 2 hours

 1

0.0061

0.0057 

0.0059 

0.0058 

0.0005 

9 

 

 2

0.0051 

0.0053

0.0052 

 

 

 

 

 3

0.0061

0.0064 

 0.0063

 

 

 

 6 hours

 1

0.0083

0.0093 

0.0088 

0.0082 

0.0019 

 24

 

 2

0.0062

0.0056 

0.0059 

 

 

 

 

 3

0.0091

0.0106 

0.0099 

 

 

 

 1 day

 1

 0.0342

0.0343

0.0347

 0.0344

0.0322 

0.0023 

7 

 

 2

0.0331

0.0328

0.0331 

0.0330 

 

 

 

 

 3

0.0289

0.0289

0.03 

0.0293 

 

 

 

 4 days

 1

0.16

0.164 

0.1620 

0.1438 

0.0162 

 11

 

 2

0.142

0.145 

0.1435 

 

 

 

 

 3

 0.127

0.125

0.1260 

 

 

 

 7 days

 1

0.274

0.263 

0.2685

0.2317 

0.0331 

 14

 

 2

0.233

0.23 

0.2315

 

 

 

 

 3

0.194 

0.196

0.1950 

 

 

 

MOP buffering

 time

 test jar

 dissolved Cu (mg.l-1)

 mean per test jar

 mean per sampling time

 total SD

 total CV

 2 hours

 1

0.0098

0.0096 

0.0097 

0.0102 

 0.0006

 6

 

 2

 0.0105

0.0099

0.0102 

 

 

 

 

 3

 0.0111

0.0105

0.0108 

 

 

 

 6 hours

 1

 0.0138

0.0126

0.0132 

0.0148 

 0.0015

 10

 

 2

0.0168

0.0152 

0.0160 

 

 

 

 

 3

0.016

0.0141 

0.0151 

 

 

 

 1 day

 1

0.0548

0.0549

0.0544 

 0.0547

0.0640 

0.0700 

11 

 

 2

 0.0673

0.069

0.068

 0.0681

 

 

 

 

 3

 0.0693

0.0693

0.0688

0.0691 

 

 

 

 4 days

 1

0.179

0.185 

0.1820 

0.2088 

0.0213 

10 

 

 2

 0.228

0.226

0.2270 

 

 

 

 

 3

 0.217

0.218

0.2175 

 

 

 

 7 days

 1

 0.32

0.323

0.3215 

0.3610

0.0311

9

 

 2

0.386

0.388 

0.3870

 

 

 

 

 3

 0.375

0.374

0.3745 

 

 

 

Table 4       Results of the pH, temperature and dissolved oxygen measurements during the acute transformation/dissolution test (7 days) : Cu2O powder, in modified ISO 6341 medium - mass loading : 1 mg.l-1 - different buffering systems.

Table 4.1       pH

 buffering system  replicate  0 hours  2 hours  6 hours  1 day  4 days  7 days
 air buffering  1  6.00 6.05   6.06  6.30  6.17  6.39
   2   6.04   6.07  6.28  6.16  6.59
   3   6.04   6.05  6.31  6.19  6.65
 mean      6.04  6.06  6.30  6.17  6.54
 sd      0.01  0.01 0.02  0.02  0.14 

 HCl/NaOH

buffering

 1

 6.00

6.03 

6.04 

 6.38

5.97 

6.08 

 

 2

 

 6.04

6.04 

6.27 

6.05 

6.15 

 

 3

 

 6.03

 6.03

6.30 

 6.03

6.25 

 mean

 

 

 6.03

6.04 

6.32 

6.02 

6.16 

 sd

 

 

 0.01

 0.01

0.06 

0.04 

0.09 

 MOP

buffering

 1

6.00 

6.00 

 6.01

5.99 

6.02 

6.04 

 

 2

 

6.00 

6.00 

6.01 

 6.00

6.04 

 

 3

 

 6.00

 6.00

 6.00

6.01

6.03 

 mean

 

 

6.00 

 6.00

 6.00

6.01 

6.03 

 sd

 

 

 0.0

0.01 

0.01 

 0.01

0.01 

Table 4.2       Temperature (°C)

buffering system  replicate  0 hours  2 hours  6 hours  1 day  4 days  7 days
 air buffering  1 22.0   21.8  21.8  21,7  21.7  21.8
   2    21.8  21.8  21.7  21.7 21.8 
   3    21.8  21.8  21.7  21.7  21.8
 mean     21.8   21.8  21.7  21.7  21.8
 sd     0.0   0.0  0.0  0.0  0.0

 HCl/NaOH

buffering

 1

22.0 

21.8 

 21.8

 21.7

 21.7

 21.8

 

 2

 

21.8 

 21.8

 21.7

 21.7

 21.8

 

 3

 

 21.8

 21.8

 21.7

 21.7

 21.7

 mean

 

 

 21.8

 21.8

 21.7

 21.7

 21.8

 sd

 

 

0.0 

 0.0

 0.0

 0.0

 0.1

 MOP

buffering

 1

 22.0

 21.8

 21.8

 21.7

 21.7

 21.7

 

 2

 

21.8 

 21.8

 21.7

 21.7

 21.8

 

 3

 

 21.8

 21.8

 21.7

 21.7

 21.7

 mean

 

 

 21.8

 21.8

 21.7

 21.7

 21.7

 sd

 

 

 0.0

 0.0

 0.0

 0.0

 0.1

Table 4.3       Dissolved oxygen concentration (mg/l)

buffering system  replicate  0 hours  2 hours  6 hours  1 day  4 days  7 days
 air buffering  1 8.03  8.01  7.96  7.89  7.93  7.83 
   2   7.96   7.97 7.93  7.92  7.85 
   3    7.95 7.95  7.99  7.99  8.03 
 mean      7.97  7.96  7.94 7.95  7.90 
 sd      0.03 0.01   0.05  0.04  0.11

 HCl/NaOH

buffering

 1

8.03 

 7.93

 7.89

7.97 

8.03 

7.96 

 

 2

 

 8.00

 7.93

 7.98

 7.91

 7.92

 

 3

 

8.02 

 7.99

 8.01

 7.90

 7.83

 mean

 

 

7.98 

 7.94

 7.99

 7.95

 7.90

 sd

 

 

 0.05

 0.05

 0.02

 0.07

 0.07

 MOP

buffering

 1

8.03 

 8.01

 8.01

 7.93

 8.00

 8.12

 

 2

 7.96

 8.00

 7.95

 7.89

 8.03

 

 3

 

 7.95

 7.93

 8.04

 7.92

 7.85

 mean

 

 

 7.97

 7.98

 7.97

 7.94

 8.00

 sd

 

 

 0.03

 0.04

 0.06

 0.06

 0.14

Applicant's summary and conclusion

Conclusions:
Refer to Executive summary
Executive summary:

The mean measured Cu concentrations after a 7 days transformation/dissolution test with different buffering systems are:

 medium  buffering system  initial pH  pH after 7 days

 measured Cu conc.

after 7 days (mg.l-1)

 Swedish water 2

 air buffering

 6.21

 6.19

 0.1013

 

 HCl/NaOH buffering

 6.21

 6.09

 0.1222

 

 MOP buffering

 6.21

 6.12

 0.1308

 modified ISO 6341

 air buffering

 6.00

 6.54

 0.1707

 

 HCl/NaOH buffering

 6.00

 6.16

 0.2313

 

 MOP buffering

6.00 

 6.03

 0.3610

The mean dissolved Cu concentrations after 7 days in the natural and artificial media are : Swedish water 2 (Brattfors) : 0.118 +/- 0.020 mg Cu/l; modified ISO 6341 medium : 0.254 +/- 0.090 mg Cu/l.

A linear increase of dissolved Cu was demonstrated over the test period of 7 days.  The dissolution kinetic can be described by the following equation

 medium  buffering sytem  equation  R2
 Swedish water

   air buffering

 mg Cu/l = 0.0138 * time + 0.0039

 0.993

 

 HCl/NaOH buffering

 mg Cu/l = 0.0169 * time + 0.0012

 0.996

 

 MOP buffering

 mg Cu/l = 0.0182 * time + 0.0030

 0.998

 modified ISO 6341

   air buffering

 mg Cu/l = 0.0239 * time + 0.0041

 0.997

 

 HCl/NaOH buffering

 mg Cu/l = 0.0335 * time + 0.0014

 0.998

 

 MOP buffering

 mg Cu/l = 0.0510 * time + 0.0050

 0.999