A borax buffer solution is one of the simplest alkaline buffers you can make. Dissolve the right weight of borax powder in good-quality water and you already have a solution that holds its pH at a little above 9, with no other ingredients. Add a little boric acid, acid or alkali and you can move it anywhere in the pH 8–10 range. This guide explains why borax buffers behave the way they do, how to weigh out common concentrations, how to adjust the pH, and how to store the result so it stays reliable.
It is written for laboratories, teaching labs and technical workshops. If you are new to working with the powder itself, our guide on how to dissolve borax powder covers solubility and mixing in more detail.
Why borax makes a good buffer
Borax is sodium tetraborate decahydrate, Na2B4O7·10H2O. When it dissolves, the tetraborate ion reacts with water to give equal amounts of boric acid and the borate ion:
B4O72− + 7 H2O → 2 B(OH)3 + 2 B(OH)4−
A buffer is simply a weak acid mixed with its conjugate base, and a 1:1 mixture sits at a pH close to the acid’s pKa. Boric acid has a pKa of about 9.14 at 25 °C, so a plain borax solution “self-buffers” at roughly pH 9.2. That is why borax is so convenient: one weighed powder gives you both halves of the buffer in exactly the right ratio.
Borate buffers work best between about pH 8 and pH 10. Outside that window their buffering capacity falls away quickly and another buffer system is a better choice.
What you need
- Borax decahydrate from a sealed, clearly labelled container, such as our 99.9% pure borax powder.
- Distilled or deionised water. Tap water adds calcium, magnesium and dissolved carbon dioxide, all of which shift the result.
- A balance reading to at least 0.01 g, a volumetric flask or measuring cylinder, and a clean glass or plastic bottle for storage.
- A calibrated pH meter if you intend to adjust the pH.
- Optional: boric acid, or dilute hydrochloric acid and sodium hydroxide solutions, for pH adjustment.
- PPE: nitrile gloves and safety glasses.
How much borax to weigh: common concentrations
The molar mass of borax decahydrate is 381.37 g/mol. The table below is straightforward arithmetic from that figure. Every value is well below the room-temperature solubility limit of borax, so all of them will dissolve fully at 20–25 °C with stirring.
| Concentration of borax | Borax decahydrate per 1 litre | Typical use |
|---|---|---|
| 0.01 mol/kg | 3.81 g per 1 kg of water | pH 9 reference-type solution |
| 0.01 M | 3.81 g, made up to 1 L | Dilute working buffer |
| 0.025 M | 9.53 g, made up to 1 L | General-purpose buffer |
| 0.05 M | 19.07 g, made up to 1 L | Stronger buffer or stock solution |
| 0.1 M | 38.14 g, made up to 1 L | Concentrated stock for dilution |
Note the difference between mol/kg (borax per kilogram of water, used for pH reference solutions) and M (moles per litre of finished solution, used for most working buffers). For dilute solutions the two are almost the same, but reference methods specify molality for a reason, so follow whichever your method states.
The weights only work for the decahydrate
These figures assume ordinary borax decahydrate. Pentahydrate and anhydrous borax contain less water per gram, so the same weight gives a stronger solution. Old stock that has been kept somewhere warm and dry can also lose some of its water of crystallisation, which makes it slightly “stronger” per gram than the label suggests. If accuracy matters, use fresh, well-sealed decahydrate. Our comparison of borax decahydrate, pentahydrate and anhydrous includes conversion factors.
How to make a borax buffer solution: step by step
- Prepare your water. Use freshly drawn deionised water, or distilled water that has been boiled for a few minutes and allowed to cool with the container covered. This drives off dissolved carbon dioxide, which would otherwise lower the pH.
- Put on gloves and glasses and work away from draughts so the powder does not drift.
- Weigh the borax for the concentration you need, scooping gently to keep dust down.
- Dissolve it in about 80% of the final volume of water, stirring until the solution is completely clear. Gentle warming speeds this up, but let the solution return to room temperature before the next step.
- Make up to volume in a volumetric flask or measuring cylinder and mix well.
- Check the pH with a calibrated meter at the temperature you will use the buffer, and adjust if needed (see below).
- Bottle and label it with “Borax buffer”, the concentration, the pH and temperature, the date and the hazard statement from the product label.
Adjusting the pH between 8 and 10
A plain borax solution sits at about pH 9.2. To move it, always add the adjusting solution slowly while stirring and watching the meter, rather than relying on a fixed recipe. Small differences in water quality and temperature make meter-based adjustment more reliable than any table.
- To lower the pH (towards 8): add a boric acid solution of the same concentration, or a few drops at a time of dilute hydrochloric acid. Adding boric acid keeps the total borate concentration steady, which many methods prefer.
- To raise the pH (towards 10): add dilute sodium hydroxide drop by drop.
- Stay inside the useful range. Below about pH 8 or above about pH 10 the buffer has little capacity left, so it will not hold its pH well.
Boric acid carries the same reproductive toxicity classification as borax, so handle it with the same precautions.
Temperature changes the pH
Borax buffers are noticeably temperature sensitive. The certified values for the US National Institute of Standards and Technology (NIST) borax pH reference material, a 0.01 mol/kg solution, show the pattern clearly:
| Temperature | pH of 0.01 mol/kg borax (NIST SRM 187f) |
|---|---|
| 5 °C | 9.401 |
| 15 °C | 9.288 |
| 20 °C | 9.239 |
| 25 °C | 9.195 |
| 30 °C | 9.155 |
| 40 °C | 9.088 |
The practical lesson: set the pH at the temperature you will actually use the buffer. A borax buffer adjusted on a warm bench will read higher in a cold room. Those certified figures apply only to the certified reference material, not to a solution made from ordinary borax powder, but the trend with temperature is the same.
Things that interfere with borate buffers
- Carbon dioxide. Alkaline solutions absorb CO2 from the air, which slowly lowers the pH. Keep bottles capped.
- Sugars and polyols. Borate binds to compounds with neighbouring hydroxyl groups, such as glycerol, mannitol and many sugars. This releases acid and lowers the pH, so a borate buffer is a poor choice for samples rich in these.
- Glass and contamination. Use clean, dedicated containers that have never held food or drink.
Where borax buffers are used
- pH meter checks. A borax solution near pH 9.2 is a classic alkaline calibration point. For formal calibration, use a certified reference buffer, and use your own borax solution for routine checks between calibrations.
- Electrophoresis. Borate is the “B” in the Tris-borate-EDTA (TBE) buffers used to run DNA gels, and sodium borate buffers are used in some gel methods.
- Coating and conjugation work. Borate buffers in the pH 8–9 range are used in laboratory coating and labelling procedures.
- Teaching laboratories. Borax is a convenient way to demonstrate buffering and the effect of temperature on pH.
For a wider view of where the powder is used professionally, see our overview of borax uses.
Which grade of borax should you use?
Technical grade borax of high purity is perfectly suitable for workshop solutions, teaching labs and routine buffers that you check with a calibrated meter. Where a method specifies an analytical reagent grade, or where a result must be traceable, use the grade the method demands and a certified reference buffer for calibration. Our technical grade borax buying guide explains the differences between grades.
Storing a borax buffer
- Store in a tightly capped, clearly labelled glass or plastic bottle, never a drinks bottle or food container.
- Keep it cool, away from food, drink and animal feed, and out of reach of children and pets.
- NIST advises discarding its prepared borax reference solution after one month, or sooner if sediment appears. That is a sensible rule for any borax buffer used for pH work.
- Do not pour unused buffer into drains or watercourses. Dispose of it according to section 13 of the Safety Data Sheet and local rules.
Safety box: working with borax and borate buffers
Borax (disodium tetraborate) and boric acid are classified under GB and EU CLP as Repr. 1B, H360FD: may damage fertility, may damage the unborn child. Borax 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 borax and preparing or using solutions.
- 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. Employers should include borates in their risk assessment for new and expectant mothers.
- Not for use on skin, in food, in cosmetics or in toys such as slime.
- Always follow the product label, the Safety Data Sheet and your COSHH assessment.
For COSHH, spills and storage of the dry powder, read our borax safety guide.
Frequently asked questions
What is the pH of a borax buffer?
A plain borax solution sits at about pH 9.2. The NIST certified value for a 0.01 mol/kg borax reference solution is 9.195 at 25 °C, rising to 9.239 at 20 °C and falling to 9.088 at 40 °C.
How do I make a 0.05 M borax buffer?
Dissolve 19.07 g of borax decahydrate in about 800 ml of distilled or deionised water, make up to 1 litre, then check and adjust the pH with a calibrated meter at the temperature you will use it.
What pH range does a borate buffer cover?
Borate buffers work best from about pH 8 to pH 10, centred on the pKa of boric acid, which is about 9.14 at 25 °C. Outside that range their buffering capacity drops sharply.
Can I calibrate a pH meter with home-made borax buffer?
It is fine for routine checks, but formal or traceable calibration should use a certified reference buffer. Solutions made from ordinary borax powder are not certified and can vary with water quality and storage.
How long does a borax buffer solution keep?
Kept tightly capped, a borax buffer is stable for weeks, but it slowly absorbs carbon dioxide from the air. NIST recommends discarding its borax reference solution after one month, or sooner if sediment appears.
Sources
- NIST, Certificate of Analysis, Standard Reference Material 187f, Sodium Tetraborate Decahydrate (Borax) pH Standard: pH values from 5 to 50 °C, water preparation, protection from CO2 and one-month shelf life of prepared solutions. tsapps.nist.gov
- Borate buffered saline: pKa of boric acid (9.14 at 25 °C), buffering range 8–10 and use in coating procedures. Wikipedia
- ECHA, substance information for disodium tetraborate: harmonised classification Repr. 1B, H360FD. echa.europa.eu
- HSE, COSHH guidance for businesses. hse.gov.uk/coshh
Need borax for the lab? Our 99.9% pure borax powder is sodium tetraborate decahydrate supplied with a Safety Data Sheet for trade, workshop and laboratory users, with free UK delivery in 3–7 working days. For larger quantities, see our industrial borax page.