Ranges and Changes of pH in Zoological Alcohol

Collections Marion Kotrba

Daniel BurckhardtKlaus Golbig

Christoph Meier

Ranges and Changes of pH in Zoological Alcohol Collections

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Comparison of pH values in museum collections of Basel (black) and Munich (white). Values arranged in rising order within taxa; 30 samples per taxon and collection i. e. 480 in total; assessed with pH electrode.

13% of samples in acidic range below pH 6 and 13% in alkaline range above pH 8. Similarities between compared collections regard e. g. the very large overall range in Formicidae and Diptera and the comparatively alkaline conditions in Crustacea. Note the small range in Basel Psyllidae (recently curated).

pH

PiscesAmphibia-Reptilia Mammalia Mollusca Crustacea Psyllidae Formicidae Diptera

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Ranges and Changes of pH in Zoological Alcohol Collections

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visually classified as probably OK

visually classified as needing curation

before curation after curation

480 specimen jars were visually classified a priori as in need for curation (series B) or not (series A) based on evaporation, colour, opacity etc. of the preservation fluid. pH values were subsequently measured with a pH electrode.

Visual assessment largely fails to recognize problematic jars. After the first pH assessment 320 jars a priori classified as needing curation (series B) were topped up with alcohol. Then the pH values were measured again.

Standard topping up procedure hardly improves the pH. Statement of problem:

Measurement and interpretation of pH in alcohol collections is time consuming and problematic. Reconstitution of desired pH is likewise problematic (usually requiring complete exchange of preservation fluid).

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Suggested approach Part I: Stabilize pH by buffering preservation fluid with solid ion-exchange material such as

or a combination of acidic and alkaline ion-exchange substrates in separate dispensers

exchange exhausted ampholyteonly if deviating pH is detected

exchange only the exhausted bout if deviating pH is detected

a substrate-bound ampholytei. e. a polymeric substrate (e.g. polystyrene or cellulose) provided with positively and negatively charged groups (e.g. carboxyl grous, sulfuric acid or phosphoric acid groups, various amino gorups).

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Ranges and Changes of pH in Zoological Alcohol Collections of Basel and Munich

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Suggested approach Part II: Combine ion-exchange material with color pH indicator for easy pH assessment using

or a combination of acidic and alkaline ion-exchange substrates in separate dispensers

a substrate-bound ampholyte

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Ranges and Changes of pH in Zoological Alcohol Collections of Basel and Munich

Suggested approach Part III: Advantages:

pH stabilized right from the start reducing risks for specimens and curatorial efforts

buffering agent and its reaction products not interacting directly with stored specimens, e.g. by forming insoluble deposits on their surface

detected pH deviations easily amended by exchanging exhausted buffering agent instead of exchanging preservation fluid, i. e. without disturbing specimens or furthering additional leaching of lipids etc.

solid ion-exchange material may be regenerated after exhaustion

recognition of problematic jars substantially facilitated by combining the ion-exchange material with pH indicator

Reference: Kotrba, M. and Golbig, K.: A new approach to stabilize the pH in fluid-preserved natural history collections. Submitted to Collection Forum.

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