Page 2 of 9

15

Shehata, A. B., AlAskar, A. R., Aldosari, R. A., & AlMutairi, F. R. (2023). Characterization of the Calibration Results of Glass Electrode pH-Meters Using

Buffer Solutions Certified by Different Producers. European Journal of Applied Sciences, Vol - 11(6). 14-22.

URL: http://dx.doi.org/10.14738/aivp.116.15791

routine pH-meters. In this work, the performance of certified buffer solutions of pH4, 7 and 10

produced by the Saudi National Metrology Institute, SASO/NMCC [10] has been verified with

regard to three sets of certified buffer solutions of pH 4, 7, and 10 produced by the Slovak

National Metrology Institute, SMU. This has been carried out by multipoint calibration of four

pH-meters. The calibration results of each pH-meter were characterized by studying the

compatibility of calibration results with the certified pH values (4, 7 and 10), bias, residuals,

and the regression of pH mean values. By this study, performance of the SASO/NMCC buffer

solutions were verified by showing similar calibration results of the three SMU buffer solutions.

Evaluation of the calibration results has been carried out in accordance with the relevant

international standards [11, 12].

EXPERIMENTAL

The 4 pH-meters with glass electrodes which were calibrated are:1) ion meter 3345, Jenway,

UK, 2), ion meter 3520, Jenway, UK, Multi 9420 Willis Towers Watson (wtw) and Mittler Toledo,

Switzerland. The pH values and their uncertainties of the four sets of CRM buffer solutions used

for calibration are shown in table below.

SMU CRM 1 SMU CRM 2 SMU CRM 3 SASO/NMCC CRM

4.002±0.020

7.001±0.020

10.000±0.020

4.002±0.020

6.999±0.020

10.001±0.020

3.999±0.020

7.006±0.020

9.998±0.020

4.001±0.019

7.005±0.015

10.002±0.023

The calibration of the 4 pH-meters was performed at 25°C using an ISO/IEC 17025 laboratory- accredited procedure.

RESULTS AND DISCUSSION

Examining Traceability of the Calibration Results of the 4 pH-meters

The traceability of measurement results can be achieved by a chain of calibrations each is

associated by an uncertainty of measurements. Calibration is defined as: operation that, under

specified conditions, in a first step, establishes a relation between the quantity values with

measurement uncertainties provided by measurement standards and corresponding

indications with associated measurement uncertainties and, in a second step, uses this

information to establish a relation for obtaining a measurement result from an indication [13].

Figure 1 shows the calibration curves of the four different pH-meters. Three of them were

calibrated by the SMU sets of buffer solutions of pH 4, 7 and 10. The fourth pH-meter was

calibrated by the SASO/NMCC buffers.

y = 0.9973x + 0.0184

R2 = 1

0

1

2

3

4

5

6

7

8

9

10

11

1 2 3 4 5 6 7 8 9 10 11

Response

CRM pH

y = 1.0088x - 0.0472

R2 = 1

0

1

2

3

4

5

6

7

8

9

10

11

1 2 3 4 5 6 7 8 9 10 11

Response

CRM pH

SMU buffer

2

SMU buffer 1

Page 3 of 9

Services for Science and Education – United Kingdom 16

European Journal of Applied Sciences (EJAS) Vol. 11, Issue 6, December-2023

Figure 1: The calibration graphs of 4 pH-meters calibrated by 4 sets of CRM buffer solutions

From this figure, one can notice that the four curves showed different slopes and intercepts

which is normal because of the random errors. They also showed R2 of approximately 1,

indicating good linearity of the calibration results. The variation of the slopes and intercepts

can be owed to the different sensitivities of the glass electrodes. The metrological traceability

of these calibration results was assured by fulfilling the criterion shown in equation 1 [12,14].

(1)

Tables 1-4 show the calculations required by this criterion. From these tables, it is clear that

the numerator │xobs-xref│ is smaller than the denominator 2 √(uobs)2 + (uref)2. This means that

the calibration results of the 4 pH-meters meet the aforementioned criterion, and confirms the

equivalence of the SASO/NMCC buffer solutions to the SMU buffers.

Table 1: Compatibility of the calibration results of pH-meter 1 with the CRM pH of SMU 1

pH xobs xref xobs - xref uobs uref

pH4 4.010 4.002 0.008 0.011 0.010 0.030

pH7 7.000 7.000 0.000 0.013 0.010 0.033

pH10 9.991 10.000 -0.009 0.015 0.010 0.036

Table 2: Compatibility of the calibration results of pH-meter 2 with the CRM pH of SMU 2

pH xobs xref xobs - xref uobs uref

pH4 3.999 4.002 -0.003 0.016 0.010 0.038

pH7 7.01 6.999 0.011 0.023 0.010 0.050

pH10 10.042 10.001 0.041 0.031 0.010 0.065

Table 3: Compatibility of the calibration results of pH-meter 3 with the CRM pH of SMU 3

pH xobs xref xobs - xref uobs uref

pH4 3.995 3.999 -0.004 0.014 0.010 0.034

pH7 7.008 7.006 0.002 0.018 0.010 0.041

pH10 9.994 9.998 -0.004 0.023 0.010 0.050

Table 4: Compatibility of the calibration results of pH-meter 4 with the CRM pH of SASO/NMCC

y = 0.9973x + 0.0184

R2 = 1

0

1

2

3

4

5

6

7

8

9

10

11

3 4 5 6 7 8 9 10 11

Response

CRM pH

y = 1.0015x - 0.0121

R2 = 1

0

1

2

3

4

5

6

7

8

9

10

11

0 1 2 3 4 5 6 7 8 9 10 11

Response

CRM pH

SASO/NMCC buffer

( ) ( ) 2 2

2 obs ref x obs x ref xx u u - £ +

( ) ( ) 2 2

2 u u x obs x ref +

( ) ( ) 2 2

2 u u x obs x ref +

( ) ( ) 2 2

2 u u x obs x ref +

SMU buffer 3