ガラス電極 ph 計での不確かさについて
DESCRIPTION
ガラス電極 pH 計での不確かさについて. 中村 進 ( NMIJ/AIST, Japan ) e-mail: [email protected]. Traceability system for pH measurement. (Notion definition). Primary (Harned cell) method. Primary pH buffer solution. (NMIJ, ….). Secondary (Glass electrode) method. Secondary pH buffer solution. - PowerPoint PPT PresentationTRANSCRIPT
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Traceability system for pH measurement
HapH lg (Notion definition)
Primary (Harned cell) method
Primary pH buffer solution(NMIJ, ….)
Secondary (Glass electrode) method
Secondary pH buffer solution
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ガラス電極法での不確かさの要因
液間電位差スターラの回転渦電流による電位変化温度効果ガラス電極表面の汚れメモリ効果測定電位のバイアスアルカリ誤差電気的なノイズ など
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2点測定法(bracketing procedure)
2 種類の pH 標準液 (pH1 と pH2）を用いて、試料の pH値を挟むように測定する。
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Two point calibration
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k
E
k
EETkpHpH jx
X
)()15.298(1[ 1
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Mathematical model
where k1 and k2 are slope factors of pH vs. temperature near T 25 C for buffers S
1 and S2 respectively;Ej = Ej1 - EjX is residual liquid junction p
otential
)}]15.298(1{)15.298(1{[ 2211
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TkpHTkpH
EEk
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Uncertainty budget for general caseFactor Unit
Standard uncertainty
Sensitivity coefficient
Sensitivity coefficient
unit
Uncertainty contribution
pH1 1 0.01 0.50 1 0.0050
pH2 1 0.01 0.50 1 0.0050
E1 V 0.0002 8.4 1/V 0.0017
E2 V 0.0002 8.4 1/V 0.0017
Ex V 0.0002 -17 1/V 0.0034
T K 0.1 -0.0191 1/K 0.00191
Ej V 0.0006 17 1/V 0.010
0.013Combined standard uncertainty
Data from the IUPAC Recommendations 2002
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pH(1)
pH(2)
EMF(1)
EMF(2)
EMF(X)
T
E(j)
Contribution of different parameters to uncertainty of pHX
60 %
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不確かさの小さい pH測定
I. ほぼ同一組成の水溶液での pH 値では、液間電位差は無視できるので、Ej 0 u(Ej) 0II. Harned セルで求めた一次標準液を使用すると、u(pH1) u(pH2) 0.002 0.005;
III. Differential - potentiometric cellを使用した場合
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Using of calibration solution ofthe same nominal composition
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Using of calibration solution ofthe same nominal composition
k
EETkpHpH x
X
)()15.298(1[ 1
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0 jE
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Uncertainty budget for case IFactor Unit
Standard uncertainty
Sensitivity coefficient
Sensitivity coefficient
unit
Uncertainty contribution
pH1 1 0.01 0.02 1 0.00021
pH2 1 0.01 0.98 1 0.0098
E1 V 0.0002 0.36 1/V 0.000072
E2 V 0.0002 17 1/V 0.0033
Ex V 0.0002 -17 1/V 0.0034
T K 0.1 -0.0191 1/K 0.0019
0.011Combined standard uncertainty
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Uncertainty budget for case I + IIFactor Unit
Standard uncertainty
Sensitivity coefficient
Sensitivity coefficient
unit
Uncertainty contribution
pH1 1 0.0015 0.02 1 0.000032
pH2 1 0.0015 0.98 1 0.0015
E1 V 0.0002 0.36 1/V 0.000072
E2 V 0.0002 17 1/V 0.0033
Ex V 0.0002 -17 1/V 0.0034
T K 0.1 -0.0191 1/K 0.0019
0.0053Combined standard uncertainty
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Contribution of different parameters to uncertainty of pHX
pH(1)
pH(2)
EMF(1)
EMF(2)
EMF(X)
T
15
2
5
2
4
2
3
2
2
2
1)( uuuuuEu
)1(
1
2
1
NN
EEu
N
ii
u2 ：ガラス電極を含む pH メータの不確かさ
Uncertainty of electromotive force測定起電力の不確かさは以下のように計算する。
平均値が期待値にどれだけ近いのか
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u3 ：短時間での起電力の安定性
u4 ：長時間での起電力の安定性
u5 ： pH メータの最小メモリの統計的誤差
Uncertainty of electromotive force
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Differential – potentiometric cell
⑥
① ②
⑤
③
④
⑥
① compartment of standard buffer solution S;② compartment of certified buffer solution X;③ hydrogen gas electrodes;④ sintered glass disk D4;⑤ sintered glass disks D0;⑥ water traps
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Pt H2 Buffer S ¦ Buffer X H2 Pt
Mathematical model
FRT
ETkpHpH sx /10ln
)]15.298(1[ 1
where E is electromotive force of differential - potentiometric cell
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Uncertainty budget for differential cell
Factor Unit Standard
uncertainty Sensitivity coefficient
Sensitivity coefficient
unit
Uncertainty contribution
pH1 1 0.0015 1.00 1 0.0015
E V 0.00006 17 1/V 0.001
T K 0.01 0.0191 1/K 0.0002
0.0018Combined standard uncertainty
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Contribution of different parameters to uncertainty of pHX
pH(S)
EMF
T
85 %
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結論
ガラス電極法による pH 測定での拡張不確かさ U (k = 2) は、 0.025 0.030pH一次標準液を用い、この標準液と同一組成と見なせる場合は 0.01;Differential – potentiometric cell を用いた場合の拡張不確かさは 0.004