A buyer choosing a diluent for a lyophilized peptide meets at least seven liquids with similar names and identical appearances. Some are grades of purity, some are sterile preparations, some contain a preservative, some contain salt, and two of them are prescription drug products in the United States. The names are used loosely in forums and in catalogues, which is how people end up asking whether bacteriostatic water and saline are interchangeable, or whether deionized water is sterile.
This post defines each one by the standard that governs it, puts them in a table, and says which belongs where. The sterile versus bacteriostatic water post covers the single most common pair in depth; the diluent post covers the choice for a given peptide. This one is the map of the whole territory.
What are the three families?
Purity grades of plain water. Deionized, distilled, Purified Water and Water for Injection describe how far water has been cleaned of ions, organics, particles and microbes, and by which process. None of them, as grades, says anything about a preservative or about salt.
Sterile water preparations. Sterile Water for Injection is Water for Injection that has been sterilised and packaged in a container that keeps it sterile. Bacteriostatic Water for Injection is the same with 0.9% benzyl alcohol added so that a multi-entry vial does not become a culture.
Sodium chloride solutions. 0.9% sodium chloride, normal saline, is water with 9 g of salt per litre, which makes it isotonic with cells. Bacteriostatic sodium chloride is the same with 0.9% benzyl alcohol.
What does each standard actually specify?
| Liquid | Governing standard | Defined by | Sterile? | Preservative? | Salt? |
|---|---|---|---|---|---|
| Deionized water | ASTM D1193, Types I to IV | Resistivity, total organic carbon, sodium, chloride, silica | Not by the grade | No | No |
| Distilled water | Process description | Water boiled and condensed; purity depends on the still and the feed | Not by the grade | No | No |
| Purified Water | USP <1231> | Conductivity, total organic carbon | Not required | No | No |
| Water for Injection | USP <1231> | As Purified Water plus an endotoxin limit of 0.25 EU/mL | Produced to be, packaged versions sterilised | No | No |
| Sterile Water for Injection | USP monograph | Water for Injection, sterilised, single-use containers | Yes | No | No |
| Bacteriostatic Water for Injection | USP monograph | Sterile water with 0.9% benzyl alcohol, multi-entry | Yes | 0.9% benzyl alcohol | No |
| 0.9% Sodium Chloride | USP monograph | 9 g/L NaCl in Water for Injection, sterile | Yes | No | 154 mM |
| Bacteriostatic 0.9% Sodium Chloride | USP monograph | As above with 0.9% benzyl alcohol, multi-entry | Yes | 0.9% benzyl alcohol | 154 mM |
Two things in this table surprise people.
The first is that deionized and distilled are not sterility claims. ASTM Type I water from a polishing system can be close to free of microbes because of its final filter, but the grade is defined by resistivity, 18.2 megohm-centimetres for Type I, and by organic carbon, not by culture. A bottle of distilled water from a supermarket is neither sterile nor low in endotoxin. For reconstituting a research peptide that will go into a cell assay, the purity grade is not enough on its own; sterility and endotoxin are separate questions, and the sterility post separates them.
The second is that Water for Injection carries an endotoxin limit and Purified Water does not. That is the whole difference between the two USP grades, and it is why a certificate line reading "endotoxin < 0.05 EU/mL" on a peptide lot is only meaningful if the diluent you add is not itself the source of endotoxin. The endotoxin post explains the units.
What does the preservative do, and which bottles have it?
Benzyl alcohol at 0.9% is bacteriostatic: it stops microbial growth in a vial that is entered more than once. It is the same preservative at the same concentration in bacteriostatic water and in bacteriostatic saline, and it is the only thing that makes either "bacteriostatic". The reconstitution solution post covers what it does and does not do to a peptide, and the cell-culture post covers why it matters for cells: benzyl alcohol is a membrane-active solvent and is not inert in a culture well.
So the rule for the preservative is simple. A vial you will enter once, or a stock you will aliquot and freeze, does not need it. A vial you will enter repeatedly over days needs it or needs to be filter-sterilised and handled aseptically, as the filter-sterilising post describes.
What does the salt do, and when does it matter?
Isotonic saline has about 308 milliosmoles per litre and an ionic strength of about 0.15 molar. Plain water has neither. For a lyophilized peptide the difference shows up in three places.
- Solubility. Most short peptides dissolve as readily in saline as in water. Some, particularly hydrophobic or aggregation-prone sequences, dissolve less well at physiological ionic strength because salt screens the charges that keep them apart; the aggregation post has the mechanism. Water first, then dilute into buffer, is the usual order.
- Assay compatibility. A stock in water added at 1% to a well changes the well's ionic strength negligibly. A stock in saline does the same. At 10% or more, the difference matters and the stock should match the assay's buffer.
- pH. Neither water nor saline is buffered. A peptide dissolved in either sits at whatever pH its own acidic and basic residues produce, which for most sequences is somewhere between 4 and 7. The pH and buffer post is the place for that.
Bacteriostatic saline exists because some multi-entry uses want both the preservative and the tonicity. For research reconstitution it is rarely the first choice, because the tonicity is usually set later by the assay buffer and the salt is one more variable in the stock.
What is the regulatory status of each?
This is the part most catalogues leave out. In the United States, Bacteriostatic Water for Injection, USP and Bacteriostatic 0.9% Sodium Chloride Injection, USP are prescription-only drug products. The DailyMed label for the Hospira bacteriostatic water carries "Rx only" in its title, and its labelled use is as a diluent for injectable drug products. Sterile Water for Injection, USP is likewise a drug product.
The reconstitution solution sold on this site is sterile water with 0.9% benzyl alcohol prepared for laboratory use: for reconstituting lyophilized research material on the bench. It is not the pharmacopoeial injection article, is not labelled as one, and is not sold for injection or for any route of administration into a person or animal. The distinction is not a technicality. It is the difference between a laboratory consumable and a prescription medicine, and a supplier should say which one it is selling. The legality post covers the wider frame.
Deionized, distilled and Purified Water are not drug products and are sold as laboratory or industrial consumables.
Which belongs where?
| Task | Use | Avoid |
|---|---|---|
| Single-entry reconstitution for an in-vitro assay | Sterile water, or the assay's own buffer | Deionized or distilled from an open container |
| Multi-entry stock kept at 4 °C for days | Laboratory reconstitution solution, 0.9% benzyl alcohol | Unpreserved water, unless filter-sterilised and handled aseptically |
| Cell culture addition at more than 1% of well volume | Stock in the assay buffer | Any benzyl-alcohol preparation |
| Diluting a stock to working concentration | The assay buffer | Water, which leaves the dilution unbuffered |
| Making the stock for a hydrophobic peptide | Water first, then buffer | Saline as the first solvent |
Frequently asked questions
Is bacteriostatic water the same as saline?
No. Bacteriostatic water is water with 0.9% benzyl alcohol. Saline is water with 0.9% sodium chloride. Bacteriostatic saline has both. The word that matches is the preservative; the solvent is different.
Is deionized water sterile?
Not by definition. Deionized describes ion removal. Sterility is a separate property that depends on filtration, sterilisation and the container, and is not part of the ASTM D1193 grade.
Can I reconstitute a peptide in saline?
For most sequences, yes, though water first and buffer second is the safer order for hydrophobic or aggregation-prone peptides. The tonicity will usually be set later by the assay buffer anyway.
Why is Water for Injection different from Purified Water?
An endotoxin limit, 0.25 EU/mL. Otherwise the two USP grades share their conductivity and organic-carbon limits.
Is the reconstitution solution on this site the USP injection product?
No. It is sterile water with 0.9% benzyl alcohol prepared for laboratory reconstitution of research material. The USP bacteriostatic injection products are prescription-only drugs with a different purpose, and this site does not sell them.
References
- ASTM International. ASTM D1193-06(2018), Standard Specification for Reagent Water. www.astm.org/d1193-06r18.html
- United States Pharmacopeia. General Chapter <1231> Water for Pharmaceutical Purposes. www.usp.org
- DailyMed, National Library of Medicine. Bacteriostatic Water for Injection, USP (Hospira), prescribing information. Read 1 October 2026. dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=87d6e9dc-fe3b-4593-ac9a-d7493d1959c7
Every product mentioned is sold for laboratory research use only and is not for human or animal use. Nothing on this page describes or recommends use of the material sold here in humans or animals.
