Where Does a Known Concentration Come From?
In the last guide we waved a hand and said "prepare standards of known concentration." But how do you ever know a concentration in the first place, with nothing to compare it against? The answer is a beautifully simple chain: weigh a pure solid on a good balance, dissolve it in a known volume of water, and arithmetic does the rest. If you dissolve exactly 1.000 gram of a pure substance in exactly one litre, you have one gram per litre — no instrument required, just a balance and a flask.
But this only works if the solid really is what the label claims. A substance pure enough, stable enough, and well-behaved enough to be weighed directly and trusted as the anchor for everything else is called a primary standard. It must not absorb water from the air, must not slowly decompose on the shelf, and must be of very high, known purity. Such substances are precious precisely because everything downstream leans on them.
When You Can't Weigh It Directly: Secondary Standards
If a substance is too unruly to weigh directly, you take a detour. You make up a solution at roughly the strength you want, then check its true concentration by reacting it against a primary standard — a process called standardization. Once you have measured what it really is, that solution becomes a secondary standard: its concentration is known not because you weighed it perfectly, but because you verified it against something that you did weigh perfectly.
Think of it as a chain of trust. The primary standard is the original measuring stick that everyone agrees on. The secondary standard is a copy whose length you carefully checked against the original. Each link adds a little uncertainty, so you keep the chain short and always trace it back to a primary standard you can defend.
One Strong Solution, Many Weak Ones
You rarely weigh a separate solid for every point on a curve. Instead you make one concentrated, carefully prepared batch — the stock solution — and store it. Whenever you need working standards, you take small, measured amounts of the stock and dilute them with water. Because every working standard is born from the same parent, they all share the same pedigree and small weighing errors don't multiply across them.
A neat way to build a row of evenly spaced standards is serial dilution: dilute the stock to make standard one, then dilute standard one to make standard two, and so on. Each step is a simple, well-controlled factor, and the chain spans a wide range of concentrations using only a pipette and flasks.
The Standard With Nothing In It: Blanks
There is one more standard you must prepare, and it is the easiest to forget because it contains none of your analyte at all. A blank is a sample that has everything except the thing you're measuring. Run it through the instrument and any signal it gives is, by definition, not coming from your analyte — it is background. Subtracting that background is what makes a low reading honest.
Two blanks deserve names. A calibration blank is the zero-concentration point of your curve — pure solvent measured the same way as the standards, anchoring where the line crosses the bottom. A reagent blank goes further: it carries every chemical you added during sample preparation, so it catches contamination hiding in your own reagents. If your acids or your water are slightly dirty, the reagent blank reveals it before it fools you.