Stock and Working Concentrations: Standard Lab Solutions and Serial Dilutions

Most laboratory solutions are kept as concentrated stocks, typically 0.5–5 M or a 10× or 100× mixture, and diluted to a working concentration in the millimolar or micromolar range when needed. This guide gives 100 mL recipes for the stocks used most often, the working concentrations they are usually diluted to and how to convert between % w/v and molar, with a serial dilution calculator for concentration series.

Serial dilution calculator

How it works

Every step dilutes by the same factor, so tube n contains stock ÷ factorⁿ. To make each tube up to the final volume V, transfer V ÷ factor from the previous tube and add V − V ÷ factor of diluent. Worked example: a 10-fold series with 1 mL per tube needs 100 µL from the previous tube plus 900 µL of diluent; starting from a 10 mM stock, the tubes hold 1 mM, 100 µM, 10 µM and so on.

• Each tube except the last passes on the transfer volume, so it ends with V − V ÷ factor (900 µL in the example). Remove the same volume from the last tube if every tube must hold the same amount.
• Errors compound along a series: a 2% error at each step becomes about 10% after five steps. Mix every tube thoroughly before the next transfer and change tips each time.
• Transfers below about 2 µL are imprecise with most pipettes, so the calculator flags them. Increase the final volume, use a smaller factor per step or make an intermediate dilution first.
• Use the same diluent as the final experiment, for example buffer, culture medium or 0.9% saline, so every tube has the same background.
• “1 : 10” here means 1 part of stock in 10 parts in total (1 part stock plus 9 parts diluent). Some protocols write 1:9 for the same mix, so check which convention a method uses.

Why make concentrated stocks?

• Stability. Concentrated salt and buffer stocks often keep longer than dilute working solutions: high salt concentrations discourage microbial growth, and small sealed portions are easy to store cold or frozen. Some stocks behave differently in the cold: 10× PBS and 10% SDS can crystallise, so warm and mix them before use.
• Convenience. One stock serves many recipes, saves weighing every time and takes less shelf space. A 10× stock is diluted 1 in 10 (1 part plus 9 parts diluent) and a 100× stock 1 in 100.
• Accuracy. Larger masses weigh more accurately than milligram amounts, and pipetting a measured volume of stock is quicker and more reliable. For example, 50 mL of 1 mM EDTA needs only 18.6 mg of the disodium salt, but 0.1 mL of a 0.5 M stock delivers the same amount.
• Consistency. Every experiment draws on the same stock, so results are easier to compare between days.
• Poorly water-soluble compounds are often kept as 1000× stocks in DMSO, so the working solution contains only 0.1% DMSO.

Common stocks and working concentrations

Each recipe makes 100 mL of stock. The volume in brackets in the second column is the amount of stock per 100 mL of working solution.

Stock	Typical working concentration	To make 100 mL of stock	Notes
5 M NaCl	150 mM (3.0 mL per 100 mL)	29.22 g sodium chloride, NaCl (58.44 g/mol)	Dissolve in about 70 mL first, because the solid takes up noticeable volume. Can be autoclaved.
1 M Tris-HCl, pH as required	10–50 mM (1–5 mL per 100 mL)	12.11 g Tris base (121.14 g/mol), then concentrated HCl to the pH needed: about 7.0 mL for pH 7.4, 6.0 mL for pH 7.6 or 4.2 mL for pH 8.0	The solution warms as acid is added; cool it to the working temperature before the final pH adjustment, because Tris pH changes by about 0.028 per °C. Starting from Tris hydrochloride instead, use 15.76 g (157.60 g/mol) and adjust with NaOH. Can be autoclaved.
0.5 M EDTA, pH 8.0	1–5 mM (0.2–1 mL per 100 mL)	18.61 g disodium EDTA dihydrate, Na₂H₂EDTA·2H₂O (372.24 g/mol), then NaOH to pH 8.0 (about 2 g of pellets, or 10 M NaOH)	It will not dissolve until the pH approaches 8. Can be autoclaved.
1 M MgCl₂	1–10 mM (0.1–1 mL per 100 mL)	20.33 g magnesium chloride hexahydrate, MgCl₂·6H₂O (203.30 g/mol)	Very hygroscopic: weigh quickly from a freshly opened container. Can be autoclaved.
1 M HEPES	10–25 mM (1–2.5 mL per 100 mL)	23.83 g HEPES free acid (238.30 g/mol), then 10 M NaOH to the pH needed (often 7.2–7.6)	Filter-sterilise and store protected from light.
10× PBS	1× (10 mL per 100 mL): 137 mM NaCl, 2.7 mM KCl, 10 mM Na₂HPO₄, 1.8 mM KH₂PO₄	8.00 g NaCl, 0.20 g KCl, 1.44 g Na₂HPO₄ (anhydrous, 141.96 g/mol) and 0.24 g KH₂PO₄ (136.09 g/mol)	The 10× stock reads below pH 7.4; check the pH after diluting to 1×. Warm to redissolve crystals formed in the cold. More in PBS and buffer recipes.
7.5% w/v sodium bicarbonate (0.893 M)	1.5–3.7 g/L in culture media, or 17.9–44.0 mM (20–49 mL per litre)	7.50 g sodium bicarbonate, NaHCO₃ (84.01 g/mol)	Filter-sterilise. Do not autoclave or heat it: CO₂ is driven off and the pH rises. Keep tightly closed.
1 M HCl (and 0.1 M)	pH adjustment	8.3 mL of 37% hydrochloric acid (about 12 M) added to about 80 mL of water, then made up to 100 mL. For 0.1 M, dilute 10 mL of 1 M to 100 mL.	Always add acid to water, with eye protection and good ventilation. Use 1 M for large changes and 0.1 M near the target pH. For fine adjustment of acetate buffers, dilute acetic acid such as 1% acetic acid can be used instead.
1 M NaOH (and 0.1 M)	pH adjustment	4.00 g sodium hydroxide pellets (40.00 g/mol). For 0.1 M, dilute 10 mL of 1 M to 100 mL.	Dissolving releases heat. Store in a polyethylene or polypropylene bottle: NaOH absorbs CO₂ from air and slowly attacks glass.
1 M KCl	2.7–150 mM (0.27–15 mL per 100 mL)	7.45 g potassium chloride, KCl (74.55 g/mol)	2.7 mM as in PBS; 50–150 mM in many enzyme and nucleic acid buffers. Potassium precipitates SDS, so do not mix the two stocks.
10% w/v SDS (0.347 M)	0.1–1% (1–10 mL per 100 mL)	10.0 g sodium dodecyl sulfate (288.38 g/mol)	Weigh with a dust mask or in a fume hood: the fine powder is an irritant. Stir gently to limit foaming. Precipitates below about 15–20 °C or with potassium salts; warm to redissolve. No need to sterilise.
1 M D-glucose	5–25 mM (0.5–2.5 mL per 100 mL)	18.02 g anhydrous D-glucose (180.16 g/mol), or 19.82 g of the monohydrate (198.17 g/mol)	Filter-sterilise: autoclaving can brown glucose, especially with phosphate or amino acids. 5.55 mM = 1 g/L and 25 mM = 4.5 g/L.

• Dissolve in about 70–80% of the final volume of purified water such as WFI water, adjust the pH, then make up to 100 mL in a volumetric flask or measuring cylinder.
• Masses are for the form named. If your container lists a different hydrate or salt, recalculate with its molecular weight; the dilution and molarity calculator does this.
• Set the pH at the temperature the solution will be used at, especially for Tris. The buffer selection guide lists pKa values and temperature effects.
• Sterilise salt, Tris, EDTA and MgCl₂ stocks by autoclaving; filter HEPES, bicarbonate and glucose through a 0.22 µm PES syringe filter.
• Label each bottle with the name, concentration, pH, date and initials, and keep working portions in sterile 10 mL vials (see the aliquoting guide).

Converting between % w/v and molar

% w/v means grams of solute per 100 mL of solution, so 1% w/v = 10 g/L = 10 mg/mL.

• % w/v to molar: M = % w/v × 10 ÷ MW. Example: 0.9% NaCl = 9 g/L ÷ 58.44 g/mol = 0.154 M (154 mM).
• Molar to % w/v: % w/v = M × MW ÷ 10. Example: 1 M glucose = 180.16 g/L = 18.0% w/v.
• mg/mL to molar: mM = mg/mL ÷ MW × 1000. Example: 1 mg/mL of a compound with MW 500 g/mol is 2 mM, and 10 µg/mL of the same compound is 20 µM.
• % w/w is different. Concentrated acids are sold as % w/w, so the density is needed: M = % w/w × density (g/mL) × 10 ÷ MW. Example: 37% HCl with a density of 1.18 g/mL is 37 × 1.18 × 10 ÷ 36.46 = 12.0 M.
• % v/v (for example 70% ethanol) means millilitres of liquid per 100 mL of solution.

Solution	% w/v	Molar
0.9% sodium chloride	0.9%	0.154 M (154 mM)
5 M sodium chloride	29.2%	5 M
1 M potassium chloride	7.45%	1 M
7.5% sodium bicarbonate	7.5%	0.893 M
10% SDS	10%	0.347 M (347 mM)
1 M D-glucose (anhydrous)	18.0%	1 M

Related: buffer selection guide, with a buffer finder · dilution and molarity calculator · PBS and buffer recipes · syringe filter selection guide · aliquoting reagents into sterile vials. Supplies: WFI water, 1× PBS, 0.9% saline, DMSO, 1% acetic acid, PES syringe filters, sterile 10 mL vials.

References

1. Sambrook J, Russell DW. Molecular Cloning: A Laboratory Manual, 3rd edn, Appendix 1: Preparation of reagents and buffers. Cold Spring Harbor Laboratory Press, 2001.
2. Phosphate-buffered saline (PBS). Cold Spring Harbor Protocols 2006; doi:10.1101/pdb.rec8247.
3. Prohaska T, Irrgeher J, Benefield J, et al. Standard atomic weights of the elements 2021 (IUPAC Technical Report). Pure Appl. Chem. 2022; 94(5); doi:10.1515/pac-2019-0603.
4. Freshney RI. Culture of Animal Cells: A Manual of Basic Technique and Specialized Applications, 7th edn. Wiley-Blackwell, 2016.

Recipes and the calculator are provided as a convenience; always check critical calculations and follow your own validated protocols where they differ. Union Biolabs products are supplied for laboratory research use only.