Distilled water is essential for making solutions in the lab primarily because it lacks impurities like dissolved ions, which could otherwise compromise the integrity and accuracy of chemical experiments.
The Purity Principle: Why Impurities Matter
In a laboratory setting, the precision and reliability of results are paramount. Distilled water is chosen because it undergoes a process that effectively removes many common contaminants found in tap water.
- Absence of Dissolved Ions: Unlike tap water, which contains various dissolved mineral ions (e.g., calcium, magnesium, iron, chlorides, sulfates) from its journey through pipes and natural sources, distilled water has these impurities removed. These ions are often responsible for "hard water" characteristics.
- Preventing Interference: The presence of dissolved ions and other impurities can interfere with chemical tests via unintended side reactions. For instance, an unwanted ion might react with a reagent, consume it, form a precipitate, or alter the pH, leading to inaccurate measurements or invalid experimental outcomes.
- Maintaining Solution Integrity: Using pure water ensures that the only substances reacting in your solution are the ones you intentionally add, allowing for controlled and predictable chemical processes.
Understanding Different Grades of Lab Water
While "distilled water" is a common term, laboratories often use various grades of purified water depending on the specific application. The purity is typically measured by conductivity (lower conductivity indicates higher purity).
Distilled Water
Distillation involves boiling water and then condensing the steam. This process leaves behind most non-volatile impurities, including minerals, salts, heavy metals, and some organic compounds. It's a fundamental purification method, but it may not remove all volatile organic compounds or dissolved gases.
Deionized (DI) Water
Deionized water is produced by passing water through ion-exchange resins that remove dissolved inorganic salts and ions. It's highly effective at removing ions but may not remove uncharged molecules like organic compounds or bacteria.
Reverse Osmosis (RO) Water
Reverse osmosis uses a semi-permeable membrane to remove ions, organic molecules, and even bacteria by forcing water through it under pressure. RO water is generally cleaner than distilled water but might still contain some dissolved gases and small organic molecules.
Ultrapure Water (Type I)
This is the highest grade of water purity, often achieved through a combination of distillation, deionization, reverse osmosis, and additional purification steps like UV sterilization and filtration. Ultrapure water is critical for highly sensitive analytical techniques and cell culture.
Consequences of Using Impure Water in Experiments
The impact of using water with impurities can range from minor discrepancies to complete experimental failures.
- Inaccurate Analytical Results:
- In titrations, dissolved ions can react with the titrant or analyte, leading to incorrect endpoints and concentration calculations.
- In spectrophotometry, impurities can absorb light at specific wavelengths, interfering with the sample's absorbance readings.
- Unexpected Side Reactions: Contaminants can participate in unwanted reactions, forming byproducts that obscure the intended chemical process.
- Precipitation: In some cases, impurities can react with components of the solution to form insoluble precipitates, clouding the solution or affecting its stability.
- Equipment Contamination and Damage: Residues from impure water can build up on sensitive laboratory equipment, such as electrodes, glassware, and chromatography columns, affecting their performance and lifespan.
- Compromised Biological Experiments: For biological applications like cell culture or molecular biology, even trace impurities can be toxic to cells or interfere with enzyme activity.
Practical Insights and Examples
Consider these common laboratory scenarios where water purity is critical:
- Preparing Standard Solutions: When creating solutions of known, precise concentrations (e.g., for calibration curves), any impurity in the water would dilute the actual concentration of the solute, leading to errors.
- Making Buffer Solutions: The pH of a buffer solution is highly sensitive. Dissolved ions in impure water could alter the ionic strength or react with buffer components, changing its pH and buffering capacity.
- Trace Analysis: In techniques designed to detect very low concentrations of substances (e.g., atomic absorption spectroscopy, inductively coupled plasma mass spectrometry), impurities in the water can introduce significant background noise or false positives.
- Cleaning Glassware: Even for the final rinse of laboratory glassware, pure water is used to prevent the deposition of mineral residues that could later contaminate experiments.
Summary of Water Grades and Uses
| Water Grade | Common Purification Methods | Typical Applications |
|---|---|---|
| Tap Water | Filtration, chlorination | General washing, initial rinsing, non-critical tasks |
| Distilled Water | Boiling and condensation | General reagent preparation, basic rinsing, non-sensitive chemical reactions |
| Deionized Water | Ion-exchange resins | Most chemical analyses, buffer preparation, general lab use, glassware rinsing |
| RO Water | Reverse osmosis membrane | Feedwater for other purification systems, general lab use, some reagent preparation |
| Ultrapure Water | Multiple stages (RO, DI, UV, filtration) | HPLC, GC, cell culture, molecular biology, trace analysis, highly sensitive experiments |
By consistently using distilled or appropriately purified water, scientists ensure that their experiments are conducted under controlled conditions, leading to reliable, reproducible, and accurate scientific discoveries.
