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Short answer
Saline-sodium citrate (SSC) is a buffered salt solution of sodium chloride and sodium citrate, adjusted to pH 7. It is made as a concentrated 20X stock and diluted to 6X, 3X, 2X or lower for nucleic-acid hybridization work such as Southern and Northern blotting and in situ hybridization. Its high ionic strength speeds probe annealing, and diluting it is one way wash stringency is controlled.
Definition and scope
SSC is a reagent, not a procedure: a defined mixture of two salts in water. The name is written as “saline-sodium citrate” by suppliers and “standard saline citrate” in the FDA Bacteriological Analytical Manual (BAM). Both describe the same sodium chloride plus sodium citrate solution. The abbreviation SSC has other meanings elsewhere; this page covers only the buffer.
What are the ingredients in SSC buffer?
FDA BAM method R77 (BAM 8th edition, Revision A, 1998; page content current as of 4 March 2025; checked 7 November 2026) gives two ingredients plus pH adjustment:
- 20X SSC, per litre: 175.3 g reagent-grade sodium chloride and 88.2 g sodium citrate, dissolved in 800 ml deionized water, adjusted to pH 7 with 10 N NaOH, then brought to 1 litre.
- 6X SSC, per litre: 52.6 g sodium chloride and 26.5 g sodium citrate, prepared the same way.
The FDA page title reads “(20%)” while the recipe it gives is the 20X stock; treat “20%” as a labelling slip, not a different concentration.
As arithmetic on the FDA quantities, 175.3 g of NaCl at 58.44 g/mol is 3.0 mol per litre, and 88.2 g at about 294 g/mol (the molar mass of trisodium citrate dihydrate) is 0.3 mol per litre, which is how 20X stock is often described as 3 M NaCl and 0.3 M citrate. The FDA page does not state which hydrate of sodium citrate is meant, so the citrate figure is a derivation, not a quoted specification.
What is the chemical composition of sodium citrate?
Sodium citrate is a sodium salt of citric acid. In SSC it supplies the citrate component alongside the sodium chloride that provides most of the ionic strength. Because “sodium citrate” can refer to different salt and hydrate forms, take the exact form and molar mass from the supplier’s certificate of analysis or safety data sheet before converting a recipe by moles rather than grams (see how to read a safety data sheet).
How SSC is used in practice
A supplier of 20X SSC (Enzo, product ENZ-GEN426, checked 7 November 2026) lists Southern blotting, in situ hybridization, DNA microarray and Northern blotting as uses. It states that the high ionic strength speeds probe annealing and that SSC is used to control stringency in in situ hybridization washes. As covered in the Southern blot guide, protocols step wash stringency by lowering the SSC strength, for example from 2X down to 0.2X. See also the Northern blot and in situ hybridization guides for where SSC fits in each method.
The dilution ladder, from one FDA recipe
The same BAM page defines the lower strengths by diluting the 6X solution. Every row below comes from that page:
| Working strength | How the FDA page makes it |
|---|---|
| 20X | 175.3 g NaCl + 88.2 g sodium citrate, pH 7, to 1 L |
| 6X | 52.6 g NaCl + 26.5 g sodium citrate, pH 7, to 1 L |
| 3X | 500 ml of 6X + 500 ml deionized water |
| 2X | 333 ml of 6X + 667 ml deionized water |
A single concentrated solution therefore supplies several working strengths.
Where it matters for lab supply decisions
Forms of SSC that reach a bench differ in what you must record:
- Made from salts (FDA recipe). You control the lots of sodium chloride and sodium citrate, but you must adjust pH to 7 and record the citrate form used. See buffer and solution preparation and chemical grades explained.
- Lyophilized salt packet. Enzo 20X SSC is supplied as lyophilized salts; the whole packet is dissolved in 250 mL of distilled or deionized water to give 20X SSC. The supplier lists ambient long-term storage and shipping and a research-use-only regulatory status.
Whichever form is used, record the concentration, pH and lot in the run notes, and note the product’s research-use-only status.
Common misunderstandings
- “20%” versus 20X. The FDA page title says 20%; the content is a 20X stock.
- Citrate elsewhere is a different context. Sodium citrate in blood-collection tubes (see coagulation tubes) or in cleaners does not make those products SSC.
Frequently asked questions
What does sodium citrate do in cleaning products?
Sodium citrate is also used outside the lab, including in cleaning products, but that is a separate use with no bearing on SSC buffer. In SSC, sodium citrate is one of the two salts that define the solution.
What are the ingredients in SSC buffer?
Sodium chloride and sodium citrate in water, adjusted to pH 7 with sodium hydroxide; quantities are in the table above.
Why is sodium citrate used instead of EDTA?
SSC is defined as sodium chloride plus sodium citrate, and the FDA recipe and the Enzo product cited on this page do not include EDTA. The supplier attributes the annealing rate and stringency control to the solution’s high ionic strength.
What is the chemical composition of sodium citrate?
It is a sodium salt of citric acid; take the exact salt and hydrate form from the supplier’s documentation, as the FDA recipe does not name it.
Sources
- FDA, BAM R77: Standard Saline Citrate (SSC) Solution, checked 7 November 2026.
- Enzo Life Sciences, 20X SSC (ENZ-GEN426), checked 7 November 2026.
- CASRAI, Southern blot, for SSC wash-stringency context, checked 7 November 2026.
