Advanced Topic 324 of 1102 States of Matter & Solutions
Hardy-Schulze Rule
Coagulating power of ions increases with their charge.
Practise States of Matter & Solutions All States of Matter & Solutions topics
In-depth explanation of Hardy-Schulze Rule
The Hardy-Schulze rule states that the coagulating power of an ion depends strongly on its charge: the higher the charge of the ion carrying the opposite charge to the colloid, the greater its ability to coagulate the colloid. For a negatively charged colloid, Al3+ is more effective than Ba2+, which is more effective than Na+. In the classical treatment the coagulating power scales roughly with the sixth power of the ion's charge. The minimum electrolyte concentration needed to cause coagulation is called the flocculation value. This rule explains why multivalent ions are used in water treatment.
Going deeper: Hardy-Schulze Rule
Key ideas: The flocculation value is lowest for the ion of highest opposite charge: for As2S3 sol (negative), approx NaCl 51, BaCl2 0.7, AlCl3 0.093 mmol/L. Effectiveness ratio roughly 1 : 30 : 700 for charge 1, 2, 3 in some tests. For positive sols like Fe(OH)3, anions matter: PO4 3- beats SO4 2- beats Cl-.
Common mistakes and connections: Students use the sign of the wrong ion; only the counterion with charge opposite to the colloid counts. Another error is thinking higher concentration is needed for higher charge; the reverse is true, lower. Flocculation value is smaller when coagulating power is larger. Links to coagulation, colloid charge, electrophoresis and delta formation.
Visualization of Hardy-Schulze Rule
Hardy-Schulze rule: higher-charge ions coagulate a colloid at far lower concentration; for As2S3 sol Al3+ is hundreds of times more effective than Na+. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Adding aluminium salts with Al3+ ions coagulates negatively charged clay particles far more effectively than adding sodium chloride.
Frequently asked questions about Hardy-Schulze Rule
What is Hardy-Schulze Rule?
Coagulating power of ions increases with their charge.
Is Hardy-Schulze Rule a beginner, intermediate or advanced topic?
Hardy-Schulze Rule is a advanced topic in States of Matter & Solutions. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Hardy-Schulze Rule?
Study these earlier topics in States of Matter & Solutions first: Freundlich Isotherm, Langmuir Isotherm, Coagulation.
What comes after Hardy-Schulze Rule?
The next topics in States of Matter & Solutions are Solid Solutions and Alloys.
Can I practise Hardy-Schulze Rule with quizzes?
Yes. The interactive lesson includes a self-check question on Hardy-Schulze Rule, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Hardy-Schulze Rule fits in States of Matter & Solutions
Hardy-Schulze Rule is topic 105 of 109 in States of Matter & Solutions.