What is benzyl alcohol's molecular weight?

108.14 Da for C7H8O, PubChem CID 244, CAS 100-51-6.

Bacteriostatic Water

Benzyl Alcohol Chemical Identity

JMWritten & reviewed by Jack Muncaster · Founder, UK PeptidesLast reviewed 2026-08-234 cited sources

Benzyl alcohol is C7H8O at 108.14 Da, PubChem CID 244, CAS 100-51-6. Under aerobic conditions it oxidises toward benzaldehyde and ultimately benzoic acid — and the aldehyde is considerably more reactive toward proteins than the alcohol it came from.

Key facts

Formula: C7H8O
Molecular weight: 108.14 Da
PubChem CID: 244
CAS: 100-51-6
Oxidation product: Benzaldehyde, CID 240, 106.12 Da
Further oxidation: Benzoic acid, CID 243, 122.12 Da
Concentration in use: 0.9% w/v

What the molecule is

A benzene ring with a hydroxymethyl group — the simplest aromatic alcohol. That structure is exactly why it works as a preservative: the ring is hydrophobic enough to partition into microbial membranes while the hydroxyl keeps the molecule water-soluble at the concentrations used.

The synonyms worth recognising

Phenylmethanol, benzenemethanol, phenylcarbinol and alpha-toluenol all appear in chemical listings and all denote the same compound. Recognising them matters when reading a certificate or a safety data sheet that does not use the common name.

Research material referenced

Bacteriostatic Water 3ml — third-party HPLC tested

View — £4.99

What it turns into

Primary alcohols oxidise to aldehydes and then to carboxylic acids, and benzyl alcohol follows that path to benzaldehyde and benzoic acid. Abend and colleagues examined its stability under aerobic conditions in the Journal of Pharmaceutical and Biomedical Analysis in 2004, reporting formation of benzaldehyde dibenzyl acetal — evidence that the oxidation proceeds in real formulations rather than only in principle.

Why the degradation product is the concerning one

This is the part usually left out. Benzyl alcohol has no reactive electrophile. Benzaldehyde does — aldehydes react with primary amines, which every peptide has at its N-terminus and at every lysine side chain. So the compound that is genuinely unreactive toward peptides can generate one that is not, and the relevant question about an old or poorly stored vial is what has formed in it.

What that implies in practice

Air exposure and time both favour oxidation, which is a second and quite separate reason not to keep a partly used vial indefinitely. The microbiological limit on an entered multiple-dose container is the usual justification for discarding it; the chemistry of its preservative is an independent one pointing the same way.

The concentration and why it is fixed

0.9% w/v is the conventional level in bacteriostatic water. It is enough to inhibit bacterial growth across repeated entries and low enough to keep the total quantity introduced small when the diluent is used in the volumes reconstitution requires.

Extended research context

The Bacteriostatic Water deep dive

Deep dive: what makes water 'bacteriostatic'

Bacteriostatic water for injection is sterile water preserved with 0.9% benzyl alcohol. The benzyl alcohol disrupts bacterial cell membranes at low concentration, preventing microbial growth once the vial has been broached. That's why BAC water can be re-entered up to about 28 days after first use — sterile water cannot, because it has no preservative to inhibit contamination.

When to use BAC water vs sterile water in peptide research

BAC water is the default for reconstituting research peptides because researchers typically draw from the same vial across multiple sessions. Sterile water is appropriate only for single-use reconstitution or where benzyl alcohol would interfere with a downstream assay (rare, but possible in some cell-culture models sensitive to preservatives).

Compatibility and interactions

Benzyl alcohol is generally inert against most research peptides. The main exceptions are peptides with free thiols or highly reactive residues where the preservative could theoretically react — check the peptide's stability data. For 99% of research peptide handling, BAC water is the correct default.

Research applications

▸Reconstitution of lyophilised research peptides
▸Preparation of stock solutions for aliquoting
▸Diluent in analytical HPLC sample prep (where preservative is acceptable)
▸Reference solvent for peptide-stability studies
▸Teaching material for aseptic-technique training

Handling checklist

✓Store vial at room temperature (15–25 °C), out of direct sunlight
✓Use within 28 days of first puncture
✓Swab the septum with 70% isopropyl alcohol before each draw
✓Never share a BAC water vial across incompatible peptide chemistries
✓Discard the vial if cloudy, discoloured, or past the 28-day window

Common research-handling mistakes

Learnt from thousands of researcher orders across our UK labs.

✗ Using tap or distilled water instead of BAC

Fix: Only bacteriostatic or sterile WFI is appropriate — tap water contains microbes and minerals.

✗ Re-using a vial past 28 days

Fix: Even preserved, contamination risk rises; discard on the 28-day mark.

✗ Assuming BAC water is medicine-grade

Fix: It is a laboratory solvent when supplied for research; do not administer to humans.

Continue researching

Peer-reviewed guides, comparators and matched reference materials.

Frequently asked questions

What is benzyl alcohol's molecular weight?: 108.14 Da for C7H8O, PubChem CID 244, CAS 100-51-6.
Does it degrade?: Yes. Under aerobic conditions it oxidises toward benzaldehyde and ultimately benzoic acid, which Abend 2004 documented in formulation.
Why does that matter?: Benzyl alcohol has no reactive electrophile; benzaldehyde does. Aldehydes react with primary amines, which peptides carry at the N-terminus and every lysine.

Primary sources & clinical trials

Peer-reviewed research and registered trials from PubMed, ClinicalTrials.gov, PubChem, FDA and NIH. All links open in a new tab and point to the primary source, so every claim can be verified at origin.

Written and reviewed by

Jack Muncaster · Founder, UK Peptides

Jack founded UK Peptides in Manchester after repeatedly receiving research compounds with missing or recycled paperwork. He is responsible for supplier selection, batch release decisions and the content published in this research library. Every article here is sourced to primary literature and every product page to a signed third-party certificate.

How we write and check this content Report a factual error in this article

Popular across the research hub

One flagship guide from every other research category — keep exploring.

← Back to Forum