LimsPulse
Drinking water · Drinking water, borewell water, packaged water

Total Dissolved Solids (TDS) — Gravimetric at 180 °C, with Conductivity Estimate

Measures the total mass of salts and other dissolved material in water. A known volume is filtered, the filtrate is evaporated in a pre-weighed dish, the residue is dried at 180 °C and weighed. A conductivity reading gives a quick estimate and a cross-check.

  • Environmental & water
  • Filtration, evaporation and drying at 180 ± 2 °C, then weighing; conductivity used as a screening estimate
  • Hot air oven
  • Analytical balance
  • Water bath
  • Conductivity meter

Reference: Typical method based on APHA Standard Methods 2540 C (total dissolved solids dried at 180 °C) and the IS 3025 part for dissolved solids. Follow your lab SOP.

PRINCIPLE

Why this method works

Everything that passes a glass-fibre filter (nominal pore size about 1.5 µm) is counted as dissolved. When the filtrate is evaporated and dried at 180 °C, water — including most mechanically trapped water — is driven off, leaving the dissolved salts. The gain in dish mass divided by the sample volume gives TDS in mg/L. Because dissolved salts carry current, electrical conductivity (EC) rises with TDS; for many natural waters TDS (mg/L) is roughly 0.55–0.75 × EC (µS/cm).

Instruments used

MATERIALS

Reagents & standards

ReagentGrade / specification
Glass-fibre filtersWithout organic binder, about 1.5 µm nominal pore size (e.g. 47 mm)
Evaporating dishesPorcelain, platinum or high-silica glass, 100–200 mL
Conductivity standardKCl standard, e.g. 1413 µS/cm at 25 °C
SAMPLE PREPARATION

Preparing the sample

  1. 01

    Prepare dish

    Heat at 180 °C, cool in desiccator, weigh to constant mass.

  2. 02

    Filter

    Filter a well-mixed sample through glass-fibre filter.

  3. 03

    Evaporate

    Evaporate the filtrate to dryness on a water bath.

  4. 04

    Dry

    Dry the residue at 180 ± 2 °C for at least 1 h.

  5. 05

    Weigh

    Cool in desiccator, weigh; repeat to constant mass.

  6. 06

    Cross-check

    Compare with conductivity × factor.

Procedure

  1. 1Heat a clean evaporating dish at 180 ± 2 °C for 1 hour, cool in a desiccator and weigh. Repeat until the mass is constant (typically within 0.5 mg).
  2. 2Choose a sample volume that will give roughly 2.5–200 mg of residue (for typical tap water, 100 mL is common).
  3. 3Filter the well-mixed sample through a pre-washed glass-fibre filter. Wash the filter with three small portions of distilled water and add the washings to the filtrate.
  4. 4Transfer the filtrate to the weighed dish and evaporate to dryness on a steam or water bath (or in the oven at a temperature at least 2 °C below boiling to avoid spattering).
  5. 5Dry the dish in the oven at 180 ± 2 °C for at least 1 hour, cool in a desiccator and weigh. Repeat drying, cooling and weighing until the mass change is within the SOP limit.
  6. 6Separately, measure the conductivity of the unfiltered sample at 25 °C (or with temperature compensation) after verifying the meter with a KCl standard.

Standard preparation

  1. 1Run a blank (distilled water through the whole procedure) with each batch.
  2. 2A check standard can be made from a known mass of sodium chloride (or a mixed salt) dissolved in water; its expected TDS is known.

Notes for analysts

  • Report the gravimetric value as the result; the conductivity figure is an estimate, not a substitute, unless your SOP allows it for screening.
  • Waters high in bicarbonate, sulphate or hygroscopic salts (calcium/magnesium chloride) can give results that drift; dry longer and weigh quickly.
  • Hand-held 'TDS meters' are conductivity meters with a fixed factor (often 0.5 or 0.7). They are useful for screening but are not the reference method.
TEST CONDITIONS

Method parameters (typical)

Drying temperature180 ± 2 °C
Drying timeAt least 1 h, then to constant mass
FilterGlass-fibre, about 1.5 µm
BalanceReadability 0.1 mg
Conductivity reference temperature25 °C
BEFORE YOU REPORT

System suitability & sequence

ParameterLimit
Constant massSuccessive weighings agree within about 0.5 mg (or 4% of the previous mass, whichever is less) — as per SOP
DuplicatesAgree within the SOP limit (commonly within 5%)
BlankResidue not more than about 0.5 mg

Typical sequence

  1. 1Dish preparation to constant mass
  2. 2Blank
  3. 3Sample in duplicate
  4. 4Check standard
CALCULATION

Calculation with a worked example

Formula
TDS (mg/L) = (A − B) × 1000 ÷ V, with A and B in mg; estimate: TDS ≈ k × EC

Where

A
Mass of dish + dried residue (mg)
B
Mass of empty dish (mg)
V
Volume of sample filtered (mL)
k
Empirical TDS/EC factor for the water type (often 0.55–0.75)
EC
Electrical conductivity at 25 °C (µS/cm)

Given

B (empty dish)
48.2153 g
A (dish + residue)
48.2412 g
V
100 mL
EC
410 µS/cm at 25 °C
k (assumed)
0.65

Working

  1. 1

    Residue = 48.2412 − 48.2153 = 0.0259 g = 25.9 mg

  2. 2
    TDS = 25.9 × 1000 ÷ 100 = 259 mg/L
  3. 3

    Conductivity estimate = 0.65 × 410 = 266.5 mg/L (close to the measured value)

  4. 4
    Observed ratio for this water = 259 ÷ 410 = 0.63 — use this to refine k for this source

TDS = 259 mg/L (gravimetric); conductivity estimate ≈ 267 mg/L

ACCEPTANCE CRITERIA

What result passes

  • As per IS 10500:2012: acceptable limit 500 mg/L, permissible limit 2000 mg/L in the absence of an alternative source; check the current edition.
  • WHO gives no health-based guideline for TDS; above roughly 1000 mg/L water may taste noticeably salty.
METHOD VALIDATION

Validation parameters (ICH Q2(R2))

ParameterTypical requirement
AccuracyRecovery of a known salt standard (typically 90–110%)
PrecisionDuplicate agreement on real samples
RangeResidue mass about 2.5–200 mg in the dish
In LimsPulse · Sample Management

Run this method in LimsPulse: specifications and STPs under version control, results calculated and checked against limits, and a signed Certificate of Analysis.

See how