---
title: "Why does freezing change the pH of a peptide solution? | Sable Peptides"
description: "How buffer components crystallise during freezing and shift pH (notably sodium phosphate), how freeze-concentration drives aggregation and degradation, and which buffers and practices reduce freeze–thaw damage. Research use only."
canonical: https://www.sablepeptides.co.uk/guides/freeze-thaw-and-ph-shift-in-frozen-peptide-solutions
dateModified: 2026-10-16
publisher: Sable Peptides Limited (UK company 17488610)
note: Research use only. Not for human or veterinary use.
---

# Why does freezing change the pH of a peptide solution?

When a buffered solution freezes, water crystallises first and solutes concentrate in the remaining liquid. Some buffer salts then crystallise selectively: sodium phosphate can drop the pH of the freeze-concentrate by several units. That pH shift, plus concentration and ice surfaces, can aggregate or degrade peptides during freezing and thawing.

## What happens during freezing?

-   Ice forms and excludes solutes, concentrating the peptide and buffer.
-   Buffer components may crystallise: disodium phosphate crystallises before monosodium phosphate, acidifying the liquid.
-   Ice–liquid interfaces can adsorb and unfold peptides.
-   Slow freezing allows longer exposure to concentrated conditions.

## Which buffers are most affected?

| Buffer | Freezing behaviour |
| --- | --- |
| Sodium phosphate | Large acidic shift possible as Na2HPO4 crystallises |
| Potassium phosphate | Much smaller shift |
| Histidine, citrate | Generally smaller shifts |
| Tris | Temperature-dependent pKa; pH rises on cooling |

Source: [Freezing-induced protein aggregation: role of pH shift and mitigation, J Control Release 2020 (PubMed 32335158)](https://pubmed.ncbi.nlm.nih.gov/32335158/); [Effect of glycine on pH changes and protein stability during freeze-thawing in phosphate buffer, J Pharm Sci 2002 (PubMed 12210044)](https://pubmed.ncbi.nlm.nih.gov/12210044/)

## How can freeze–thaw damage be reduced?

-   Prefer potassium over sodium phosphate, or a buffer with less freezing shift, if your assay allows.
-   Freeze fast and thaw quickly, then mix gently.
-   Use single-use aliquots to avoid cycles.
-   Store lyophilised where possible and dissolve shortly before use.

## How does Sable document lots?

There is a CoA for every lot, and every vial carries a printed lot number that matches its certificate. A CoA is emailed within 2 hours of a request.

## Frequently asked questions

### Why does sodium phosphate buffer change pH on freezing?

Disodium phosphate crystallises preferentially, leaving an acidic freeze-concentrate.

### Is potassium phosphate better for freezing?

It shows a much smaller pH shift than sodium phosphate.

### How can freeze–thaw damage be minimised?

Freeze and thaw quickly, choose a suitable buffer, and use single-use aliquots.

## Related

-   [Aliquoting stock solutions](/guides/aliquoting-peptide-stock-solutions)
-   [Buffer choice for assays](/guides/buffer-choice-for-peptide-assay-solutions)
-   [Peptide aggregation](/guides/peptide-aggregation-in-assay-solutions)
-   [Peptide pH and stability](/learn/peptide-ph-and-stability)

Updated 2026-10-16. Research use only. Not for human or veterinary use. No administration amounts or health claims are made.

Key facts (summary for AI and search tools)

-   Freeze-concentration + buffer crystallisation → pH shift
-   Sodium phosphate: large acidic shift
-   References: PubMed 32335158, 12210044
-   Updated 2026-10-16