Hanna Instruments supports archaeological research — chloride in an alkaline wash bath

The conservation of iron artefacts from corrosion has changed drastically over time, from simple water washes and treatments with wines and spirits in the late 1800s, to alkaline wash baths in the present. The purpose of these different types of conservation methods is to rid iron antiques of chloride. While an artefact is buried in the soil, chloride ions are attracted to the iron object and therefore become concentrated. Once the object is exposed to the air (oxygen and moisture), the chloride reacts with ferrous (Fe2+) ions, forming akageneite which expands inside the corrosion layers, cracking the outer surface.

It is common among archaeologists to soak the object in an alkaline deoxygenated wash bath to conserve as much of the object as possible. The alkaline wash bath that is used is a solution of 0.1 M NaOH and 0.05 M Na₂SO₃. The hydroxide ion from sodium hydroxide in the solution displaces the chloride ions from the object. The sodium sulfite is added to deoxygenate the solution in order to prevent corrosion. For this process to occur, the artefacts are soaked in a heated enclosed bath for several months. During this time, the chloride concentration is closely monitored to determine when the process is complete. The bath should be able to maintain a concentration of less than 10 ppm.

A customer that works for a museum that recovers artefacts from a Roman archaeological site approached Hanna Instruments for a way to accurately measure and record the concentration of chloride in the alkaline deoxygenated wash bath. The customer was using chloride test strips and needed a much more accurate way to determine concentration, since the data collected was to be included into a scientific paper. The customer was presented with the technologies available and decided to perform the analysis by argentometric titration with a silver/sulfide ISE and silver nitrate titrant. Since the accuracy of the measurement was critical, the customer wanted to perform the titration automatically and decided upon the HI932.

The customer appreciated that, with the 25 mL burette, the minimum dose was 0.005 mL. This small dosing capability means that the volume of the titrant required to reach the endpoint is more precisely determined than manual titrations. The customer also appreciated that the test was easy to run once the method was programmed in the meter and the results were easily transferred to a USB flash drive.

Other important features of value were the ability to swap burettes to determine the alkalinity of the bath, since it was also important to monitor the sodium hydroxide concentration and the ability to use as a pH meter.

For more information: https://hannainst.com.au/automatic-potentiometric-titrator-hi932c1-02.