Why should we avoid preparing a standard solution of ethanedioic acid?

Some lab books (e.g., Cheng & Chow, 1999; Chung, 1997; Fung, 2000) provide students with procedures to prepare a standard solution of ethanedioic acid. The experiment is dangerous because ethanedioic acid (oxalic acid) is a strong poison. Patnaik (1999) has summarized its health hazards as follows:

Oxalic acid is a strong poison. The toxic symptoms from ingestion include vomiting, diarrhea, and severe gastrointestinal disorder, renal damage, shock, convulsions, and coma. Death may result from cardiovascular collapse. ...... .Oxalic acid may be absorbed into the body through skin contact. It is corrosive to the skin and eyes, producing burns. (pp. 86-88)

If the purpose of the practical work is merely to prepare a standard solution (e.g., Cheng & Chow, 1999, p. 89; Chung, 1997, p. 92), then the use of toxic ethanedioic acid is not essential. Chemistry teachers may replace ethanedioic acid by chemicals such as sodium carbonate. If you really want your students to prepare a primary standard of an acid, potassium hydrogen phthalate (C₈H₅O₄K) is much better than ethanedioic acid. It is a monoprotic acid (HOOCC₆H₄COO^- K⁺) with low toxicity. Hong Kong teachers are allowed to buy this acid even though it is not on the list of chemicals compiled by the Education Department. Potassium hydrogen phthalate is a good primary standard because of the following properties (Lainchbury, Stephens & Thompson, 1997):

• High purity
• Stable in air
• Easily soluble in water
• High molar mass
• Complete and rapid acid-base reaction

What a pity that students pour the standard solution into the sink after the experiment! Is there a way to use the standard solution of potassium hydrogen phthalate in acid-base titrations? Yes, certainly! The standard solution of potassium hydrogen phthalate may be used to determine the concentration of an aqueous solution of NaOH. This acid-base titration is suitable for secondary 4 - 7 students. Dry the potassium hydrogen phthalate in an oven before use.



References

Cheng, E. & Chow, J. (1999). Chemistry: A modern view, experiment workbook 1B (4^th ed.). Wilson Welsh.
Chung, L. H. M. (1997). Integrated chemistry today: Experimental workbook 1B (2^nd ed.). Jing Kung.
Fung, C. L. (2000). New way chemistry for Hong Kong advanced level: Practical book 1. Manhattan.
Lainchbury, A., Stephens, J. & Thompson, A. (1997). ILPAC advanced practical chemistry (2^nd ed.). London: John Murray.
Patnaik, P. (1999). A comprehensive guide to the hazardous properties of chemical substances (2^nd ed.). New York: John Wiley & Sons.

---