Borax powder looks as if it should vanish the moment it touches water, yet anyone who has stirred a spoonful into a cold jug knows it often sits at the bottom as a cloudy sludge. That is not a sign of poor quality. Borax (sodium tetraborate decahydrate) is only moderately soluble in cold water, and its solubility rises sharply with temperature. Once you understand that curve, making a clear, consistent borax solution for flux, laboratory or workshop use is straightforward.
This guide explains how much borax dissolves at different temperatures, how to make a solution step by step, why crystals sometimes reappear as the liquid cools, and how to store and label what you make.
How soluble is borax in water?
At room temperature, a saturated solution of borax decahydrate contains roughly 6% borax by weight. In practical terms, that is only about 6 g of borax in 100 g of water before no more will dissolve. Warm the water and the picture changes completely.
| Water temperature | Borax in a saturated solution (% by weight) | Approx. borax per 100 g water |
|---|---|---|
| 0 °C | 1.99% | about 2 g |
| 25 °C | 5.80% | about 6 g |
| 50 °C | 17.91% | about 22 g |
| 75 °C | 40.18% | about 67 g |
The percentages are published solubility data for borax (see Sources). The right-hand column is our own conversion, rounded, so treat it as a working guide rather than a lab value. The key point is clear: water at around 50 °C holds several times more borax than cold tap water.
Why cold water leaves a cloudy layer
If you add more borax than the water can hold at that temperature, the excess simply stays undissolved. Fine powder also tends to clump when it is dumped in all at once, because the outside of each lump wets and slows water from reaching the middle. Both problems are solved by using warm water, adding the powder gradually and stirring.
Decahydrate, pentahydrate and anhydrous borax
The figures above refer to ordinary borax decahydrate, the form sold as 99.9% pure borax powder. Anhydrous (fused) borax dissolves much more slowly because it is a glassy solid, and both it and pentahydrate contain more borate per gram, so the weights are not interchangeable. Our comparison of borax decahydrate, pentahydrate and anhydrous includes a conversion table.
How to dissolve borax powder: step by step
The method below is for preparing a borax solution for professional technical use, such as a flux dip or a workshop cleaning solution. Laboratory methods (buffers, standards) should always follow the exact procedure in the method you are using.
- Put on your PPE first. Nitrile gloves and safety glasses. Weighing powder is the dustiest step, so work away from draughts.
- Choose the right container. Use a clean glass, stainless steel or heat-resistant plastic vessel that is used only for chemicals, never a kitchen jug or food container.
- Measure warm water. Water at roughly 40–60 °C is ideal. It dissolves borax quickly without the splash risk of boiling water. Distilled or deionised water gives the clearest result.
- Weigh the borax. Use scales rather than spoons. Scoop gently instead of pouring to keep dust down.
- Add the powder gradually while stirring. Sprinkle it in a little at a time and stir with a dedicated glass rod or plastic spatula until the liquid clears.
- Let it cool and check. If fine crystals appear as it cools, the solution was more concentrated than the water can hold at room temperature (see below).
- Label the container immediately with “Borax solution”, the concentration, the date and the hazard statement from the product label.
Working out concentrations
Most technical instructions give a borax solution as a percentage. Two conventions are common, so check which one your method uses:
- % w/v (weight per volume): grams of borax per 100 ml of final solution. A 4% w/v solution is 40 g of borax made up to 1 litre with water.
- % w/w (weight per weight): grams of borax per 100 g of total solution. A 4% w/w solution is 40 g of borax plus 960 g of water.
For dilute solutions the difference is small, but for anything above a few percent it matters. Write down the method on the label so the next person can repeat it.
Why crystals form when the solution cools
Because solubility falls so steeply with temperature, a solution made strong in hot water becomes supersaturated as it cools, and the excess borax comes back out as crystals on the bottom and sides of the container. This is normal and does not mean the borax was impure.
To avoid it:
- Stay below the room-temperature limit if the solution will be stored or used cold. Around 4–5% by weight leaves a margin below the roughly 6% saturation point at 25 °C.
- Keep it warm in use if you need a stronger solution, for example a flux dip on the bench, and accept that it will crystallise overnight.
- Re-dissolve when needed. Gently warming the closed, labelled container in warm water usually takes the crystals back into solution.
- Remember the cold. A solution that is clear in a heated workshop can crystallise in an unheated store in winter, when solubility drops towards 2% near 0 °C.
pH and buffering: what a borax solution does
Borax dissolves to give a mildly alkaline solution. Its pH stays relatively steady across a range of concentrations, which is why sodium tetraborate is used as a buffering agent and as a primary standard in acid-base titrations. A 0.01 mol/kg solution of borax decahydrate (about 3.81 g per kilogram of water) is a recognised pH reference standard with a value of 9.18 at 25 °C.
In practice this means a borax solution resists sudden pH changes and can neutralise small amounts of acid. For more on where this is used professionally, see our overview of borax uses.
Does hard water matter?
For general workshop solutions, ordinary tap water is usually fine, although very hard water can leave a slight haze as borax interacts with the calcium and magnesium already present. For laboratory buffers, standards and anything where clarity or exact concentration matters, use distilled or deionised water.
Common uses for borax solutions
Flux for soldering and brazing
Jewellers and metalworkers often keep a borax solution or paste to coat work before heating, so that the flux spreads evenly and protects the metal from oxidation. Using warm water makes a smoother, stronger mix. Our guide to borax flux for soldering, brazing and forge welding covers application in detail.
Laboratory buffers and standards
Borate buffers in the pH 8–10 range are prepared from borax and, where specified, boric acid. Always follow the published method for weights, water quality and pH adjustment. Our step-by-step guide to making a borax buffer solution covers common concentrations and the effect of temperature on pH.
Industrial cleaning and water conditioning
In professional settings, dilute borax solutions are used to help shift grease and mineral deposits and to condition water. For household laundry and cleaning, use a borax substitute instead, which is designed for home use.
Storing and disposing of borax solutions
- Store solutions in closed, clearly labelled containers, never in drinks bottles or food containers.
- Keep them in a cool, dry, frost-free place, away from food, drink and animal feed, and out of reach of children and pets.
- Make up only what you need. Small batches stay consistent and reduce waste.
- Do not pour borax solutions into drains or watercourses. Dispose of waste according to section 13 of the Safety Data Sheet and local rules.
For COSHH assessments, spills and storage of the dry powder, read our borax safety, handling and storage guide.
Safety box: handling borax and borax solutions
Borax (disodium tetraborate) is classified under GB and EU CLP as Repr. 1B, H360FD: may damage fertility, may damage the unborn child. It is supplied for professional, industrial and laboratory use only.
- Keep out of reach of children and away from pets, food and drink.
- Wear nitrile gloves and safety glasses when weighing, dissolving or using borax.
- Do not swallow. Do not eat, drink or smoke where borax is handled, and wash hands afterwards.
- Avoid contact with eyes. If it gets into the eyes, rinse cautiously with water for several minutes and follow the first-aid section of the SDS.
- Not for use by anyone who is pregnant, may be pregnant or is trying to conceive.
- Not for use on skin, in food, in cosmetics or in toys such as slime.
- Always follow the product label and Safety Data Sheet.
Frequently asked questions
Does borax dissolve in cold water?
Yes, but only a little. At around 25 °C a saturated solution holds roughly 6% borax by weight, and near 0 °C only about 2%. Warm water at 40–60 °C dissolves it much faster and in far greater amounts.
Why is my borax solution cloudy?
Usually because more borax was added than the water can hold at that temperature, or because the powder clumped. Warm the solution gently and stir, or add more water. Very hard tap water can also cause a slight haze.
Why have crystals formed in my borax solution?
The solution was stronger than the water can hold once it cooled. Borax is far more soluble in warm water, so a strong solution crystallises as it cools. Gently warm it to re-dissolve, or make a weaker solution if it will be stored cold.
Can I use boiling water to dissolve borax?
It is not necessary. Water at 40–60 °C dissolves borax well and is safer to handle. Very hot solutions increase the risk of splashes and burns, and a strong hot solution will crystallise heavily as it cools.
What is the pH of a borax solution?
Borax gives a mildly alkaline solution with a fairly stable pH. A 0.01 mol/kg solution of borax decahydrate is used as a pH reference standard at 9.18 (25 °C).
Sources
- Borax: solubility table (% by weight of saturated solution at 0, 25, 50 and 75 °C), use as a buffering agent and primary standard. Wikipedia
- U.S. Borax, Borax Decahydrate product information: about 5.93% by weight in saturated solution at room temperature, mildly alkaline solutions, buffering, caking behaviour. borax.com
- ECHA, substance information for disodium tetraborate: harmonised classification Repr. 1B, H360FD. echa.europa.eu
Need borax for professional use? Our 99.9% pure borax powder is supplied with an SDS for trade, workshop and laboratory users, and larger quantities are available via our industrial borax page. Orders ship from the EU in 3–7 working days. For home laundry and cleaning, choose our borax substitute.