Conical Joints

Ground-glass conical joints seal a multi-neck electrochemical cell. You can swap a stopper for a septum or a gas adapter. This fit lets you purge and blanket the cell with inert gas. See Standard Operating Procedure. You keep the seal at every electrode port.

Joint nomenclature

A "24/40" conical joint has 24 mm diameter at the top of the taper. It has 40 mm along the ground-glass surface. The taper angle is standardized. It is 1:10 on the diameter. This equals about 2.86° half-angle. Glassware from different manufacturers pairs if it has the same number. The standard series includes these sizes (Quickfit/ISO 383): 10/19, 14/23, 19/26, 24/29, 29/32, 34/40, 45/50, and larger sizes. Electrochemistry cells use these common sizes: 14/23 (small vials, micro-cells), 24/29 (standard conical flask necks), and 29/32 (three-neck round-bottom flasks for multiple electrodes).

See the Camlab guide to interchangeable laboratory glassware (linked below) for the full Quickfit sizing table.

Joint types in electrochemical cells

Not every neck on a cell needs the same hardware. The choice affects reference-electrode stability and purge quality:

• Inner (cone) and outer (socket). The cone goes into the socket. Most electrodes and adapters are cones. They fit into sockets on a multi-neck flask. A 24/29 cone on a reference electrode holder fits a 24/29 socket on a cell lid.
• Threaded adapters. Some modern cells use threaded PEEK or PTFE caps instead of bare glass joints. Example: Pine Research and Gamry flat cells. They use threaded caps for the working electrode. Threads let you tighten a reproducible seal. They compress an O-ring for a defined exposed area. These caps are not interchangeable with bare conical joints. Check what your cell expects.
• O-ring joints (SVT/GL threads). These are increasingly common on reference-electrode and sparge-tube ports. A thread cap compresses an O-ring. The cap replaces the grease-and-clamp arrangement of bare glass joints. O-ring joints are easier to clean. They leave no grease contamination in the electrolyte. Older glassware does not have them.

Joint hardware

Clamps and springs

A bare conical joint relies on friction. You often add a thin film of grease or a PTFE sleeve. A bare joint is not enough for a pressurized cell. Sparging or movement can break the seal. You need a mechanical retainer:

• Keck clips. These are plastic spring clamps. They are most common for 14/23 and 19/26 joints. They are polyacetal spring clips. A clip snaps over the joint flanges. Keck clips are available for 24/29 and 29/32 joints. Larger sizes need stronger springs. Print your own Keck clips: Thingiverse 6851203 (24/29 and 29/32 sizes).
• Metal tension springs. These are hook-ended wire springs. They connect one flange to the other. They are more secure than plastic clips for larger joints (29/32 and up). Use them for warm cells or cells with stir-plate vibration. An STL file at Thingiverse 4199682 is a 3D-printable version.
• Screw-thread sleeve joints. The socket has an external thread. A plastic nut threads over the cone. Tightening the nut pulls the cone down. These are common on reference-electrode bodies and solvent-still heads. They are more secure than clips. Not all glassware has this thread.

Grease vs PTFE sleeves

Use high-vacuum grease (Apiezon, Dow Corning) or silicone stopcock grease for bare ground-glass joints. Smear grease on the cone. Seat it in the socket. Grease fills gaps in the ground surfaces. It prevents joints from freezing or leaking. Grease is not inert for electrochemistry. Trace grease dissolves into the electrolyte. This contaminates the working electrode. It creates spurious signals in CV. An adsorbing film shows as an extra peak or shifted baseline.

Two alternatives avoid grease:

• PTFE sleeves. Wrap a thin, unsintered PTFE film over the cone. Insert the cone into the socket. PTFE has low friction and inertness. This provides a leak-tight seal without grease. PTFE sleeves are not compatible with all solvents. THF, dichloromethane, and other polar organic solvents swell PTFE. Swelling deforms the sleeve, causes leaks, or jams the joint. Check solvent compatibility before assembly (see Drying Solvents). A swollen sleeve is hard to remove and contaminates the electrolyte. The sleeve must fit the joint exactly. PTFE sleeves are single-use or limited-reuse. They deform by installation.
• O-ring seals (GL-thread / SVL systems). An O-ring compresses radially by the threaded cap. The joint never relies on glass-on-glass friction. These are cleaner and more reproducible. They are more expensive.

Joint warming

Ground-glass joints seize ("freeze") after assembly with some reagents. Concentrated bases etch the glass surface. They can form a silicate bond. Do not force a seized joint. Glass is thin at the joint. It will crack before it releases. The joint typically releases with a heat gun or gentle flame on the socket. Pull apart with padded pliers. Apply balanced torque (not a straight pull). Thermal expansion of the outer glass breaks the seal. Clear liquid from the cell first. Heating a sealed glass vessel with solvent is a pressure hazard.

Joints in the sparging and solvent-drying workflow

See SOP (cell assembly, sparging / inert purge) and Drying Solvents (solvent prep). Conical joints recur in these workflows. Cells assemble on multineck flasks. Solvent stills use joints for the condenser, receiver, and adapter necks. Reference electrodes mount through a joint or threaded port. Know your glassware joint sizes before you buy adapters or electrodes.

Joint and labware links

• ILPI --- Organic Chemistry Glassware Gallery: pictures and descriptions of every standard glass joint, adapter, and flask. Browse this if you acquire secondhand or mismatched glassware. Not every "24/40" joint has the same taper tolerance. Some antique German glass (Normschliff) uses a different standard. NS 29/32 means 29 mm by 32 mm. It looks interchangeable with ISO 29/32. But it can be slightly incompatible.
• Camlab --- Understanding Quickfit Connections: the Quickfit sizing with a compatibility table.
• Glassware --- general glassware notes.
• Magnetic Stir Plate --- stir-bar selection and spinout physics. Choose a stir bar that clears electrodes.

3D-printable joint hardware

• Glassware Clip 29/32 --- 3D-printable clip for NS 29/32 joints.
• Joint clamp for NS 29/32 lab glassware --- Keck type.
• Clip for lab glassware 24/40 Chinese standard.
• Jointed Quickfit glassware clips --- four sizes (29/32, 24/29, 19/26, 14/23). Use 100% infill. Print with supports.