微粒子計數器

美國TSI AEROTRAK™微粒子計數器,符合ISO 21501-4的嚴格規定。 這些粒子計數器採用PSLs校正,追溯NIST,是公認的標準粒子測量。

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免疫組織化學染色試劑

有效解決non-specific binding造成的Background。
可用於2種物種來源相同的抗體進行染色。
經濟實惠,且可避免Ms on Ms,Rb on Rb 干擾。

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組織包埋、切片、染色、判讀掃片

應用於藥物開發、健康食品功能性,或研究單位探討疾病時透過不同動物模型加以實驗評估;對實驗動物全身組織器官全面性檢查或細部分析,以瞭解藥物與食品的毒性或功效。

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智能鞋底清潔機

※獨家具有彩色螢幕,觸控面板,可程序化控制自動或手動進出水。※並且多重感應器,可偵測及顯示內部維修保養訊息。工廠通過ISO9001、ISO4001、ISO13485、ISO17025等國際認證。

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使用pH meter來檢測超純水會有什麼樣的問題


 

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,其原因如下:

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時,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的,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3M的KCl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,對pH meter的工作能力是極具挑戰的。
 
3. 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。

 

 

 

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.