Resource SN -- Assay versus Purity

[Quarky00]

TLDR: For SN , it's pretty much the same .

Assay means purity, right?
The verb assay means to "examine a chemical to determine its purity." In our context, assay provides a metric that refers to the chemicals purity, usually expressed as a percent assay. On occasion, assay refers to the purity of the chemical, e.g., 99% or 99.9% pure. There is no test to determine the purity of a chemical, so in practice, the amount of impurities is quantitatively analyzed and the assay is determined by subtracting the sum of impurities from 100, to infer the assay of the chemical. This leads to a caveat emptor or "Let the Buyer Beware" situation since the supplier can choose to measure only certain impurities and if the customer does not know how to perform the analysis, they will have no clue about the actual purity of the material.

Assay
In chemistry, an assay is an analysis carried out to determine the level of impurities a sample has. It is a quantitative determination. In a sample, after identifying the main material present in it, its concentration is measured in an assay. Assay methods normally incorporate accurate and precise analytical methods. Depending on the sample to be analyzed and the other requirements, you can choose the most appropriate assay type. Chemical assays are done using techniques like chromatography, titrations, etc.

Purity
Pure means absence of contaminants or other materials, which we do not expect to have in a sample. Purity is some kind of a measurement to indicate how pure the sample is. This measurement can be qualitative or quantitative. If the purity is low, that means there are lots of contaminants. If the purity is high, then the contaminants are low. Pure substance cannot be separated into two or more substances by any mechanical or physical method. Pure substance is, therefore, homogenous. It has a uniform composition throughout the sample. Further, the properties of it are also uniform throughout the sample.

Assay means analyzing the substance for impurities . It searches for the impurities . It is an inference (deduction) : if 1% impurities were found then we probably have 99% . Since it searches for most impurities it may miss some types of impurities , though at small amounts.
Purity actually measures how much of the desired substance actually exists in the pack .

I am intentionally simplifying that . Plus that needs a further scientific clarification or verification . Unless someone requires high levels of purity , for example I need 99.99% and 99.8% is not good enough for me -- the difference with reliable chemical companies is usually negligible for our purposes (SN as poison) . It is that 0.5% difference . It may mean that a stated "99.0% assay" is actually 98.5% purity . I think that assay > 95% is fine and similar to level of purity required for this practice . We don't care about 0.5% difference .

Regarding SN , assay is accurate:

The most common assay for sodium Nitrite purity is with a very accurate technique of titration called permanganate redox titration. This is a titration that finds the endpoint with the reduction of potassium permanganate by color and it is very sensitive.

Furthermore , members had no issues of "impure SN" . Some cases of failed SN intake that showed other symptoms were revealed to be nitrate (NaNO3) rather than Nitrite (NaNO2) . So there isn't a case of NaNO2 purity/assay stated >95% , with or without MSDS , that resulted in failure due to substance . To make it clearer: potential impurities at those levels did not cause a ctb to fail . It is such a simple common compound that chemical suppliers would not tinker with . The purity maybe not accurate , but not to a concerning level , as long as it's above 95% (and considering the toxicity of SN even slightly lower would probably still be fatal )

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@Tintypographer for more advice . @WhatIsMyLife may be interested in answers .