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CCQM-K9, CCQM-K9.1, CCQM-K9.2, and APMP.QM-K9

MEASURAND : pH value of phosphate buffer
Sample 1 : [0.025 mol kg-1 KH2PO4 + 0.025 mol kg-1 Na2HPO4]
Measurements at 15 °C

NOMINAL VALUE : pH = 6.9 at 25 °C

Degrees of equivalence Di and expanded uncertainty Ui (k = 2)

Red diamonds: CCQM-K9 participants
Green triangle: SMU "subsequent" (CCQM-K9.1)
Orange circles: CCQM-K9.2 participants only
Blue squares: APMP.QM-K9 participants only

DITDI = -0.0377, and UITDI = 0.0502

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CCQM-K9, CCQM-K9.1, CCQM-K9.2, and APMP.QM-K9

MEASURAND : pH value of phosphate buffer
Sample 1 : [0.025 mol kg-1 KH2PO4 + 0.025 mol kg-1 Na2HPO4]
Measurements at 25 °C

NOMINAL VALUE : pH = 6.9 at 25 °C

Degrees of equivalence Di and expanded uncertainty Ui (k = 2)

Red diamonds: CCQM-K9 participants
Green triangle: SMU "subsequent" (CCQM-K9.1)
Orange circles: CCQM-K9.2 participants only
Blue squares: APMP.QM-K9 participants only

DMSL = 0.0272, and UMSL = 0.0040;
DITDI = -0.0642, and UITDI = 0.0530;
UVMI-STAMEQ = 0.0840

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CCQM-K9, CCQM-K9.1, CCQM-K9.2, and APMP.QM-K9

MEASURAND : pH value of phosphate buffer
Sample 1 : [0.025 mol kg-1 KH2PO4 + 0.025 mol kg-1 Na2HPO4]
Measurements at 37 °C

NOMINAL VALUE : pH = 6.9 at 25 °C

Degrees of equivalence Di and expanded uncertainty Ui (k = 2)

Red diamonds: CCQM-K9 participants
Green triangle: SMU "subsequent" (CCQM-K9.1)
Orange circles: CCQM-K9.2 participants only
Blue squares: APMP.QM-K9 participants only

DITDI = -0.0794, UITDI = 0.0538

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CCQM-K9, CCQM-K9.1, CCQM-K9.2, and APMP.QM-K9

MEASURAND : pH value of phosphate buffer
Sample 1 : [0.025 mol kg-1 KH2PO4 + 0.025 mol kg-1 Na2HPO4]
Measurements at 15 °C

NOMINAL VALUE : pH = 6.9 at 25 °C

   
Lab iDiUi
NRCCRM-0.00250.0061
PTB0.00170.0022
DPL0.00080.0019
KRISS-0.00340.0022
CENAM0.00150.0061
GUM0.00150.0022
VNIIFTRI0.00050.0039
SMU-0.00450.0022
NIST-0.00020.0014
   
SMU(s)0.00270.0031
   
DFM-0.00010.0024
NMIJ-0.00130.0032
INMETRO-0.00970.0065
CMI0.02640.0040
   
NIMT*-0.00610.0094
GL*0.00130.0114
MSL*--
KIM-LIPI*--
NML-SIRIM*0.00410.0042
ITDI*-0.03770.0502
VMI-STAMEQ*--

'(s)' means 'subsequent bilateral comparison'.
Italics characters indicate that the erroneous result was used for computing the degrees of equivalence involving SMU.
Blue indicates participants in CCQM-K9.2 only.
* indicates participants in APMP.QM-K9 only.


Results are presented under A4 printable format in Summary Results (.PDF file).

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CCQM-K9, CCQM-K9.1, CCQM-K9.2, and APMP.QM-K9

MEASURAND : pH value of phosphate buffer
Sample 1 : [0.025 mol kg-1 KH2PO4 + 0.025 mol kg-1 Na2HPO4]
Measurements at 25 °C

NOMINAL VALUE : pH = 6.9 at 25 °C

   
Lab iDiUi
NRCCRM-0.00330.0042
PTB0.00100.0025
DPL0.00100.0020
KRISS-0.00360.0023
CENAM-0.00530.0121
GUM0.00120.0021
VNIIFTRI0.00070.0038
SMU-0.00430.0023
NIST-0.00030.0017
   
SMU(s)0.00210.0033
   
DFM0.00140.0024
NMIJ-0.00090.0029
INMETRO-0.00350.0062
CMI0.01580.0040
   
NIMT*-0.00410.0070
GL*-0.00120.0118
MSL*0.02720.0040
KIM-LIPI*-0.01920.0134
NML-SIRIM*-0.00450.0040
ITDI*-0.06420.0530
VMI-STAMEQ*-0.00420.0840

'(s)' means 'subsequent bilateral comparison'.
Italics characters indicate that the erroneous result was used for computing the degrees of equivalence involving SMU.
Blue indicates participants in CCQM-K9.2 only.
* indicates participants in APMP.QM-K9 only.


Results are presented under A4 printable format in Summary Results (.PDF file).

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CCQM-K9, CCQM-K9.1, CCQM-K9.2, and APMP.QM-K9

MEASURAND : pH value of phosphate buffer
Sample 1 : [0.025 mol kg-1 KH2PO4 + 0.025 mol kg-1 Na2HPO4]
Measurements at 37 °C

NOMINAL VALUE : pH = 6.9 at 25 °C

   
Lab iDiUi
NRCCRM-0.00240.0042
PTB0.00120.0025
DPL0.00130.0020
KRISS-0.00340.0024
GUM-0.00120.0022
VNIIFTRI-0.00040.0040
SMU-0.00440.0024
NIST0.00120.0045
   
SMU(s)0.00290.0036
   
DFM0.00150.0026
NMIJ-0.00080.0032
INMETRO-0.00490.0062
CMI0.01700.0048
   
NIMT*-0.00190.0092
GL*-0.00040.0116
MSL*--
KIM-LIPI*--
NML-SIRIM*-0.00530.0042
ITDI*-0.07940.0538
VMI-STAMEQ*--

'(s)' means 'subsequent bilateral comparison'.
Italics characters indicate that the erroneous result was used for computing the degrees of equivalence involving SMU.
Blue indicates participants in CCQM-K9.2 only.
* indicates participants in APMP.QM-K9 only.


Results are presented under A4 printable format in Summary Results (.PDF file).

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CCQM-K9.2
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Metrology area, Sub-field Chemistry and Biology, Electrochemistry
Description pH measurement of a phosphate buffer solution
Time of measurements 2006 - 2007
Status Approved for equivalence
Final Reports of the comparisons
Metrologia, 2008, 45, Tech. Suppl., 08006
Measurand pH 6.9 at 25 °C
Parameters Temperature: 15 °C, 25 °C and 37 °C
Transfer device CCQM-K9 Sample 1: 0.025 mol/kg of KH2PO4 and Na2HPO4
Comparison type Key Comparison
Consultative Committee CCQM (Consultative Committee for Amount of Substance)
Conducted by CCQM (Consultative Committee for Amount of Substance)
Comments

This is a subsequent comparison to CCQM-K9, first scheduled to be carried out between Radiometer A/S and PTB, and then extended to other participants.

Results published on 13 April 2008

pH measurement of phosphate buffer solutions

CCQM-K9.2 and APMP.QM-K9 results are linked to those of CCQM-K9 (sample 1)
Pilot institute PTB
Physikalisch-Technische Bundesanstalt
Germany
Contact person P. Spitzer

+49 (0) 531 592 3130
First Name Last Name
wwww@ww.www +356719836 Institute 1 Institute 1 Khmelnitskiy
Pilot laboratory
PTB

Physikalisch-Technische Bundesanstalt, Germany, EURAMET

CMI

Czech Metrology Institute, Czechia, EURAMET

DFM

Danish Fundamental Metrology A/S, Denmark, EURAMET

INMETRO

Instituto Nacional de Metrologia, Qualidade e Tecnologia, Brazil, SIM

NMIJ AIST

National Metrology Institute of Japan, Japan, APMP

VNIIFTRI

Institute of Physical Technical and Radiotechnical Measurements, Rosstandart, Russian Federation, COOMET

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CCQM-K9, CCQM-K9.1, CCQM-K9.2, and APMP.QM-K9

  • CCQM-K9 and CCQM-K9.1 (subsequent bilateral SMU - PTB)

MEASURAND : pH value of phosphate buffer
Sample 1 : [0.025 mol kg-1 KH2PO4 + 0.025 mol kg-1 Na2HPO4]
Measurements at 15 °C

NOMINAL VALUE : pH = 6.9 at 25 °C

The key comparison reference value, pHR, is obtained as the variance-weighted mean of the results from the participants. The standard uncertainty, uR, of pHR is calculated according to the external consistency concept (see page 11 of the CCQM-K9 Final Report).

pHR = 6.8975 and uR = 0.0005

The degree of equivalence of each laboratory with respect to the reference value is given by a pair of terms: Di = (pHi - pHR) and Ui, its expanded uncertainty (k = 2),
Ui = 2(ui2 + uR2)1/2. Note that this gives a conservative estimate of Ui.

The degree of equivalence between two laboratories is given by a pair of terms:
Dij = Di - Dj = pHi - pHj and Uij, its expanded uncertainty (k = 2),
Uij = 2(ui2 + uj2)1/2.

Within its uncertainty, the performance of PTB is assumed to be similar in the original full comparison and in the subsequent bilateral comparison between SMU and PTB. Following this bilateral comparison:

– the new degree of equivalence of SMU with respect to the key comparison reference value is DSMU(s) = pHSMU(s) - pHPTB(s) + DPTB and
USMU(s) = 2(uSMU(s)2 + uPTB(s)2 + uR2)1/2;

– the new degree of equivalence between SMU and any other laboratory j is
DSMU(s) j = DSMU(s) - Dj and USMU(s) j = 2(uSMU(s)2 + uPTB(s)2 + uj2)1/2.

  • Linking CCQM-K9.2 results to CCQM-K9 results
The linkage process is detailed in Section 13 of the CCQM-K9.2 Final Report and is based on the performance of the common participants, PTB and VNIIFTRI, in both key comparisons. It leads to the computation of the degrees of equivalence of participants in CCQM-K9.2 only, relative to the CCQM-K9 key comparison reference values, and allows to extend the graph of equivalence obtained in CCQM-K9 at temperature 15 °C.

No pair-wise degrees of equivalence involving participants in CCQM-K9.2 only are computed.
  • Linking APMP.QM-K9 results to CCQM-K9 results
The linkage process is detailed in Section 8 of the APMP.QM-K9 Final Report and is ensured by the performance of SMU, NIM and NMIJ, which participated in APMP.QM-K9 and also in one of the key comparisons CCQM-K9, CCQM-K9 (subsequent) or CCQM-K9.2.
It leads to the computation of the degrees of equivalence of participants in APMP.QM-K9 only, relative to the CCQM-K9 key comparison reference values, and allows to extend the graph of equivalence obtained in CCQM-K9 at temperature 15 °C.

No pair-wise degrees of equivalence involving participants in APMP.QM-K9 only are computed.
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CCQM-K9, CCQM-K9.1, CCQM-K9.2, and APMP.QM-K9

  • CCQM-K9 and CCQM-K9.1 (subsequent bilateral SMU - PTB)

MEASURAND : pH value of phosphate buffer
Sample 1 : [0.025 mol kg-1 KH2PO4 + 0.025 mol kg-1 Na2HPO4]
Measurements at 25 °C

NOMINAL VALUE : pH = 6.9 at 25 °C

The key comparison reference value, pHR, is obtained as the variance-weighted mean of the results from the participants. The standard uncertainty, uR, of pHR is calculated according to the external consistency concept (see page 11 of the CCQM-K9 Final Report).

pHR = 6.8633 and uR = 0.0006

The degree of equivalence of each laboratory with respect to the reference value is given by a pair of terms: Di = (pHi - pHR) and Ui, its expanded uncertainty (k = 2),
Ui = 2(ui2 + uR2)1/2. Note that this gives a conservative estimate of Ui.

The degree of equivalence between two laboratories is given by a pair of terms:
Dij = Di - Dj = pHi - pHj and Uij, its expanded uncertainty (k = 2),
Uij = 2(ui2 + uj2)1/2.

Within its uncertainty, the performance of PTB is assumed to be similar in the original full comparison and in the subsequent bilateral comparison between SMU and PTB. Following this bilateral comparison:

– the new degree of equivalence of SMU with respect to the key comparison reference value is DSMU(s) = pHSMU(s) - pHPTB(s) + DPTB and
USMU(s) = 2(uSMU(s)2 + uPTB(s)2 + uR2)1/2;

– the new degree of equivalence between SMU and any other laboratory j is
DSMU(s) j = DSMU(s) - Dj and USMU(s) j = 2(uSMU(s)2 + uPTB(s)2 + uj2)1/2.

  • Linking CCQM-K9.2 results to CCQM-K9 results
The linkage process is detailed in Section 13 of the CCQM-K9.2 Final Report and is based on the performance of the common participants, PTB and VNIIFTRI, in both key comparisons. It leads to the computation of the degrees of equivalence of participants in CCQM-K9.2 only, relative to the CCQM-K9 key comparison reference values, and allows to extend the graph of equivalence obtained in CCQM-K9 at temperature 25 °C.

No pair-wise degrees of equivalence involving participants in CCQM-K9.2 only are computed.
  • Linking APMP.QM-K9 results to CCQM-K9 results
The linkage process is detailed in Section 8 of the APMP.QM-K9 Final Report and is ensured by the performance of SMU, NIM and NMIJ, which participated in APMP.QM-K9 and also in one of the key comparisons CCQM-K9, CCQM-K9 (subsequent) or CCQM-K9.2.
It leads to the computation of the degrees of equivalence of participants in APMP.QM-K9 only, relative to the CCQM-K9 key comparison reference values, and allows to extend the graph of equivalence obtained in CCQM-K9 at temperature 25 °C.

No pair-wise degrees of equivalence involving participants in APMP.QM-K9 only are computed.
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CCQM-K9, CCQM-K9.1, CCQM-K9.2, and APMP.QM-K9

  • CCQM-K9 and CCQM-K9.1 (subsequent bilateral SMU - PTB)

MEASURAND : pH value of phosphate buffer
Sample 1 : [0.025 mol kg-1 KH2PO4 + 0.025 mol kg-1 Na2HPO4]
Measurements at 37 °C

NOMINAL VALUE : pH = 6.9 at 25 °C

The key comparison reference value, pHR, is obtained as the variance-weighted mean of the results from the participants. The standard uncertainty, uR, of pHR is calculated according to the external consistency concept (see page 11 of the CCQM-K9 Final Report).

pHR = 6.8394 and uR = 0.0006

The degree of equivalence of each laboratory with respect to the reference value is given by a pair of terms: Di = (pHi - pHR) and Ui, its expanded uncertainty (k = 2),
Ui = 2(ui2 + uR2)1/2. Note that this gives a conservative estimate of Ui.

The degree of equivalence between two laboratories is given by a pair of terms:
Dij = Di - Dj = pHi - pHj and Uij, its expanded uncertainty (k = 2),
Uij = 2(ui2 + uj2)1/2.

Within its uncertainty, the performance of PTB is assumed to be similar in the original full comparison and in the subsequent bilateral comparison between SMU and PTB. Following this bilateral comparison:

– the new degree of equivalence of SMU with respect to the key comparison reference value is DSMU(s) = pHSMU(s) - pHPTB(s) + DPTB and
USMU(s) = 2(uSMU(s)2 + uPTB(s)2 + uR2)1/2;

– the new degree of equivalence between SMU and any other laboratory j is
DSMU(s) j = DSMU(s) - Dj and USMU(s) j = 2(uSMU(s)2 + uPTB(s)2 + uj2)1/2.

  • Linking CCQM-K9.2 results to CCQM-K9 results
The linkage process is detailed in Section 13 of the CCQM-K9.2 Final Report and is based on the performance of the common participants, PTB and VNIIFTRI, in both key comparisons. It leads to the computation of the degrees of equivalence of participants in CCQM-K9.2 only, relative to the CCQM-K9 key comparison reference values, and allows to extend the graph of equivalence obtained in CCQM-K9 at temperature 37 °C.

No pair-wise degrees of equivalence involving participants in CCQM-K9.2 only are computed.
  • Linking APMP.QM-K9 results to CCQM-K9 results
The linkage process is detailed in Section 8 of the APMP.QM-K9 Final Report and is ensured by the performance of SMU, NIM and NMIJ, which participated in APMP.QM-K9 and also in one of the key comparisons CCQM-K9, CCQM-K9 (subsequent) or CCQM-K9.2.
It leads to the computation of the degrees of equivalence of participants in APMP.QM-K9 only, relative to the CCQM-K9 key comparison reference values, and allows to extend the graph of equivalence obtained in CCQM-K9 at temperature 37 °C.

No pair-wise degrees of equivalence involving participants in APMP.QM-K9 only are computed.
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CCQM-K9, CCQM-K9.1, CCQM-K9.2, and APMP.QM-K9

  • CCQM-K9 and CCQM-K9.1 (subsequent bilateral SMU - PTB)

MEASURAND : pH value of phosphate buffer
Sample 1: [0.025 mol kg-1 KH2PO4 + 0.025 mol kg-1 Na2HPO4]
Measurements at 15 °C

NOMINAL VALUE : pH = 6.9 at 25 °C

pHi : result of measurement carried out by laboratory i
ui : combined standard uncertainty (k = 1) of pHi

 Lab i Temperature Date of measurement
15 °C
pHi ui
 NRCCRM 6.8950 0.0030 99-12
 PTB 6.8992 0.0010 99-11
 DPL 6.8983 0.0008 99-10
 KRISS 6.8941 0.0010 99-11
 CENAM 6.8990 0.0030 99-11
 GUM 6.8990 0.0010 99-12
 VNIIFTRI 6.8980 0.0019 99-10
 SMU* 6.8930 0.0010 00-02
 NIST 6.8973 0.0005 00-01
 SMU corrected 6.8970 0.0009 00-02

*SMU discovered a calculation error. The erroneous result is used for computing the degrees of equivalence involving SMU.
The erroneous result is corrected before it is used for evaluating the key comparison reference value.

A subsequent bilateral comparison was carried out between SMU and PTB in December 2000.
The composition of the measured sample was very similar to that of sample 1.
Measurements were also carried out at 25 °C and 37 °C.

 PTB(s) 6.9004 0.0011 00-12
 SMU(s) 6.9014 0.0010 00-12

'(s)' means 'subsequent bilateral comparison'.

  • CCQM-K9.2

MEASURAND : pH value of phosphate buffer
Sample 1: [0.025 mol kg-1 KH2PO4 + 0.025 mol kg-1 Na2HPO4]
Measurements at 15 °C

NOMINAL VALUE : pH = 6.9 at 25 °C

pHi : result of measurement carried out by laboratory i
ULab i : expanded uncertainty (k = 2) of pHi

 Lab i Temperature Date of measurement
15 °C
pHi ULab i
 CMI 6.9245 0.0036 Dec 2006
 DFM 6.8980 0.0016 Nov 2006
 INMETRO 6.8884 0.0062 Jan 2007
 NMIJ 6.8968 0.0026 Dec 2006
 PTB 6.8977 0.0020 Nov 2006
 VNIIFTRI 6.9008 0.0022 Jan 2007
  • APMP.QM-K9

MEASURAND : pH value of phosphate buffer
Sample : [0.0217 mol kg-1 KH2PO4 + 0.0219 mol kg-1 Na2HPO4]
Measurements at 15 °C

NOMINAL VALUE : pH = 6.86 at 25 °C

pHi : result of measurement carried out by laboratory i
u i : combined standard uncertainty (k = 1) of pHi

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CCQM-K9, CCQM-K9.1, CCQM-K9.2, and APMP.QM-K9

  • CCQM-K9 and CCQM-K9.1 (subsequent bilateral SMU - PTB)

MEASURAND : pH value of phosphate buffer
Sample 1 : [0.025 mol kg-1 KH2PO4 + 0.025 mol kg-1 Na2HPO4]
Measurements at 25 °C

NOMINAL VALUE : pH = 6.9 at 25 °C

pHi : result of measurement carried out by laboratory i
ui : combined standard uncertainty (k = 1) of pHi

 Lab i Temperature Date of measurement
25 °C
pHi ui
 NRCCRM 6.8600 0.0020 99-12
 PTB 6.8643 0.0011 99-11
 DPL 6.8643 0.0008 99-10
 KRISS 6.8597 0.0010 99-11
 CENAM 6.8580 0.0060 99-11
 GUM 6.8645 0.0009 99-12
 VNIIFTRI 6.8640 0.0018 99-10
 SMU* 6.8590 0.0010 00-02
 NIST 6.8630 0.0007 00-01
 SMU corrected 6.8640 0.0009 00-02

*SMU discovered a calculation error. The erroneous result is used for computing the degrees of equivalence involving SMU.
The erroneous result is corrected before it is used for evaluating the key comparison reference value.

A subsequent bilateral comparison was carried out between SMU and PTB in December 2000.
The composition of the measured sample was very similar to that of sample 1.
Measurements were also carried out at 15 °C and 37 °C.

 PTB(s) 6.8657 0.0011 00-12
 SMU(s) 6.8668 0.0011 00-12

'(s)' means 'subsequent bilateral comparison'.

  • CCQM-K9.2

MEASURAND : pH value of phosphate buffer
Sample 1: [0.025 mol kg-1 KH2PO4 + 0.025 mol kg-1 Na2HPO4]
Measurements at 25 °C

NOMINAL VALUE : pH = 6.9 at 25 °C

pHi : result of measurement carried out by laboratory i
ULab i : expanded uncertainty (k = 2) of pHi

 Lab i Temperature Date of measurement
25 °C
pHi ULab i
 CMI 6.8786 0.0036 Dec 2006
 DFM 6.8642 0.0016 Nov 2006
 INMETRO 6.8593 0.0059 Jan 2007
 NMIJ 6.8619 0.0022 Dec 2006
 PTB 6.8632 0.0020 Nov 2006
 VNIIFTRI 6.8642 0.0021 Jan 2007
  • APMP.QM-K9

MEASURAND : pH value of phosphate buffer
Sample : [0.0217 mol kg-1 KH2PO4 + 0.0219 mol kg-1 Na2HPO4]
Measurements at 25 °C

NOMINAL VALUE : pH = 6.86 at 25 °C

pHi : result of measurement carried out by laboratory i
u i : combined standard uncertainty (k = 1) of pHi

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CCQM-K9, CCQM-K9.1, CCQM-K9.2 and APMP.QM-K9

  • CCQM-K9 and CCQM-K9.1 (subsequent bilateral SMU - PTB)

MEASURAND : pH value of phosphate buffer
Sample 1 : [0.025 mol kg-1 KH2PO4 + 0.025 mol kg-1 Na2HPO4]
Measurements at 37 °C

NOMINAL VALUE : pH = 6.9 at 25 °C

pHi : result of measurement carried out by laboratory i
ui : combined standard uncertainty (k = 1) of pHi

 Lab i Temperature Date of measurement
37 °C
pHi ui
 NRCCRM 6.8370 0.0020 99-12
 PTB 6.8406 0.0011 99-11
 DPL 6.8407 0.0008 99-10
 KRISS 6.8360 0.0010 99-11
 CENAM - - -
 GUM 6.8382 0.0009 99-12
 VNIIFTRI 6.8390 0.0019 99-10
 SMU* 6.8350 0.0010 00-02
 NIST 6.8406 0.0022 00-01
 SMU corrected 6.8397 0.0009 00-02

*SMU discovered a calculation error. The erroneous result is used for computing the degrees of equivalence involving SMU.
The erroneous result is corrected before it is used for evaluating the key comparison reference value.

A subsequent bilateral comparison was carried out between SMU and PTB in December 2000.
The composition of the measured sample was very similar to that of sample 1.
Measurements were also carried out at 15 °C and 25 °C.

 PTB(s) 6.8422 0.0011 00-12
 SMU(s) 6.8439 0.0013 00-12
'(s)' means 'subsequent bilateral comparison'.

  • CCQM-K9.2

MEASURAND : pH value of phosphate buffer
Sample 1: [0.025 mol kg-1 KH2PO4 + 0.025 mol kg-1 Na2HPO4]
Measurements at 37 °C

NOMINAL VALUE : pH = 6.9 at 25 °C

pHi : result of measurement carried out by laboratory i
ULab i : expanded uncertainty (k = 2) of pHi

 Lab i Temperature Date of measurement
37 °C
pHi ULab i
 CMI 6.8564 0.0043 Dec 2006
 DFM 6.8409 0.0016 Nov 2006
 INMETRO 6.8345 0.0058 Jan 2007
 NMIJ 6.8386 0.0024 Dec 2006
 PTB 6.8403 0.0020 Nov 2006
 VNIIFTRI 6.8393 0.0026 Jan 2007
  • APMP.QM-K9

MEASURAND : pH value of phosphate buffer
Sample : [0.0217 mol kg-1 KH2PO4 + 0.0219 mol kg-1 Na2HPO4]
Measurements at 37 °C

NOMINAL VALUE : pH = 6.86 at 25 °C

pHi : result of measurement carried out by laboratory i
u i : combined standard uncertainty (k = 1) of pHi