Stability and Accuracy of Conductivity Standards

Questions often come up regarding liquid conductivity standards and their usage. The purpose of this article is to discuss topics such as stability, accuracy, and best practices for using these products.

What is meant by a conductivity standard's stability?

Specifications for all Hamilton liquid conductivity standards include a statement on stability. The stability can range from 12 to 36 months depending on the solution. Stability is defined as the liquid solution's ability to comply with the stated accuracy from time of manufacturing. Often the terms stability and shelf life are used interchangeably.

For example, the graph below provides test data for 5 µS/cm standard (Ref 238926). This standard has an accuracy of ±1% which equates to deviation of less than ± 0.05 µS/cm. Samples from the same production lot were measured over a three year period. Testing was conducted by the National Metrology Institute of Germany PTB (Physikalisch-Technische Bundesanstalt). During this time it was found that the conductivity standard kept its accuracy of ±1%.

How does Hamilton verify the accuracy of our conductivity standards?

A sample of each production batch is sent to the accredited independent laboratory DFM (Danish Metrology Institute) for testing. The lab verifies the liquid standard on test equipment built in collaboration with NIST (National Institute of Standards and Technology) and certified to ASTM (American Society for Testing and Materials) standard D1125-(2014).

The actual conductivity value, standard deviation of the batch, and expected accuracy value during the stability period are listed on the declaration of quality. A copy of this document is shipped with each bottle of conductivity standard or can be provided by Hamilton. The lot (batch) calibration certificate from DFM is also included for additional traceability.

An example of batch calibration certificate from DFM

What influences stability?

Stability is dependent on the chemical make-up of the solution and the bottle / cap used for packaging. Most conductivity standard rely upon inorganic salts such as KCl or NaCl, or acids such HCl. These chemicals are chosen since they remain stable for indefinite periods of time.

The bottle and cap offer a much larger influence on stability. Over time, evaporation of the liquid solution or penetration of outside vapors through the packaging can cause a shift in conductivity. Choice of material such as glass or plastic defines the vapor permeability. For this reason, the majority of Hamilton conductivity standards uses impermeable glass bottles with specialty lined caps. Some products are also available with polyethylene bottles. Polyethylene is a plastic recognized by NIST for very low permeability.

Examples of bottle types use with Hamilton liquid conductivity standards can be seen above. The plastic Calpack bottle seen on the left uses low-permeability polyethylene to ensure stability of the solution. The bottle has several unique features including a small 15mL calibration chamber with a one-way valve. The chamber is used for calibration thus a separate beaker is not needed. Removing the need for a separate container lessens the chance for contamination due to an improperly cleaned beaker.

What happens once the bottle is opened?

Once opened, the conductivity solution may be influenced by exposure to air as well as the whims of the human end-user. Atmospheric gases such as CO₂ are well known to form carbonic acid H₂CO₃ by dissolving into the aqueous liquid. The net result is an increase in dissolved ions and a change in accuracy of the solution. Other contributors to loss of accuracy include contamination from the conductivity sensor during calibration, improper cleaning of glassware used during calibration, dust, and general human handling issues.

Hamilton recognizes that accuracy may be unpredictable once the bottle is opened. As a general recommendation, if a user is following good laboratory practices then a liquid standard may be used repeatedly under the condition that a bottle is not left open for longer than one hour total.

What are best practices for using conductivity standards?

Store the bottle at ambient temperatures between 5 to 35°C (41 to 95°F)
Protect the bottle from direct sunlight
When using a beaker or graduated cylinder for calibration make sure to clean and rinse with distilled or deionized water. Shake out any residual droplets prior to adding the standard.
Avoid fluctuating temperature. Let the standard stabilize to ambient temperature prior to usage or consider using a water bath to control temperature.
If calibrating at temperatures other than 25°C then refer to our conductivity standard temperature charts for reference.