What is a key characteristic of a secondary standard?

A key characteristic of a secondary standard is that mass or volume alone is unreliable. Secondary standards are substances that are used for calibration and require comparison with a primary standard for accurate measurement. The defining aspect of a secondary standard is that its concentration or purity may not be well established, making it necessary to utilize it in conjunction with a primary standard to ensure accuracy in experimental results.

In laboratory settings, primary standards are highly pure chemical substances with a well-known concentration, and because secondary standards can vary in their properties, relying on mass or volume alone does not provide a reliable basis for quantitative work. Therefore, a secondary standard's reliability comes from using it alongside a primary standard to confirm concentration.

The other options describe characteristics that may or may not apply to specific secondary standards but do not capture their essential definition. For example, high molecular weight and stability in pure form might be true for some secondary standards, but this isn’t universally applicable. Similarly, while being non-hygroscopic could be an asset for some secondary standards, it is not a defining characteristic.