pH Electrode Recommendations by Sample

The types of available electrodes, desired electrode features, and the compatibility of the electrode with the sample are all determining factors in selecting the right pH electrode.

The following section provides the information needed to select the best pH electrode for your applications. The pH electrodes are available with a glass or epoxy body and a variety of electrode body styles, fill types, references, junctions, and connectors that suit your application. Other factors to consider when selecting a pH electrode include:

• The reference junction should provide a stable and reproducible reading under a wide variety of sample conditions. Thermo Scientific™ electrodes include a large selection of reference junctions and filling solutions that are designed to provide fast and reliable readings in all types of samples. Electrodes with a Thermo Scientific™ Sure-Flow™ reference junction can be used in a variety of samples, including difficult samples such as dirt, colloids, sludge, and viscous material.

• The electrode filling solution should not interfere with the electrode measurements. Electrodes with a double junction reference prevent silver ions from coming in contact with the sample, which is key when measuring samples that contain silver-binding agents such as TRIS buffer, proteins, or sulfides. Refillable Thermo Scientific™ Orion™ ROSS Ultra™ and Thermo Scientific™ Orion™ ROSS™ electrodes do not contain any silver and allow the operator to modify the outer filling solution. This is useful when the regular filling solution contains ions that interfere or react with the sample that is being measured.

• The filling solution should flow freely with no fouling or clogging of the junction by the sample. The Sure-Flow reference junction on many Thermo Scientific pH Electrodes improve the electrode performance by allowing a constant flow of filling solution into the sample. The enhanced flow rate produces stable reference potentials for faster response and better stability. In addition, pressing on the electrode cap flushes the reference, which allows for easy cleaning and refilling of the electrode. Thermo Scientific™ AquaPro™ pH electrodes incorporate a high-performance polymer that isolates the silver internal reference from the sample. These low maintenance pH electrodes offer a rapid response due to their open junction design, even in dirty or viscous samples.

Sample Type	Electrode Recommendations
Biological/Pharmaceutical – TRIS Buffer, Proteins, Enzymes	A ROSS or double junction Ag/AgCl reference(no sample contact with silver)
Education/Student Use	An epoxy body for added durability
Emulsions – Foods, Cosmetics, Oils	A Sure-Flow or open junction to prevent the electrode from clogging
Emulsions – Petroleum Products, Paint	A glass body that resists damage from the sample and a Sure-Flow or open junction to prevent the electrode from clogging
Flat Surfaces – Cheese, Meat, Agar	A flat-surface tip and ROSS or double junction Ag/AgCl reference (no sample contact with silver)
Flat Surfaces – Paper	A flat-surface tip
General Purpose – Most Sample Types	All electrodes are suitable for general purpose measurements
Harsh Environments – Field or Plant Use, Rugged Use	An epoxy body for added durability and polymer or gel filled for easy maintenance
High Ionic Strength - Acids, Bases, Brines, pH > 12 or pH < 2	A Sure-Flow or open junction for better contact with the sample and more stable measurements
High Temperatures	A ROSS reference for longer life and/or a quick flow junction for fast response
HF Samples	A rugged or durable glass bulb and a double junction to protect the reference
Large Sample Sizes - Tall Flasks	A long body that fits the container
Low Ionic Strength - Treated Effluent, Deionized Water, Distilled Water	Refillable for better contact with the sample and more stable measurements
Nonaqueous – Solvents, Alcohols	A glass body that resists damage from the sample and a Sure-Flow junction for better contact with the sample and more stable measurements
Semi-Solids - Fruit, Meat, Cheese	A spear tip for piercing samples and a ROSS or double junction Ag/AgCl reference
Small Sample Size - Micro-titer Plates	A small diameter that fits the container
Small Sample Size – NMR Tubes	A small diameter that fits the container
Small Sample Size – Test Tubes, Small Flasks and Beakers	A small diameter that fits the container
Small Sample Size – TRIS Buffer, Proteins, Sulfides	A small diameter that fits the container and a ROSS or double junction Ag/AgCl reference
Titration	A Sure-Flow or sleeve junction for better contact with the sample and more stable measurements
Viscous Liquids - Slurries, Suspended Solids Sludges	A Sure-Flow or open junction to prevent the electrode from clogging
Waters – Acid Rain, Boiler Feed Water, Distilled Water, Rain Water, Well Water	A ROSS or double junction Ag/AgCl reference that is refillable for better contact with the sample
Waters – Drinking Water, Tap Water	An epoxy body for added durability
Waters – Wastewater, Seawater	A ROSS or double junction Ag/AgCl reference and have an epoxy body for added durability

Sources:

Thermo Scientific 2020

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