A burette is a specialized piece of volumetric glassware designed for the accurate dispensing of variable liquid volumes, most commonly used in titration experiments. Maintaining the cleanliness of this instrument is paramount because even minute residues can react with the dispensed solution, introducing systematic errors that compromise quantitative analysis. Proper cleaning prevents contamination, preserves the precise calibration of the glass walls, and ensures the longevity of the burette’s components.
Routine Rinsing Procedure
The immediate cleaning protocol is a sequential series of rinses designed to quickly remove the bulk of the chemical. The initial step involves rinsing the burette with the solvent used in the experiment to dissolve and flush out the remaining solution. This is followed by multiple rinses using tap water, which serves to dilute and carry away the dissolved chemicals. The technique requires tipping and rolling the burette to ensure the water contacts the entire inner surface, followed by draining the water through both the top opening and the stopcock tip.
The final and most crucial step of the routine rinse involves at least three rinses with distilled or deionized (DI) water. Tap water contains dissolved minerals, such as calcium and magnesium ions, which can precipitate and leave a film on the inner glass surface upon drying. This mineral film interferes with “sheeting,” the formation of a continuous, uniform sheet of liquid that indicates a clean burette. Using DI water for the final rinse prevents these mineral deposits, ensuring the glass remains pristine for accurate volume measurements.
Addressing Stubborn Residue
When a simple water rinse is insufficient, indicated by non-sheeting water droplets or a visible film, a deeper cleaning is necessary. Specialized laboratory detergents can be used to prepare a warm, soapy solution for washing the glassware. The detergent solution should be applied gently with a long-handled burette brush, scrubbing the interior walls without scratching the glass surface. Highly concentrated detergent should be avoided, as it can be difficult to rinse completely and the residue can interfere with subsequent chemical reactions.
For residues that resist mild detergent, such as grease or sticky precipitates, a soaking procedure may be necessary. Dilute solutions of acid or base, selected based on the nature of the residue, can be used to dissolve stubborn deposits. This procedure requires caution and thorough subsequent rinsing to avoid damaging the glassware or creating a safety hazard. Harsh abrasive materials, scouring pads, or strongly corrosive chemicals must never be used, as they will permanently scratch the glass and destroy the burette’s volumetric accuracy.
Stopcock Disassembly and Care
The stopcock, the valve mechanism that controls the liquid flow, is a sensitive component that requires specialized attention separate from the main glass tube. Stopcocks are typically made of either ground glass or polytetrafluoroethylene (PTFE), commonly known as Teflon. Glass stopcocks require a thin, uniform layer of stopcock grease to create a liquid-tight seal and allow smooth turning. This grease must be applied sparingly to the polished glass plug, ensuring none of the lubricant obstructs the bore or contaminates the solution.
PTFE stopcocks do not require lubrication because the material itself provides a low-friction seal against the glass barrel. For these types, the key components, including the plug and any associated washers or nuts, should be carefully disassembled for cleaning. Each piece must be thoroughly rinsed with the appropriate solvent and DI water to remove any chemical traces before reassembly. When reassembling, it is important to check that the plug tension is set so the valve turns smoothly without leaking, yet is not overtightened, which could potentially strain the glass.
Final Drying and Storage
After the cleaning process is complete, the burette must be properly dried and stored to maintain its condition. Air drying is the preferred method, often by inverting the burette on a dedicated drying rack or stand to allow gravity to drain all moisture. Excessive heat, such as from an oven or a heat gun, should be avoided as it can stress the glass and potentially affect the volumetric calibration.
For storage, the burette should remain in a vertical position, ideally on a stand or in a drawer, to prevent physical damage. It is good practice to leave the stopcock in the open position to prevent the plug from seizing to the barrel, especially for glass stopcocks where grease may dry out. To protect the clean interior from dust and airborne particles, a small, clean beaker or a light protective cap should be placed over the top opening.