Overview
A standard in biochemistry is a material containing a substance of known concentration, expressed with definite numbers and units, which is used to find the concentration of that same substance in an unknown material. Every calibration, every titration and every quality control programme in a clinical laboratory ultimately traces back to a standard.
This lesson explains the two types of standard — primary and secondary — their defining properties, how they are used for calibration and titration, and how they fit together in the flow from national reference laboratories down to the routine clinical bench.
Learning Objectives
After this lesson you will be able to…- Define a standard and explain its main uses in a biochemistry laboratory
- List and explain the properties that qualify a chemical as a primary standard
- Explain why certain chemicals (e.g. NaOH, KMnO4) can only serve as secondary standards
- Describe calibration by titration using a worked ascorbic acid example
- Trace the flow of primary and secondary standards from national to end-user level
Clinical Story
Why This MattersA student technologist is asked to standardize a sodium hydroxide solution before using it for an acid-base titration. She quickly learns that NaOH itself cannot be weighed directly and trusted, because it absorbs moisture from the air the moment its container is opened. Instead, she must standardize it against a primary standard — potassium hydrogen phthalate — before it can be trusted for any patient-related titration.
Core Concepts
A standard (or calibrator) is a material containing a known concentration of a substance of interest, expressed with definite numbers and units. Standards serve four main functions:
- Provide a reference to determine an unknown concentration
- Standardize volumetric solutions
- Allow preparation of secondary standards
- Calibrate an instrument
Standards are divided into two types: primary standards, which come first and act as the ultimate reference, and secondary standards, which are calibrated against the primary standard and used in day-to-day laboratory work.
A primary standard is a chemical or reagent that is:
- Extremely pure — preferably 99.98% or higher, exceeding American Chemical Society (ACS) grade
- Highly stable — very low reactivity, so it does not change over time
- Anhydrous — contains no water of crystallization (e.g. anhydrous MgSO₄, not MgSO₄·7H₂O)
- Less hygroscopic — does not readily absorb atmospheric moisture
- Of very high molecular weight, so small weighing errors have minimal effect
- Easily weighed, ready to use, non-toxic and inexpensive
Common primary standards include sodium carbonate and Tris (for acid titration), potassium hydrogen phthalate — KHP (for base titration), and potassium dichromate or sodium oxalate (for redox titration).
A secondary standard has less purity, is less stable and more reactive than a primary standard, but its solution remains stable once prepared, and it must be titrated (standardized) against a primary standard before use. Classic examples are anhydrous sodium hydroxide (NaOH), which is extremely hygroscopic, and potassium permanganate (KMnO₄), which is reactive and easily contaminated by its own oxidation product (MnO₂).
Calibration (also called standardization) is the process of comparing the measurement of a standard or instrument (primary) against another standard or instrument (secondary) to eliminate variation. Secondary standards are used by companies making reagents and kits, and by external quality control programmes for smaller laboratories, which in turn calibrate their own internal QC against the secondary standard.
Laboratory Principle
A primary standard acts as the ultimate reference material at national level; a reference measurement procedure is calibrated against it. Companies then prepare secondary standards, which their own measurement procedures use to assign values to product calibrators (external quality control). End-user laboratories standardize their routine/field method against this product calibrator, enabling value assignment, internal quality control, and analysis of unknown patient samples — a traceable chain from national reference laboratories all the way to the patient's report.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Sodium Carbonate (Na2CO3) or Tris | ACS/analytical reagent grade, >99.98% purity | Primary standard for acid titration | Desiccator, room temperature |
| Potassium Hydrogen Phthalate (KHP) | Anhydrous, >99.98% purity | Primary standard for base titration | Desiccator, room temperature |
| 2-Di-chloro Indophenol (2-DCIP) Indicator | Dilute aqueous solution | Indicator for ascorbic acid (vitamin C) titration/calibration | 2–8°C, protect from light |
Step-by-Step Procedure
Weigh an anhydrous, high-purity primary standard chemical (e.g. sodium carbonate or KHP) on an analytical balance.
Dissolve the weighed chemical in pure, deionized water in a volumetric flask to prepare a standard solution of known concentration.
Add the secondary standard (or unknown) solution drop by drop to the primary standard/indicator system until the endpoint (e.g. complete decolourization) is reached.
Use the volume and known concentration relationship (e.g. 100 mL standard has 40 mg ascorbic acid, so 1 mL = 0.4 mg) to calculate the unknown concentration.
Record the standardized value and use the secondary standard as the working internal calibrator for routine testing until the next re-standardization.
Flow Diagram
Quality Control
Internal QC for standards involves periodically re-verifying the secondary standard against a fresh primary standard titration, and checking expiry dates, storage conditions and seal integrity before each use.
External quality control uses secondary reference materials assigned values by national institutes such as the Institute for Reference Materials and Measurements (IRMM), against which commercial kit and reagent manufacturers calibrate their own product calibrators.
Reference Values
Normal Ranges⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Chemical purity below ACS reference grade | Unsuitable for use as a primary standard | Use only for secondary standard preparation or routine reagent work |
| Chemical is hygroscopic (e.g. NaOH) | Cannot be weighed reliably as a primary standard; must be standardized before use | Titrate against a suitable primary standard immediately before use |
| Chemical contains water of crystallization (e.g. MgSO4.7H2O) | Inaccurate weight-to-concentration calculation if mistaken for the anhydrous form | Always confirm anhydrous, high-molecular-weight form before use as primary standard |
Common Errors & How to Avoid Them
Cause: Weighing MgSO4·7H2O (MW 234) but calculating as if it were anhydrous MgSO4 (MW 108)
Prevention: Always check the label for water of crystallization; only use the anhydrous, analytical-grade form for primary standards
Cause: Using NaOH or KMnO4 directly from the bottle without first titrating it against a primary standard
Prevention: Always standardize secondary standards against a primary standard immediately before use, since they degrade or absorb moisture over time
Cause: Using a standard chemical past its expiry date, with a broken seal, or stored outside recommended conditions
Prevention: Check manufacture date, expiry date, seal integrity, and transport/storage conditions before using any standard chemical
Laboratory Tips from the Bench
Never open a hygroscopic chemical like NaOH near a humid environment for longer than necessary — weigh it quickly, in a closed container, right before use.
When preparing standard solutions, always use deionized water; ordinary tap water contains dissolved minerals that will introduce error into your calculated concentration.
Remember 'PASH' for primary standard properties — Pure, Anhydrous, Stable, High molecular weight — the four pillars that make a chemical trustworthy enough to be a primary reference.
Important Notes
A secondary standard is not 'less important' — it is simply less pure and less stable than a primary standard, and therefore must be standardized against one before it can be trusted for routine use.
The term 'calibration' and the term 'standardization' both refer to the same process: comparing a measurement against a standard or instrument to eliminate variation, just as a shopkeeper compares vegetables against a known weight.
Interactive Quiz
Test Your KnowledgeFlashcards
Tap to flipClick or tap any card to reveal the answer. Use arrow keys to navigate in single-card mode.
Clinical Case Study
Apply Your KnowledgePriya is assigned to standardize a fresh batch of sodium hydroxide (NaOH) solution before it can be used for routine acid-base titrations in the clinical chemistry section, as required by the lab's monthly QC schedule.
The visibly moist NaOH crystals confirm that sodium hydroxide is highly hygroscopic and cannot be trusted by weight alone. By titrating it against the primary standard KHP, Priya determined the true, standardized concentration of the NaOH solution (0.098 mol/L) before releasing it for use in routine laboratory titrations.
- →NaOH must always be standardized against a primary standard like KHP before use, never trusted by weight alone
- →Visible moisture on a hygroscopic chemical is a warning sign to standardize before use, not to discard automatically
- →Recording the standardized concentration allows the working solution to be used reliably until its next re-standardization
Frequently Asked Questions
Because NaOH is extremely hygroscopic — it begins absorbing atmospheric moisture the instant its container is opened, so its true weight (and therefore true concentration) can never be trusted without standardizing it against a primary standard first.
A primary calibrator has a stated purity above 99.98% and is used to calibrate the reference measurement procedure, typically only at national reference institutes. A secondary calibrator (or product calibrator) is prepared and supplied by companies, standardized against the primary calibrator, and used by routine clinical laboratories for everyday calibration.
Because ordinary tap water contains dissolved minerals and ions that would introduce unknown contamination into the standard solution, making the calculated concentration inaccurate and unreliable.
Quick Revision
10-Minute ReviewKey Takeaways
- Common primary standards include sodium carbonate, Tris, KHP, potassium dichromate and sodium oxalate.
- Standard solutions must always be prepared using pure, deionized water.
- A standard is a substance of known concentration used to determine unknown concentrations, standardize solutions, and calibrate instruments.
- Primary standards sit at the top of the calibration chain; secondary standards are standardized against them.
- The flow of standards runs from national reference institutes to companies to routine clinical laboratories.
- Calibration by titration (e.g. ascorbic acid vs 2-DCIP) is a practical way to determine unknown concentration.
Competency Checklist
Track Your MasteryReferences
- National Institute of Open Schooling (NIOS). Biochemistry — Module: Primary and Secondary Standards (Lesson 29).
- Skoog DA, West DM, Holler FJ. Fundamentals of Analytical Chemistry.
- CLSI. Application of a Quality Management System Model for Laboratory Services (QMS01).