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I was reading about gold number and a book defined gold number as following:

It is the minimum mass in milligrams of a stabilizing agent which is added to the 10 mL of red gold sol to protect it against coagulation caused by 1 mL of 10% (by mass) NaCl.

Is this the definition? Because I am not able to understand above definition. Is NaCl used as the standard?

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A red gold sol is a colloidal suspension of gold nanoparticles with average size of particles less than 100 nm. The suspension is quite stable — the nanoparticles don't aggregate because of the existence of an electrical double layer on the surface of particles which causes electrostatic repulsion between the particles.

If you add a sufficient amount of an electrolyte (e.g., NaCl) to the colloid, the ions will disrupt the electric double layer which will cause aggregation (coagulation) of gold particles. A stabilizing agent (e.g., citric acid) is a compound that is able to adsorb on the surface of gold nanoparticles and protect them from aggregation (it acts like a shield), even in a high electrolyte concentration.

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    $\begingroup$ Why does colour change from red to violet? $\endgroup$ – Abcd Dec 1 '18 at 8:01
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Practically it means:

Dissolve different amounts of your reagent in 10 mL water. Make a solution of colloidal gold. Specifically this colloidal should be "red gold" (there are different ways to prepare colloidal gold, but "red gold" is a common standard. Add 1 mL of solution of NaCl. It will cause precipitation in a sample with no stabilizing reagent, but if the amount of stabilizing reagent is sufficient then the gold will remain in "red gold" colloidal state. The amount of required stabilizing agent (in grams) is the gold number of this reagent.

The reason they do it is because it provides an standard, easy and reproducible way to compare different stabilizing agents. Part about 10 (by mass) is unclear.

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