Overview
ISO 1577:1987 - Tea - Determination of acid‑insoluble ash specifies a laboratory method to quantify the fraction of tea ash that is insoluble in hydrochloric acid. The procedure starts from the total ash generated according to ISO 1575, treats that ash with HCl, filters, ignites and weighs the remaining residue. Results are reported as a percentage of the sample on a dry‑matter basis.
Keywords: ISO 1577:1987, tea testing, acid‑insoluble ash, ash determination, laboratory method, ISO tea standards
Key topics and technical requirements
- Scope: Determination of acid‑insoluble ash in tea samples using the total ash from ISO 1575 as the starting material.
- Primary reagents:
- Hydrochloric acid solution (dilute by mixing 1 volume concentrated HCl [ρ20 ≈ 1.16–1.18 g/mL] with 2.5 volumes water).
- Silver nitrate solution (~17 g/L) for testing absence of chloride in washings.
- Apparatus: 50–100 mL capsules (platinum, porcelain or inert material), adjustable furnace (525 ± 25 °C), boiling water bath, ashless filter paper, desiccator, analytical balance.
- Procedure summary:
- Add 25 mL HCl to total ash in capsule; gently boil 10 min; cool and filter.
- Wash until no chloride is detected (AgNO3 test). Return filter and residue to capsule, evaporate, and ignite at 525 ± 25 °C until residue is free of char.
- Cool in desiccator and weigh; reheat 30 min and repeat until consecutive weights differ by ≤ 0.001 g.
- Replicates and repeatability: Two independent determinations are required; the arithmetic mean is taken if repeatability criteria are met. The difference between paired determinations must not exceed 0.02 g of acid‑insoluble ash per 100 g of ground sample.
- Calculation: Results are expressed as % (m/m) of acid‑insoluble ash relative to dry matter; dry matter content is determined per ISO 1572.
Applications and users
- Quality control and specification testing in tea production and blending operations.
- Analytical laboratories performing food safety, trade and conformity testing for tea imports/exports.
- Regulatory agencies evaluating contamination or mineral residue levels (acid‑insoluble ash indicates siliceous or mineral contaminants such as sand).
- Research labs studying tea composition and processing impacts on inorganic residues.
Related standards
- ISO 1575 - Tea - Determination of total ash (starting point for this method).
- ISO 1572 - Tea - Preparation of a ground sample for determination of known dry matter (used for sample prep and dry‑matter reporting).
This standard is essential for consistent, reproducible measurement of acid‑insoluble ash in tea and for harmonized quality and compliance testing across laboratories.
