🌡️ Dry Residue / TDS

Physicochemistry PN-EN 12880

Overview

Dry residue (Total Dissolved Solids — TDS) is the total mass of substances remaining after evaporation and drying of a water sample at 105±2°C to constant mass. This parameter includes mineral salts (chlorides, sulfates, bicarbonates), organic substances, and other non-volatile compounds at this temperature. It is one of the most important indicators of water mineralization.

Determination of dry residue is crucial in drinking water quality control (permissible value 500 mg/L according to WHO, in Poland no normative limit but recommended value <1000 mg/L), wastewater monitoring (parameter important in wastewater discharge settlements), assessment of water suitability for industrial purposes (boilers, coolers), and control of desalination processes (reverse osmosis, distillation).

The gravimetric method according to PN-EN 12880 is a classical analytical technique used for over a hundred years. Its advantages are simplicity, low cost, and high reliability of results. Disadvantages — long analysis time (several hours of drying) and large sample volume required at low concentrations. Alternatively, moisture analyzers are used, which significantly shorten the determination time.

The result is expressed in mg/L (for water) or in % mass (for sewage sludge). For drinking and surface waters, typical values are 100–500 mg/L, for wastewater 500–5000 mg/L, and for sewage sludge 1–40% dry mass.

Method principle

The gravimetric method involves evaporating a measured volume of water sample in a dried and weighed dish (evaporating dish) on a water bath, then drying the residue in an oven at 105±2°C to constant mass (difference between successive weighings <0.5 mg or <1% mass). Dry residue is calculated from the formula: TDS [mg/L] = (m₂ − m₁) × 1000 / V, where m₁ — mass of empty dish [mg], m₂ — mass of dish with residue [mg], V — sample volume [mL].

Applications

• Drinking water mineralization control
• Municipal and industrial wastewater monitoring
• Assessment of dry mass content of sewage sludge
• Control of desalination processes (RO, distillation)
• Testing of water suitability for industrial purposes (steam boilers)
• Monitoring of surface and groundwater
• Swimming pool water quality control

Key parameters

Standard

Standard number

PN-EN 12880:2004

Title (PL)

Charakterystyka osadów ściekowych — Oznaczanie suchej pozostałości i zawartości wody

Title (EN)

Characterization of sludges — Determination of dry residue and water content

Step-by-step procedure

1. Preparation of dishes

   Wash porcelain dishes, dry in oven at 105°C for min. 1 h. Cool in desiccator (30 min). Weigh with accuracy to 0.1 mg (m₁).

   ⏱ Time: 2 h • 🌡 Temperature: 105°C

2. Sample homogenization

   Mix water sample thoroughly. For sludge — homogenize mechanically. Measure known volume (e.g., 100 mL) with pipette or cylinder.

   ⏱ Time: 5 min

3. Evaporation on water bath

   Transfer measured sample to dish. Evaporate to dryness on water bath (approx. 100°C). Avoid vigorous boiling and splashing.

   ⏱ Time: 1–3 h • 🌡 Temperature: ~100°C

4. Drying in oven

   Transfer dish with residue to oven set at 105±2°C. Dry for minimum 1 hour.

   ⏱ Time: 1 h • 🌡 Temperature: 105°C

5. Cooling in desiccator

   Remove dish with tongs and place in desiccator. Cool to room temperature (min. 30 min).

   ⏱ Time: 30 min

6. First weighing

   Weigh dish with residue on analytical balance with accuracy to 0.1 mg. Record mass.

   ⏱ Time: 3 min

7. Repeated drying

   Place dish again in oven for 30 min at 105°C. Cool in desiccator. Weigh again.

   ⏱ Time: 1 h • 🌡 Temperature: 105°C

8. Checking constant mass

   Compare results of successive weighings. If difference >0.5 mg — repeat drying. Constant mass = difference <0.5 mg.

   ⏱ Time: 5 min

9. Calculation of result

   TDS [mg/L] = (m₂ − m₁) × 1000 / V. Where m₁ — mass of empty dish [mg], m₂ — mass of dish with dry residue [mg], V — sample volume [mL].

   ⏱ Time: 5 min

10. Quality control

    Perform duplicate determination (RPD <10%). Perform blank test (dish + distilled water). Blank test result <2 mg/L.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Hot dishes and oven — risk of burns, use tongs and heat-resistant gloves
• Water bath — avoid contact with water vapor
• Safety goggles and laboratory coat mandatory
• Wastewater samples — use additional protective measures (gloves, mask for sludge)

Frequently asked questions