A reference (standard) solution must always be calibrated for the pH meter before measuring pH. A buffer solution whose pH is unlikely to vary is used as the reference solution. Without a reliable reference solution, erroneous results can be expected.
Next, we will present five types of reference solutions specified in JIS, and the pH values of these solutions at each temperature.
Although it is not difficult to make these reference solutions, the process is troublesome if you do not have equipment for the purpose. It is more economical to purchase commercially available solutions.

Standard pH of Reference Solution, by Temperature
(JIS Z 8802 confirmed in 2011)

 

Temp(.C)

 

Nitrate Salt

 

Phthalate

Reference Solution

Neutral Phosphate

 

Borate

 

Carbonate

0

1.67

4.01

6.98

9.46

10.32

5

1.67

4.01

6.95

9.39

(10.25)

10

1.67

4.00

6.92

9.33

10.18

15

1.67

4.00

6.90

9.27

(10.12)

20

1.68

4.00

6.88

9.22

(10.07)

25

1.68

4.01

6.86

9.18

10.02

30

1.69

4.01

6.85

9.14

(9.97)

35

1.69

4.02

6.84

9.10

(9.93)

38

---

---

---

---

9.91

40

1.70

4.03

6.84

9.07

---

45

1.70

4.04

6.83

9.04

---

50

1.71

4.06

6.83

9.01

---

55

1.72

4.08

6.84

8.99

---

60

1.73

4.10

6.84

8.96

---

70

1.74

4.12

6.85

8.93

---

80

1.77

4.16

6.86

8.89

---

90

1.80

4.20

6.88

8.85

---

95

1.81

4.23

6.89

8.83

---

Names and Composition of Reference Solutions

Name

Composition

Oxalate reference solution

0.05 mol/L potassium tetraoxalate
KH3 (C2O4) 2·2H2O solution

Phthalate reference solution

0.05 mol/L potassium hydrogenphthalate
C6H4(COOK)(COOH)solution

Neutral phosphate reference solution

0.025 mol/L monopotassium phosphate KH2 PO4-
0.025 mol/L dibasic sodium phosphate Na2HPO4 solution

Borate reference solution

0.01 mol/L sodium borate (borax)
Na2B4O7·10H2O solution

Carbonate reference solution

0.025 mol/L sodium bicarbonate NaHCO3-
0.025 mol/L sodium carbonate Na2CO3 solution