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超純水技術筆記 首頁 > 純水知識 > 水質的檢測與標準
使用pH meter來檢測超純水會有什麼樣的問題!
2011-09-09

iStock_000015780862XSmall-Germ boxing.jpg

  當使用一般的pH meter來檢測超純水時其讀值是
  非常不穩定的原因如下:

1.                一般pH meter的原理設計是針對使用在高離子強度 (High ionic strength) 的溶液中相對的超純水卻是離子強度極低 (Ultra-low ionic strength) 的溶液

2.                實際上市面上確實有低離子強度溶液專用的電極及高靈敏度的主機(但價格很高)如果不是使用這類型儀器偵測超純水的pH 讀值就會亂跳十分難以確認

3.                所以當使用一般的pH meter來檢測超純水時其讀值是非常不穩定的並且以檢測讀值的趨勢來看分偏高或偏低兩種如果遇到偏低的情況則是因為二氧化碳溶入的原因偏低是正常的並且如果持續觀察的話會發現pH會持續下降至最低4.5左右

4.                但如果所測到的超純水pH為不合理的偏高時(多為在9~11左右)就可能是有關pH電極的問題

5.                常識來說電極是需要定期保養的否則會造成電極上鹽橋(同義字: salt bridge /diaphragm / junction)的阻塞也會造成應有功能的喪失使檢測低離子強度的超出水時更加不穩定

6.                鹽橋乃使用疏鬆多孔的陶磁或鐵弗龍材質做成主要是做為電極內外陰陽離子平衡所用但因缺乏定期清潔在低離子強度的溶液下如果遇到上述的偏高情況可以加入一小匙中性鹽(Neutral salt  1g KCl /100ml)以提高離子在鹽橋上的擴散能力在大部份的情況下pH值會在幾秒之內會掉到7以下

7.                理論上中性的KCl不會改變pH pH所以會被中性鹽的加入而改變只是因為受到離子強度的改變而已所以如果偵測超純水而測到高pH的情況可能只是假象而已

iStock_000010782075XSmall--analyzing-samples.jpg結論

1.      一般實驗室的pH meter常出現的問題如下: (a) pH標準液過期及重複使用. (b)電極不用時未浸泡在3MKCl溶液中. (c) 未常更換電極內的參考電極溶液 (d) 未定期清潔電極 (e) 電極的清潔方法不對.

2.      如果 pH讀值偏鹼表示可能是電極出了問題(電極膜污染或老化鹽橋阻塞參考液污染等)如果要測超純水的pH等同於讓pH meter在極限條件下工作pH meter的工作能力是極具挑戰的

 

3.      純水老博士(彩+logo)-小圖.jpg我們不建議使用pH meter來証明水質的好壞,因為牽扯的因素太多了,所以以不接觸空氣的(On-line)方式,檢測超純水的導電度,是最準確並最穩定的做法,但技術層次要求更高。

 

如果您有任何水質方面的問題,請與我們連絡,謝謝 !!

 

   台灣艾爾加生命科學股份有限公司  系統知識傳遞經理  孫承儒

 

 

 

補充閱讀資料

pH Theory

pH is the universally accepted scale for the concentration of hydrogen ions in aqueous solution. It is an indication of acidity (pH<7) alkalinity (pH>7) or neutrality (pH=7). pH is defined as the negative logarithm of the molar concentration of the active hydrogen ions (activity).

pH = -log10[aH+]

Pure water undergoes autopyrolysis to yield equal numbers of hydrogen and hydroxide ions in very low quantities.

H2O <--> H+ + OH-

This is an equilibrium reaction for which an equilibrium constant has been determined.

Kw = [H+] x [OH-] = 10-14 at 25oC

If pOH is defined as the negative logarithm of the hydroxide concentration then the equilibrium expression can be rewritten as:

pKw = pH + pOH = 14

This equation applies for any system containing water and explains the balance between acidity and alkalinity and the reason for the pH scale ranging from 0 to 14.

pH is probably the most common of all routine measurements with extensive application in laboratories industries of all kinds and the environment. The most common mode of measurement is the electrode method. It requires measuring the voltage developed between two electrodes immersed in the sample and comparing that measurement to a calibration derived from the same electrode pair and known standards. The voltage developed by the electrode pair has very low power and requires a special high impedance voltmeter.


 TechpH2 TechpH3

The two electrodes have special qualities that enable them to work together to specifically measure pH. Most electrode pairs are enclosed in a single electrode body. Figure 6 shows the components of the IJ Combination pH electrode.

The glass half cell electrode consists of a pH sensitive lithium glass membrane attached to a sealed insulating tube containing a solution of fixed pH in contact with a silver-silver chloride element (Figure 7). It develops a voltage across the thin pH sensitive glass proportional to the activity of hydrogen ions in the solution. The relationship between the voltage and the hydrogen activity follows the Nernst equation:

E = E0 - (RT/nF) ln[H+].

This equation can be rewritten in linear form by substituting in the definition of pH and grouping all the constants to give:

E = E0 + SLOPE (T) x pH

E0 is referred to as the offset zero potential point or isopotential point since theoretically it is defined as the pH which has no temperature dependence. Most pH electrode manufacturers design their isopotential point to 0mV at pH 7 to correspond with the temperature compensation software in most meters. The offset potential is often displayed after calibration as an indication of the condition of the electrode. The IJ44/64 should read 0 +/- 30mV in a pH 7 buffer. In reality E0 is composed of several single potentials each of which have slight temperature coefficients and are sources of error in temperature compensation algorithms. For greatest accuracy it is advisable to calibrate at the same temperature as the sample measurement.

The SLOPE(T) factor is a function of temperature and contains the conversion of the natural logarithm to the base ten logarithm. It is defined as the number of mV per unit of pH and is the factor which is adjusted in temperature compensation algorithms in pH meters. The slope is another electrode status indicator often displayed after calibration on most pH meters and should read 58+/-3mV per pH unit at 25oC for the IJ44/46. Table 1 shows how the ideal SLOPE(T) factor varies with temperature.

Table 1: Values of 2.303RT/F 0o-50oC (mV) 

ToC

RTln(10)/F 

ToC

RTln(10)/F 

0

54.197

30

60.149

5

55.189

35

61.141

10

56.181

38

61.737

15

57.173

40

62.133

20

58.165

45

63.126

25

59.157

50

64.118

 

The potential developed across the membrane requires a reference electrode to complete the circuit. The reference half cell ideally maintains a constant potential regardless of other species in solution. Stability and non-selectivity are maintained by making electrical contact between the sample and reference half cell via an inert salt bridge. Typically the salt bridge is composed of concentrated potassium chloride the same salt used to form the Ag/AgCl half cell but since the IJ is a double junction design it has the option of using a variety of inert salts. This electrical contact must allow uninhibited movement of electrolyte between the sample and reference half cell to assure a repeatable constant reference potential. At the same time it must not grossly contaminate the sample with electrolyte. Therefore a restriction (typically a porous ceramic or plastic frit) is used to slow the flow.

If the restriction becomes clogged and movement of ions becomes inhibited the electrode system will appear to be stable in buffer solutions but produce errors in non-ideal samples (e.g. low ionic strength samples) (1). The IJ reference system addresses this problem by allowing free movement of electrolyte past the restriction and by allowing the junction to be easily cleaned and refreshed when needed. The result is a reference electrode system of assured reliability.

References:
(1)John A. Illingworth "A common source of error in pH measurement" Biochem. J. (1981) 195 p. 259-262.

 

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