Reviewed by Er. Inderjit Sharma (Retd.), Punjab PWD
Why test sulphates? Some soils and groundwater contain dissolved sulphate salts (sodium, magnesium or calcium sulphate). Sulphate attacks concrete: it reacts with the cement and forms new crystals that expand, crack and soften the concrete of foundations, piles and buried pipes.
What this test tells you: how much soluble sulphate the soil holds. The result decides the type of cement, the minimum cement content and the water–cement ratio for concrete in contact with that soil.
This guide follows the volumetric (titration) method of IS 2720 (Part 27).
How Sulphate Damages Concrete
The Idea Behind the Test
We add a known amount of barium chloride, slightly more than needed. Barium joins with all the sulphate and forms an insoluble white powder (barium sulphate). Then we measure the extra barium left over using potassium chromate. The difference tells us how much sulphate was there.
Apparatus Required
Balance (0.001 g)
Weighs the soil accurately.
Sieve (425 micron)
The dried soil is sieved through it.
Burettes (2)
One for barium chloride, one for potassium chromate.
Conical flask and pipette
10 ml of soil extract is pipetted into the flask.
Hot plate
Boils the extract during titration.
Filter funnel and paper
Separates the clear extract from the soil.
Reagents
🧪 Barium chloride (N/4)
30.54 g in 1 litre of distilled water. Reacts with sulphate to form white BaSO₄.
🧪 Potassium chromate (N/4)
24.275 g in a little water, a few drops of silver nitrate added, filtered and made up to 250 ml. Used to back-titrate the extra barium.
🧪 Silver nitrate (0.5%)
500 mg in 100 ml distilled water. External indicator for the end point.
🧪 Dilute ammonium hydroxide
Ammonia (sp. gr. 0.888) mixed 1:2 with distilled water. Neutralises the solution.
🧪 Hydrochloric acid
Concentrated (sp. gr. 1.11). A few drops make the extract slightly acidic.
N/4 means a quarter-normal solution. Prepare reagents with distilled water and store them in labelled, closed bottles.
Procedure
Step 1: Prepare the Soil
- Oven-dry about 100 g of soil and sieve it through the 425 micron IS sieve.
Step 2: Make the Soil Extract
- Weigh 10 g of the prepared soil into a beaker.
- Add about 50 ml of distilled water, stir well and let the soil settle.
- Filter the liquid. Wash the soil on the filter with a little water and make the filtrate up to 100 ml. Each 10 ml of this extract now contains the sulphate from 1 g of soil.
Step 3: Titrate
- Pipette 10 ml of the extract into a conical flask. Add a few drops of HCl to make it slightly acidic and heat it to boiling.
- While it boils, add N/4 barium chloride from the burette until no more white precipitate forms, then a little extra. Note the volume as X ml.
- Neutralise the solution with a few drops of dilute ammonium hydroxide.
- Titrate the extra barium with N/4 potassium chromate. Note the volume as Y ml.
- End point check: put a drop of the solution next to a drop of silver nitrate on a white tile. When a reddish-brown colour appears, chromate is in slight excess and the titration is complete.
Calculation
X = volume of N/4 barium chloride added (ml); Y = volume of N/4 potassium chromate used in the back titration (ml). This formula assumes the standard amounts above: 10 g soil in 100 ml extract, with 10 ml titrated (that is, 1 g of soil).
💡 Why 0.0177?
1 ml of N/4 barium chloride reacts with about 0.0177 g of sodium sulphate. Dividing by 1 g of soil and multiplying by 100 gives a percentage.
🔁 As SO₃
Concrete codes such as IS 456 give limits as SO₃. With the same amounts, SO₃ (%) = 1.0 × (X − Y), so it’s easy to convert.
📝 Worked example
X = 6.0 ml, Y = 5.6 ml → X − Y = 0.4 ml
Na₂SO₄ = 1.77 × 0.4 = 0.71%
SO₃ = 0.40%
Try It: Sulphate Calculator
Enter your readings. Change the soil mass or volumes if you used different amounts. The values filled in are the worked example.
Precautions
🔥 Keep it boiling
Add barium chloride while the solution boils, so the barium sulphate forms fully and settles well.
➕ Slight excess only
Add barium chloride just past the point where precipitation stops. A large excess makes the back titration long and less accurate.
🧼 Clean glassware
Rinse everything with distilled water. Tap water may contain sulphate and give a false high result.
☠️ Handle safely
Barium chloride is poisonous and potassium chromate is toxic. Wear gloves and goggles, never pipette by mouth, and dispose of waste safely.
Practical Tips for Site Engineers
1. Sulphate Classes (IS 456, Table 4)
| Class | Total SO₃ in soil | What to do |
|---|---|---|
| 1 | Below 0.2% | Normal cement |
| 2 | 0.2–0.5% | Higher cement content, lower w/c |
| 3 | 0.5–1.0% | Sulphate-resisting cement (or PPC/PSC, richer mix) |
| 4 | 1.0–2.0% | Sulphate-resisting cement, rich mix |
| 5 | Above 2.0% | Sulphate-resisting cement + protective coating |
IS 456 also gives limits for sulphate in groundwater. Use the higher of the two classes, and follow Table 4 for the exact cement content and water–cement ratio.
2. Signs of Sulphate on Site
⚪ White crust
White salt deposits on the soil surface or on old walls and foundations nearby.
🏚️ Damaged old concrete
Nearby old footings or pipes that are cracked, swollen or crumbling at the edges.
💧 Salty groundwater
High water table, brackish water, or areas near the sea, industrial waste or gypsum-bearing soils.
3. Good Site Habits
- Test before design: sample soil and groundwater at foundation level during soil investigation, before the mix design and cement are finalised.
- Test the groundwater too: water carries sulphate to the concrete. A low soil value with high groundwater sulphate is still a risk.
- Sample at several points and depths: sulphate can vary a lot across a site. Don’t rely on a single sample.
- Dense concrete is the best defence: low w/c, proper compaction and curing matter as much as cement type. Keep the specified cover.
- Use a protective coating (bitumen or epoxy) on below-ground concrete in severe cases, as specified.
- Check before lime or cement stabilisation: sulphate in soil can react with lime and cement and cause swelling of stabilised layers.
- Keep the record: note sample location, depth, date and result in the soil report so the designer can choose the right exposure class.
Key Takeaways
- Sulphate attacks concrete; this test sets the cement type and mix for foundations.
- Extract: 10 g soil in water, made up to 100 ml; titrate 10 ml (= 1 g soil).
- Add excess N/4 BaCl₂ (X) while boiling; back-titrate with N/4 K₂CrO₄ (Y); silver nitrate shows the end point.
- Na₂SO₄ (%) = 0.0177 × 100 × (X − Y); SO₃ (%) ≈ 1.0 × (X − Y).
- On site: above 0.2% SO₃, follow IS 456 Table 4; test groundwater too; make concrete dense.
References: IS 2720 (Part 27): Methods of Test for Soils – Determination of Total Soluble Sulphates; IS 456: Plain and Reinforced Concrete – Code of Practice (Table 4).