Application of Ultra-pure water in Laboratory–Change of Water Sample and Selection of Water Sample Container

Ultrapure water plays an essential role in laboratory analysis, environmental testing, chemical experiments, microbiology, and precision measurement. To ensure accuracy, water samples must be properly collected, stored, and analyzed. However, once a water sample leaves its natural environment, biological, chemical, and physical processes begin altering its composition almost immediately. Understanding why water samples change and how to minimize these changes is critical for achieving reliable laboratory results.

Once a water sample is taken out of its original environment, various internal and external factors begin to affect its components. These changes can distort analytical results if not properly controlled.

Microorganisms such as bacteria, algae, and protozoa continue to metabolize inside the stored water sample.

Biological activities can:

• Consume oxygen
• Produce carbon dioxide
• Change nitrogen, phosphorus, or silicon concentrations
• Generate new organic compounds
• Alter overall water chemistry

These effects occur rapidly and can significantly change the sample within minutes to hours.

Components within the water sample often interact chemically when isolated from natural conditions.

Common chemical changes:

• Dissolved oxygen oxidizes iron (Fe²⁺ → Fe³⁺), sulfides, etc.
• Polymer compounds may depolymerize
• Monomers may polymerize
• pH changes may cause precipitation or dissolution

These reactions alter the concentration and properties of target analytes.

Environmental conditions strongly influence water sample stability.

Physical factors include:

• Light exposure — accelerates photochemical reactions
• Temperature fluctuations — affects gas solubility
• Standing time — causes precipitation of CaCO₃, Mg₃(PO₄)₂, Al(OH)₃
• Container materials — may adsorb metals or organics
• Vibration or shaking — increases volatilization of oxygen, cyanide, mercury

Even sealed samples can undergo rapid physical changes.

The rate and degree of sample alteration depend on:

• Water type (drinking water, surface water, wastewater, etc.)
• Storage temperature and light exposure
• Container material
• Transportation and climatic conditions

Wastewater changes the fastest, while drinking water is relatively stable.

Because changes occur rapidly, timely analysis and proper preservation techniques are essential.

Correct preservation minimizes changes and ensures analytical accuracy.

Filling containers to overflowing reduces contact with air and prevents:

• pH fluctuations
• DO (dissolved oxygen) interference
• Carbon dioxide absorption
• BOD and chemical changes due to oxygen exposure

Exception:
Do not fill containers completely if samples will be frozen (to avoid bursting).

Refrigeration slows biological and chemical activity.

Guidelines:

• Store in the dark
• Use ice-water bath immediately after sampling
• Avoid long-term refrigerated storage
• Wastewater holds for less time

Freezing extends storage time.

Requirements:

• Use plastic containers (expansion during freezing)
• Ensure rapid & uniform thawing before analysis
• Not suitable for all analytes (e.g., gas analysis)

Fixatives stabilize specific components.

Common preservatives:

• Acids and alkalis
• Biological inhibitors
• Fixing agents for metals or organics

Usage rules:

• Add to empty bottle before sampling or immediately after sampling
• Must not interfere with measured components
• Volume of preservative must be considered in final calculations
• Perform blank tests when needed (e.g., trace metals)

Safety note:
Some preservatives (like HgCl₂, acids) are hazardous—handle with protective equipment.

Proper container selection prevents contamination, adsorption, or chemical reactions.

1. No New Contamination

Glass containers release Na, Ca, Mg, Si, B into water, so avoid them when analyzing these elements.

2. No Adsorption or Absorption

• Glass may adsorb metals
• Polyethylene and plastics may adsorb organics, phosphates, oils
  Choose container material based on target analytes.

3. No Chemical Reaction with Sample

Example:
Glass reacts with fluoride → do not store fluorine samples in glass.

4. Dark Containers for Light-Sensitive Samples

Amber glass reduces photochemical decomposition.

Container cleanliness directly impacts data accuracy.

For trace analysis:

• Use new or thoroughly cleaned containers
• Standard cleaning sequence:
  1. Water + laboratory detergent
  2. Acid cleaning (e.g., sulfuric acid-chromic acid lotion)
  3. Rinse with tap water
  4. Rinse with distilled or deionized water

Important:

• Avoid phosphate detergents when testing for phosphate
• Avoid chromic acid for sulfate or chromium testing
• Heavy metal analysis containers must be soaked in nitric or hydrochloric acid (1 mol/L) for 1–2 days

• Use brown glass bottles
• Plastics (except PTFE) interfere with organics
• After normal cleaning, dry at 180°C for 4 hours
• Rinse with purified hexane or petroleum ether

Containers must be:

• Heat-sterilizable
• Free from chemicals that inhibit biological activity
• Washed with nitric acid and DI water to remove metal residues

Add sodium thiosulfate to neutralize chlorine (0.1 mL of 10% Na₂S₂O₃ per 125 mL).

Ultrapure water is essential for accurate laboratory testing, but water samples begin to change immediately after collection due to biological, chemical, and physical factors. Proper preservation—such as using the right container, controlling temperature, sealing correctly, and applying suitable preservatives—is critical for ensuring analytical reliability. Understanding container selection and cleaning protocols further minimizes contamination risks. By following standardized preservation and handling procedures, laboratories can maintain sample integrity and ensure precise, reproducible results.

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Biological metabolism, chemical reactions, and physical influences begin immediately once water leaves its natural environment.

Refrigeration at 2–5°C in the dark is generally sufficient for short-term storage.

Avoid glass; use acid-washed polyethylene containers to prevent adsorption and contamination.

No. Freezing is not recommended for gas analysis or samples sensitive to precipitation upon thawing.

Amber (dark) bottles protect light-sensitive analytes from photodegradation.

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• Characteristics and Principles of Laboratory Ultra-Pure Water