How to Make 0.5 M EDTA, 1 M Tris and Other Lab Stocks
8 min read · Updated September 24, 2026
Every molecular biology bench runs on the same dozen stock solutions, and every one of them comes down to a single equation: mass = concentration × volume × formula weight. The equation never goes wrong. The formula weight does — usually because the bottle on the shelf is a different hydrate from the one in the recipe.
This guide lists the standard stocks with the mass to weigh for each, computed from each reagent's molecular formula rather than copied from a table, covers the pH-adjusted ones that need more than weighing, and shows how large the error is when a hydrate is mixed up.
One equation, and the one number to get right
Mass in grams = molarity (mol/L) × volume (L) × formula weight (g/mol). For 500 mL of 0.5 M EDTA disodium dihydrate that is 0.5 × 0.5 × 372.24 = 93.06 g.
Concentration and volume are what you decided; the formula weight is the only number you look up, and it belongs to the exact chemical form in your bottle. Water of crystallisation counts towards the mass you weigh but not towards the moles of the compound, so a hydrate and its anhydrous form need different masses for the same molarity. Suppliers print the formula weight of that exact form on the label — use it.
The hydrate trap, in numbers
Four reagents account for most of these mistakes. Each is sold in two forms whose formula weights differ by the factor shown, and weighing by the wrong one puts the stock out by that same factor:
- EDTA: disodium dihydrate (C10H14N2Na2O8·2H2O) 372.24 g/mol; free acid (C10H16N2O8) 292.24 g/mol — a factor of 1.27.
- Magnesium chloride: hexahydrate (MgCl2·6H2O) 203.30 g/mol; anhydrous (MgCl2) 95.21 g/mol — a factor of 2.14.
- Sodium acetate: trihydrate (C2H3NaO2·3H2O) 136.08 g/mol; anhydrous 82.03 g/mol — a factor of 1.66.
- Calcium chloride: dihydrate (CaCl2·2H2O) 147.01 g/mol; anhydrous 110.98 g/mol — a factor of 1.32.
What the wrong entry costs
186.1 g of EDTA free acid made up to a litre is 0.64 M, not 0.5 M; 146.1 g of the disodium dihydrate is 0.39 M. Magnesium chloride is the extreme case: weigh 95.2 g of the hexahydrate believing it is anhydrous and the '1 M' stock is 0.47 M, an error that carries into every reaction set up from it.
Once made, none of these stocks looks wrong: the solution is clear, the pH is plausible and the bottle is labelled with the concentration you intended. EDTA is the one that gives you a warning while you work — the free acid has three protons to lose on the way to pH 8, not one, so it stays a suspension after the usual 20 g of NaOH and needs three to four times that. If a '0.5 M EDTA' will not dissolve, read the bottle before adding more base. Otherwise the only defence is reading the formula on the label before weighing.
The stocks most labs keep, and what to weigh
Each mass below is molarity × volume × a formula weight computed from IUPAC standard atomic weights, for 1 L unless stated. Dissolve in well under the final volume, adjust pH where noted, then make up to volume.
- 5 M NaCl — 292.2 g NaCl (58.44 g/mol). Close to saturation, so it dissolves slowly; stir and warm gently.
- 1 M KCl — 74.55 g KCl (74.55 g/mol).
- 1 M Tris-Cl — 121.1 g Tris base (121.14 g/mol), then set the pH with concentrated HCl (see below). Not the same as the Tris-HCl salt on a bottle, which is 157.59 g/mol.
- 0.5 M EDTA, pH 8.0 — 186.1 g EDTA disodium dihydrate (372.24 g/mol); it will not dissolve until NaOH brings the pH to about 8.0.
- 1 M MgCl2 — 203.3 g MgCl2·6H2O (203.30 g/mol), or 95.21 g anhydrous MgCl2.
- 1 M CaCl2 — 147.0 g CaCl2·2H2O (147.01 g/mol), or 111.0 g anhydrous CaCl2.
- 3 M sodium acetate, pH 5.2 — 408.2 g sodium acetate trihydrate (136.08 g/mol), or 246.1 g anhydrous; pH with glacial acetic acid.
- 1 M MgSO4 — 246.5 g MgSO4·7H2O (246.47 g/mol).
- 1 M HEPES — 238.3 g HEPES free acid (238.30 g/mol), titrated up to the working pH with NaOH or KOH.
- 1 M imidazole — 68.08 g (68.08 g/mol).
- 1 M DTT — 1.542 g in 10 mL (154.24 g/mol). Made in small volumes, filter-sterilised rather than autoclaved, and stored frozen in aliquots.
- 1 M IPTG — 2.383 g in 10 mL (238.30 g/mol). Also filter-sterilised and stored frozen.
- 10% (w/v) SDS — 100 g per litre. A mass-per-volume stock rather than a molar one; it works out at about 0.35 M (288.38 g/mol).
Stocks that need a pH step, and the order to do it in
Dissolve in well under the final volume, adjust the pH, and only then make up to volume. The acid or base you add takes up volume, which is why you do not start at 1 L; for most 1 L stocks about 800 mL is the usual start, and that is what the Cold Spring Harbor Protocols recipes below use. Concentrated HCl and glacial acetic acid fume, so add them in a fume hood with eye protection; NaOH pellets, and anhydrous MgCl2 and CaCl2, heat the solution as they dissolve, so add them gradually.
0.5 M EDTA, pH 8.0. Stir 186.1 g of the disodium dihydrate into 800 mL of water and add NaOH. The salt stays as a suspension until the pH approaches 8.0, and a litre takes roughly 20 g of NaOH pellets to get there — about one NaOH per EDTA, the proton the disodium salt still has to give up at pH 8.
1 M Tris-Cl (often labelled Tris-HCl). Dissolve 121.1 g of Tris base in 800 mL and bring the pH down with concentrated HCl: roughly 70 mL for pH 7.4, 60 mL for pH 7.6 and 42 mL for pH 8.0. Let the solution cool to room temperature before the final adjustment. Tris is unusually temperature-sensitive — its pH falls by about 0.03 units for every degree of warming — so a buffer set to pH 8.0 at room temperature reads noticeably higher on ice and lower at 37 °C. Set it at the temperature you will use it. And do not confuse this with the Tris-HCl salt: Tris hydrochloride is 157.59 g/mol, dissolves acidic, and has to be brought up with NaOH rather than down with HCl. A calculator asked for 'Tris-HCl' gives the salt's mass.
3 M sodium acetate, pH 5.2. Here 800 mL is too much to start with. The 408 g of trihydrate carries 162 g of water of crystallisation and the salt takes up volume of its own, so dissolved in 800 mL it already comes to more than a litre before any acid goes in, and the stock can no longer reach 3 M. Start from about 500 mL of water, adjust with glacial acetic acid — acetic acid is the buffer's own weak acid, so it adds no chloride — and then make up to 1 L. Cold Spring Harbor Protocols prints 408.3 g of the trihydrate (and 800 mL); computing from current atomic weights gives 408.2 g, a rounding difference that is immaterial.
Diluting stocks into working solutions
Stocks exist so you weigh once and pipette afterwards, and the conversion from stock to working solution is C1 × V1 = C2 × V2. For 100 mL of TE (10 mM Tris-Cl pH 8.0, 1 mM EDTA), take 1 mL of 1 M Tris-Cl pH 8.0 and 0.2 mL of 0.5 M EDTA pH 8.0 and make up to 100 mL.
Stocks are also the answer when the mass would be too small to weigh. A 10 µM solution of NaCl in 1 mL needs 0.58 µg, far below what a laboratory balance can resolve; make a concentrated stock and dilute it instead.
Frequently asked questions
How do I make 0.5 M EDTA pH 8.0?
For 1 L, stir 186.1 g of EDTA disodium dihydrate (372.24 g/mol) into about 800 mL of water and add NaOH until the pH reaches 8.0 — the salt only dissolves as the pH approaches 8, and a litre typically takes around 20 g of NaOH pellets. Then make up to 1 L. For 500 mL, halve everything: 93.06 g.
What is the molecular weight of EDTA?
It depends on the form. The free acid (C10H16N2O8) is 292.24 g/mol; the disodium dihydrate (C10H14N2Na2O8·2H2O) that most labs use for stock solutions is 372.24 g/mol. Use the formula weight printed on your bottle — weighing one by the other's value is a 27% error in one direction or 21% in the other.
How do I make 1 M Tris-HCl?
The usual '1 M Tris-HCl' stock is Tris base titrated with HCl, not the Tris-HCl salt. Dissolve 121.1 g of Tris base in about 800 mL of water, bring the pH down with concentrated HCl (roughly 70 mL for pH 7.4 or 42 mL for pH 8.0), let it cool to room temperature, make the final pH adjustment and make up to 1 L. Tris pH falls by about 0.03 units per degree of warming, so set it at the temperature you will use it.
How much NaCl do I need for 5 M NaCl?
292.2 g per litre: 5 mol × 58.44 g/mol. 5 M is close to NaCl's solubility limit, so it dissolves slowly; stir well and warm gently if needed.
Does it matter whether I use MgCl2 hexahydrate or anhydrous?
Yes, by more than a factor of two. The hexahydrate is 203.30 g/mol and anhydrous MgCl2 is 95.21 g/mol, so 1 M needs 203.3 g of one or 95.21 g of the other. Both forms take up water from the air, so keep the bottle closed and discard crystals that have gone wet; anhydrous MgCl2 also releases a lot of heat as it dissolves, which is one reason the hexahydrate is the usual choice for stocks.
How do I make 3 M sodium acetate pH 5.2?
Dissolve 408.2 g of sodium acetate trihydrate (or 246.1 g anhydrous) in about 500 mL of water — 800 mL leaves no room, because the trihydrate brings its own water and bulk — adjust the pH to 5.2 with glacial acetic acid, and make up to 1 L. Recipe books often print 408.3 g, from a formula weight rounded to 136.1 g/mol; the difference is immaterial.
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