⚗️ Total dissolved solids (TDS) in water and eluates — filtration through 0.45 µm, evaporation and drying to constant mass at (105 ± 5) °C

Physicochemistry PN-EN 15216

Overview

EN 15216:2021 specifies a method for determining the total dissolved solids (TDS) in water and in eluates (annex A), provided the constituents are not volatile under the conditions of the determination and do not give up their water of hydration. It applies to waters and eluates containing more than 100 mg/l of dissolved substances; samples with a lower content can be analysed by repeating the drying step. TDS is the mass of the dissolved constituents per unit volume of water remaining after a defined filtration and drying, expressed in mg/l; it is a common sum parameter in the analysis of water and eluates. The determination should be carried out without delay, at the latest a week after the sample was taken or the eluate prepared; if they have to be stored, the samples are kept at 2–8 °C. In Poland it is in force as PN-EN 15216:2022-03 (English version; it replaced PN-EN 15216:2010 "Charakteryzowanie odpadów — Oznaczanie całkowitej substancji rozpuszczonej (TDS) w wodzie i eluatach" [Characterisation of waste — Determination of total dissolved solids (TDS) in water and eluates]).

The related determination of "dissolved solids" in the PWr textbook "Chemia wody" [Water chemistry] (the gravimetric method) consists in filtering, evaporating on a water bath and drying a defined volume of the sample to constant mass: the crystallising dishes are dried for 2 h at 105 °C to constant mass (a difference between weighings of no more than 0.5 mg), 100 cm³ of the filtrate is measured out, evaporated, dried for 2 h at 105 °C, cooled in a desiccator and weighed; the result is X = (m₂ − m₁)·1000/V in mg/dm³, rounded to 1; the samples are preserved with 2 cm³ of chloroform per 1 dm³ and kept at about 4 °C, the determination being made within 24 h.

Method principle

The test sample is filtered and then dried in an evaporating dish to constant mass in a drying oven at (105 ± 5) °C. Equipment: a drying system thermostatted at (105 ± 5) °C (a drying oven, an infrared system), a desiccator with an active drying agent (e.g. silica gel), an analytical balance accurate to 1 mg or better, a membrane filter with a pore size of 0.45 µm with a filtration set, and an evaporating dish or crucible of glass, ceramic, porcelain, aluminium or another material inert towards the sample and able to withstand 105 °C. Procedure: the sample is filtered through a 0.45 µm membrane filter (unless it has already been filtered, e.g. eluates according to the EN 12457 series); the evaporating dish is dried at (105 ± 5) °C and, once it has cooled, weighed to 1 mg (m_a); a homogenised portion of the filtrate (V_FB) — in stages if necessary — is transferred into the dish so that after evaporation to dryness not less than 20 mg and not more than 1000 mg of dry matter m_D remains; it is evaporated to dryness in the drying system and, after cooling in the desiccator, weighed (m_b). The mass is regarded as constant when after a further half hour of drying it differs by no more than 0.5 % of the previous value or by 2 mg (whichever is the greater); otherwise the drying is repeated; if after the third drying the mass is still not constant, the last value is taken and a note is made in the test report. Where the sample is not homogeneous or behaves unusually, it is recommended to carry out the determination in duplicate. Calculation: ρ_TDS = m_D/V_FB, m_D = m_b − m_a (mg/l); the result is rounded to 3 significant figures.

Applications

• Waters with a mineralisation above 100 mg/l — drinking water, groundwater, mine waters and brines
• Eluates from leaching tests on waste (the EN 12457 series) — characterisation of waste and of materials
• Wastewater and process waters — control of the salinity and the salt balance in installations (reverse osmosis, evaporation)
• Surface waters — a sum parameter alongside the conductivity
• Gravimetric laboratories — a simple method with a drying oven and an analytical balance of 1 mg

Key parameters

Standard

Standard number

PN-EN 15216:2022-03 (EN 15216:2021)

Title (PL)

Stałe składniki środowiska — Oznaczanie całkowitej substancji rozpuszczonej (TDS) w wodzie i eluatach

Title (EN)

Environmental solid matrices — Determination of total dissolved solids (TDS) in water and eluates

Step-by-step procedure

1. Sample

   Carry out the determination without delay, at the latest a week after sampling (for an eluate — after it has been prepared); store it at 2–8 °C in the meantime.

2. Filtration

   Filter the sample through a 0.45 µm membrane filter (omit this if it has already been filtered through a membrane, e.g. an eluate according to EN 12457).

3. Evaporating dish

   Dry the dish at (105 ± 5) °C, cool it to ambient temperature and weigh it to 1 mg (m_a).

4. Portion of the filtrate

   Homogenise the filtrate and transfer a portion V_FB (in stages if necessary) so that 20–1000 mg of dry matter remains after evaporation.

5. Evaporation and drying

   Evaporate to dryness in the drying system at (105 ± 5) °C, cool in the desiccator and weigh (m_b). Dry for a further 30 min and weigh again — the mass is constant when the difference is ≤ 0.5 % or ≤ 2 mg; otherwise repeat (after the third drying take the last value and note it in the test report).

6. Calculation

   ρ_TDS = (m_b − m_a)/V_FB [mg/l]; round to 3 significant figures. For a non-homogeneous sample carry out the determination in duplicate. In the test report give the reference to the standard, the identification of the sample, the result and any deviations.

Required equipment and apparatus

Health and safety (OHS)

• Hot dishes from the drying oven at 105 °C — tongs, heat-resistant gloves
• Eluates of waste and wastewater may contain harmful substances — gloves, extraction during the evaporation
• Filtration under vacuum — sound suction flasks

Frequently asked questions