实验室用水的种类和区别

水是实验室内一个常常被忽视但至关重要的试剂。实验室用水有那些种类？能达到什么级别？不同实验对水的要求有那些？

实验室常见的水的种类：

1、蒸馏水（Distilled Water ）：

实验室最常用的一种纯水，虽设备便宜，但极其耗能和费水且速度慢，应用会逐渐减少。蒸馏水能去除自来水内大部分的污染物，但挥发性的杂质无法去除，如二氧化碳、氨、二氧化硅以及一些有机物。新鲜的蒸馏水是无菌的，但储存后细菌易繁殖；此外，储存的容器也很讲究，若是非惰性的物质，离子和容器的塑形物质会析出造成二次污染。

2、去离子水（Deionized Water ）：

应用离子交换树脂去除水中的阴离子和阳离子，但水中仍然存在可溶性的有机物，可以污染离子交换柱从而降低其功效，去离子水存放后也容易引起细菌的繁殖。

3、反渗水（Reverse osmosis Water）：

其生成的原理是水分子在压力的作用下，通过反渗透膜成为纯水，水中的杂质被反渗透膜截留排出。反渗水克服了蒸馏水和去离子水的许多缺点，利用反渗透技术可以有效的去除水中的溶解盐、胶体，细菌、病毒、细菌内毒素和大部分有机物等杂质，但不同厂家生产的反渗透膜对反渗水的质量影响很大。

4、超纯水（Ultra-pure grade water）：

其标准是水电阻率为18.2MΩ-cm。但超纯水在TOC、细菌、内毒素等指标方面并不相同，要根据实验的要求来确定，如细胞培养则对细菌和内毒素有要求，而HPLC则要求TOC低。

评价水质的常用指标：

1、电阻率（electrical resistivity）：

衡量实验室用水导电性能的指标，单位为MΩ-cm，随着水内无机离子的减少电阻加大则数

值逐渐变大，实验室超纯水的标准:电阻率为18.2MΩ-cm。

2、总有机碳（Total Organic Carbon ，TOC）：

水中碳的的浓度，反映水中氧化的有机化合物的含量，单位为ppm 或 ppb。

3、内毒素（Endotoxin）：

革兰氏阴性细菌的脂多糖细胞壁碎片，又称之为“热原”，单位cuf/ml。

1、自来水（Tapwater）

Tap water is usually of uncontrolled quality, may have seasonal variations suchas level of suspended sediment depending on the source (municipal reservoir,river, well), may contain other chem-icals purposely added to drinking water(chlorine, fluoride), and is generally unsuitable for use in important experiments.Tapwater is fine for washing glassware but should always be followed by a rinsewith a higher-grade water (distilled, deionized, etc.).

2、蒸馏水（DistilledWater）

Distillation generally eliminates much of the inorganic

con-taminationandparticularly sediments present in tap water feedstock. Itwill also help reduce the level of some organic con-taminants in the water.Double distilling simply gives a slightly higher grade distilled water, butcannot eliminate either inorganic or organic contaminants. Distilled water is often produced in large stills that serve an

entiredepartment, or building. The quality of the water is dependent on how well theequipment is maintained. A significant stir occurred within a large university’sbiochemistry department when the first mention of a problem with the housedistilled water was a memo that came out from the maintenance department thatstated: “We would like to inform you that the repairs have been made to thestill serving the department. There is no longer any radium in the water.” Thenext day, a follow-up memo was issued that stated:“Correction—there is nolonger any sodium in the dis-tilled water.”

3、去离子水（DeionizedWater）

Deionized water can vary greatly in quality depending on the type and ef ficiencyof the deionizing cartridges used. Ion exchange beds used in home sy stems, forinstance, are used primarily to reduce the “hardness” of the water usually dueto high levels of divalent cations such as magnesium and calcium. The resin bedconsists of a cation exchanger, usually in the sodium form, which releasessodium into the water in exchange for removing the diva-lent ions. (Rememberthat when you attempt to reduce your sodium intake!) These beds therefore donot reduce the ionic content of the water but rather exchange one type of ionfor another.

Laboratory deionizing cartridges are usually mixed-bed cartridges designed toeliminate both anions and cations from the water. This is accomplished bypreparing the anion-exchange bed in the hydroxide (OH-) form and thecation-exchange resin in the acid (H+) form. Anions or cations in the water(including monovalent) are exchanged for OH-or H+, respectively, which combineto form neutral water. Any imbalance in the removal of the ions can result in apH change of the water.Typically water from deion-izingbeds is slightly acidic, often between pH 5.5 to 6.5. The deionizing resins can themselves increase the organiccon-taminant level in the water by leaching of resin contaminants, monomer, andso on, and should always be followed by a bed of activated carbon to eliminatethe organics so introduced.

4、18MΩ水 (ReverseOsmosis/MilliQTM)

The highest grade of water available is generally referred to as 18MW water.This is because when the inorganic ions are completely removed, the ability ofthe water to conduct electric current decreases dramatically, giving aresistance of 18 -mercial systems that produce this grade of waterusually apply a multiple-step cleanup process including reverse osmosis,mixed-bed ion exchangers, carbon beds, and filter disks for particulates. Somemay include filters that exclude microorganisms, resulting in a sterile waterstream. High-grade 18 MW water tends to be fairly acidic—near pH 5.Necessary pH adjustments of dilute buffer solutions preparedusing 18 MW water could cause discrep-ancies in the final ionic concentration ofthe buffer salts relative to buffers prepared