The question "can I just use tap water" is asked more often than laboratories admit, and reverse-osmosis water from the wall unit is a common quiet substitute. This guide explains what each contains, what RO removes, and why neither is a laboratory substitute for bacteriostatic water.
What is in tap water
- Disinfectant: free chlorine or chloramine, added to keep the distribution system safe. Both are reactive oxidants that will attack many dissolved materials and will slowly oxidize benzyl alcohol if the two are mixed.
- Minerals: calcium, magnesium, sodium, chloride, sulfate, bicarbonate, silica; typically hundreds of mg/L in total.
- Trace metals from pipes: copper, iron, zinc, occasionally lead.
- Organics: natural organic matter and disinfection by-products.
- Microorganisms and endotoxin: low but non-zero bacterial counts, and endotoxin from the biofilm present in every distribution system.
Tap water is safe to drink and unsuitable as a solvent for anything that needs to be defined, sterile or endotoxin-free.
What reverse osmosis removes
RO forces water through a membrane that rejects most dissolved ions (typically 95 to 99%), most organics above a few hundred daltons, and most microorganisms and endotoxin by size exclusion. The product is much purer than tap water: low conductivity, low organics, reduced endotoxin. RO is in fact one of the accepted ways to make WFI-grade water, but only as part of a validated system with pretreatment, additional polishing (electrodeionization, ultrafiltration), sanitization, and continuous monitoring.
What a bench RO unit does not deliver
- Sterility. RO membranes are not absolute barriers and the downstream tubing, storage tank and tap are colonized by bacteria within days of installation. RO water at the point of use routinely carries bacterial counts.
- Endotoxin control. Whatever the membrane removed, the biofilm downstream adds back. Without monitoring and sanitization, the endotoxin level at the tap is unknown.
- Specification. A wall unit is not tested lot by lot; there are no lots. A bacteriostatic water vial comes with a certificate of analysis for exactly the liquid inside it.
- Preservation and a sealed container. RO water dispensed into a bottle is exposed to air and to the bottle, and has nothing in it to inhibit growth.
The comparison
| Property | Tap water | Bench RO water | Bacteriostatic water |
|---|---|---|---|
| Dissolved minerals | High | Low | Negligible (WFI-grade base) |
| Chlorine / chloramine | Present | Removed by pretreatment | Absent |
| Sterile | No | No | Yes, sealed and tested |
| Endotoxin controlled | No | No | < 0.5 EU/mL, tested |
| Preservative | No | No | 0.9% benzyl alcohol |
| Lot documentation | No | No | Lot-specific COA |
Where each belongs
Tap water: washing, cooling, non-critical rinsing. RO water: feeding polishers and stills, general glassware rinsing, non-sterile bench preparations that do not need defined purity. Bacteriostatic water: repeated small withdrawals of sterile, endotoxin-controlled, preserved water for materials that will not touch living cells or UV-based analysis. See laboratory uses.
Adding benzyl alcohol to RO water does not make bacteriostatic water
It produces an unsterile, untested solution of unknown preservative content in an unsealed container. The reasons are set out in can you make bacteriostatic water yourself. The purification ladder that bacteriostatic water actually sits on is explained in USP water grades, and the related comparisons with distilled and deionized water follow the same logic.
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.