Buret selection directly affects titration quality because it determines how steadily you can deliver titrant, how clearly you can read the meniscus, and how repeatable your delivered volumes are from run to run. A well-chosen glass burette plus a consistent reading and handling routine reduces common accuracy killers like parallax, trapped tip bubbles, and slow leaks through the valve.
This guide gives you (1) a quick selector you can use for purchasing and (2) a short technique section that helps technicians and students get the same volume result from the same endpoint. You’ll also learn what “Class A/AS vs Class B” means in practical tolerance terms, when a Schellbach stripe is worth it, and how stopcock choice can make or break endpoint control.
Quick Selector (Most Valuable)
1) Decision table
2) 7-step checklist (shopping-ready)
- Capacity (how to choose 10 vs 25 vs 50 mL): Choose the smallest capacity that still covers your typical titrant volume with headroom; 10 mL for low-volume endpoints, 25 mL for general teaching and many assays, 50 mL when volumes often exceed ~25–30 mL.
- Class A/AS vs Class B (what it changes in practice): ISO 385 specifies two accuracy classes-Class A (including AS) as the higher grade and Class B as the lower grade-and provides maximum permissible errors by capacity and subdivision (for example, 25 mL with 0.10 mL subdivisions is ±0.05 mL for A/AS vs ±0.10 mL for B).
- Graduation readability (Schellbach stripe, colored scale): ISO 385 describes Schellbach tubing and how the meniscus is read using the stripe’s optical “arrowheads,” which can make consistent readings easier in busy labs.
- Stopcock type (PTFE key vs needle valve vs glass): Pick based on how fine your endpoint approach must be and how much maintenance you can support; procurement should confirm the valve is designed for smooth, precise outflow control.
- Leakage control + smooth flow: ISO 385 includes leakage test expectations (expressed in scale subdivisions over time) and emphasizes that stopcocks must prevent leakage beyond specified limits.
- Material/color (clear vs amber) and when it matters: If light can change titrant concentration or indicator response, choose amber; otherwise clear improves general visibility and training.
- Accessories (stand, clamp, funnel, tip guard): Plan for a rigid stand/clamp (vertical alignment matters for reading), a small filling funnel to reduce spills, and tip protection to prevent chips that can trap bubbles.
Types of Glass Burettes (What You’ll See on Product Pages)
Class A / Class AS vs Class B
ISO 385 defines Class A (subdivided into A and AS) as the higher grade and Class B as the lower grade, and it lists maximum permissible errors by nominal capacity and scale subdivision. A practical “tolerance mindset” is to match class to decision risk: if a titration result releases product, fails a batch, or supports accreditation-driven reporting, tighter class control generally reduces measurement uncertainty contribution from the buret itself.
Class AS is the variant where a waiting time applies: ISO 385 specifies a 30 s waiting time for Class AS and notes it is “critical to the accuracy of the reading,” while also noting that in many titrations the endpoint step naturally consumes that time. In teaching labs and basic work where the dominant error is technique (reading, endpoint overshoot, inconsistent swirling), Class B is often sufficient, as long as leakage and reading discipline are controlled.
Schellbach stripe burettes (readability upgrade)
ISO 385 states that for burettes constructed from Schellbach tubing, the meniscus is read “at the point of contact of the tips of the apparent two arrowheads created by the distortion of the rear dark stripe by the meniscus.” That defined visual reference can reduce disagreement between operators, especially when solutions are slightly colored or lighting varies.
When is it worth paying for? If your lab sees high operator variability (multiple shifts, students, rotating analysts) or if you routinely read close to 0.01–0.02 mL by interpolation, the readability benefit can pay back in fewer repeats and fewer “was that 18.36 or 18.38?” disputes. ISO 385 also specifies durability expectations for the stripe and limits the stripe width to not exceed 40% of the tube circumference.
Capacity and graduation
Common capacities you’ll see are 10, 25, 50, and 100 mL, and ISO 385 provides typical subdivision patterns and permissible errors tied to those sizes (e.g., 50 mL often uses 0.10 mL subdivisions with ±0.05 mL permissible error for A/AS and ±0.10 mL for B). Selection logic is simple: smaller capacity supports better control for low-volume work; larger capacity reduces refills and meniscus reset steps for high-volume work.
Keep graduation thinking practical: you never “measure” only to the printed line-operators interpolate between lines, so readability and consistent eye-level alignment often matter as much as the nominal subdivision. ISO 385 also specifies that burettes are graduated downward from zero at the top.
Stopcock Choices Explained (This Is Where Accuracy Often Fails)
PTFE stopcock vs glass stopcock (what changes for the user)
ISO 385 allows stopcocks made entirely of glass as well as other designs made from inert alternative materials such as plastics, provided they allow smooth, precise control and meet leakage requirements. In day-to-day use, the difference is usually felt as “feel” (smoothness), sealing stability over time, and how much routine attention is needed to keep the valve operating consistently.
From a maintenance standpoint, many labs prefer designs that avoid grease-related variability and contamination risk; a ptfe stopcock is commonly chosen when you want a chemically inert seal without lubrication steps. Borosil’s catalog notes that PTFE-key designs can avoid lubrication, reducing seizure and grease contamination risk in operation.
ptfe key stopcock (why labs like it)
A ptfe key stopcock is popular because it supports controlled delivery and consistent closing torque without relying on greasing, which helps with repeatability and reduces one common source of leaks and stick–slip flow. Borosil specifically lists operational advantages of PTFE keys including “lubrication is never required,” “excellent flow control,” and reduced risk of contaminating greases.
As a default for routine titration benches, a ptfe key stopcock is often a sensible starting point when procurement needs one configuration that most analysts can use with minimal retraining. If your lab runs very slow, near-endpoint additions (drop-by-drop over extended time), validate that the valve gives stable micro-flow without “sudden release” behavior.
Needle valve vs key stopcock
Needle valves can give very fine control for creeping up on an endpoint, which is helpful when the reaction is slow, the indicator transition is sharp, or the cost of overshoot is high. ISO 385 emphasizes smooth and precise outflow control as a construction requirement, regardless of the valve style.
Trade-offs are practical: needle valves may be slower for bulk delivery and can increase the learning curve for new users, while a key stopcock can be faster for most of the titration but requires good hand control to avoid over-delivery near the endpoint. In teaching settings, pick the style that minimizes “panic adjustments” and makes it easy to demonstrate leak checks and bubble removal.
Technique Basics for Accurate Titration Volumes
Conditioning/rinsing is about consistency: after cleaning with water, rinsing with a small portion of titrant helps remove residual water that would otherwise dilute the titrant and change its effective concentration during delivery. This is one of the simplest ways to prevent systematic bias, especially when multiple people share the same buret.
Filling and removing air bubbles near the tip is non-negotiable because bubbles can compress or dislodge during delivery, causing apparent “jumps” in volume. ISO 385 also highlights construction expectations to avoid cavities that trap air bubbles at joins and requires a jet design that avoids turbulence and bubble traps.
Meniscus reading: read at eye level to avoid parallax error; LibreTexts explicitly warns that reading from above or below the meniscus introduces parallax error, and it recommends being at eye level with the bottom of the meniscus. For visibility, the same source suggests placing a dark piece of paper behind the buret to improve contrast when reading.
Recording initial/final readings: write both values (don’t rely on “starting at zero”) and use the same interpolation habit every time (e.g., estimate to the same decimal place your method supports). For Class AS instruments, ISO 385 defines a 30 s waiting time before the final setting/reading in the adjustment concept, even though titration endpoint time often exceeds it.
Drain behavior and repeatability: keep the buret vertical and avoid touching the tip to the flask wall while delivering, because contact can alter flow and leave inconsistent droplets. ISO 385 describes delivery time determination with the jet not in contact with the receiving vessel wall, reinforcing that contact-free delivery is part of standardized behavior.
Common Problems & Troubleshooting
- Leaky stopcock symptoms and what to check (general): If the level drifts while “closed,” check that the valve closes smoothly and that sealing surfaces are clean; ISO 385 includes leakage limits expressed in scale subdivisions over time and requires stopcocks to prevent leakage beyond those limits.
- Bubbles that cause jumps in volume delivery: Look for bubbles at the tip/jet and any joints; ISO 385 calls out avoiding cavities at joins that can trap air bubbles.
- Flow too fast / too slow: Verify the tip is not chipped or constricted and that the valve motion is smooth; ISO 385 specifies jets should avoid sudden constriction that could create turbulent outflow.
- Meniscus hard to see: Improve contrast with a dark card behind the tube and control glare by adjusting lighting angle; LibreTexts suggests using dark paper behind the buret to better read the meniscus.
- Student-to-student variability (teaching lab tips): Standardize three things-eye level position, the same background card, and a shared rule for endpoint approach (fast to near endpoint, then dropwise with consistent swirling).
Maintenance & Storage (Safe, General)
Basic cleaning: for typical aqueous titrants, rinse promptly after use and avoid letting residues dry; the goal is to maintain smooth flow and a clean meniscus line without deposits. ISO 385 also emphasizes permanent, clearly visible graduations and inscriptions, so avoid abrasive cleaning that damages markings.
Preventing stuck parts: choose valve designs that reduce the need for grease and frequent re-fitting; Borosil notes PTFE-key systems where lubrication is not required, which helps reduce seizure and grease contamination. For long-term storage, drain fully, leave the valve in a neutral position recommended by your lab SOP, and protect the tip from knocks.
When replacement is more practical: if leakage persists after cleaning and inspection, or if the jet is chipped/rough (bubble trapping, erratic droplets), replacement of the valve components or the buret can be faster and more reliable than repeated troubleshooting. ISO 385’s leakage and jet construction requirements provide a useful reference point for deciding what “acceptable” looks like.
FAQ
What is a burette used for in titration?
A burette is used to deliver a measured volume of titrant into a flask so you can calculate an unknown concentration from the volume at the endpoint.
How do you read a burette accurately?
Read at eye level at the meniscus to avoid parallax error, and use a consistent background for contrast; LibreTexts explicitly notes eye-level reading and warns that above/below viewing causes parallax error.
What is a Class A burette?
Under ISO 385, Class A (and AS) is the higher accuracy grade, with maximum permissible errors listed by capacity/subdivision and tighter limits than Class B.
What’s the difference between a burette and a pipette?
A burette delivers variable volumes under controlled flow (you read initial and final scale values), while a volumetric pipette typically delivers one fixed calibrated volume in a single transfer.
Do I need a Schellbach burette?
You “need” it when reading consistency is a recurring problem; ISO 385 defines a specific Schellbach meniscus reading point based on the stripe’s arrowhead distortion, which can reduce operator disagreement.
What stopcock type is easiest for beginners?
Beginners usually do best with a valve that moves smoothly, seals reliably, and makes leaks obvious during checks; ISO 385 emphasizes stopcocks that allow smooth, precise outflow control and meet leakage limits.
Key Takeaways
- Pick capacity based on typical titrant volume: smaller (10 mL) for low-volume work, larger (50 mL) for high-volume routines.
- Use Class A/AS when tighter tolerance matters; ISO 385 defines A/AS as higher grade and B as lower grade.
- Know typical permissible errors; ISO 385 lists them by capacity/subdivision (e.g., 25 mL with 0.10 mL subdivisions: ±0.05 mL for A/AS vs ±0.10 mL for B).
- Schellbach stripes improve consistent meniscus reading; ISO 385 defines the “arrowhead” contact point method.
- Stopcock quality matters as much as class; ISO 385 requires smooth, precise control and leakage limits.
- A ptfe stopcock is often chosen to reduce grease-related variability and contamination risk in routine use.
- Use consistent conditioning/rinsing to avoid dilution bias from residual water in the bore.
- Remove tip bubbles before starting; ISO 385 addresses bubble-trap avoidance in jet construction.
- Read at eye level to avoid parallax; LibreTexts explicitly warns about parallax from above/below viewing.
- If repeatability is poor, troubleshoot in order: leaks, bubbles, reading method, then endpoint technique.