pH of MS-222 Using Tris

Observation: The water here at the Stock Center in San Marcos, TX, does not buffer the acidifying effects of MS-222 (3-Aminobenzoic Acid Ethyl Ester C₉H₁₁NO₂ * CH₄SO₃). This results in a drastic change in pH of the solution when a higher concentration of MS-222 is desired. This can adversely affect fish health. Also phosphate buffer does not buffer effectively when used in lower concentrations.

Question: Will Tris buffer a 0.06% solution of MS-222 when used in low concentrations and keep the pH around neutral?

Experiment: Make a 1% stock solution of MS-222 using 5g MS-222 salt to 500ml conditioned water (tap water with prescribed amount of Jungle Brand Start Right added.) Test the pH. Also test the pH of the conditioned tap water. Using a 1M stock solution of Tris (pH=8.13) make 250ml of the following dilutions: 500mM, 50mM, 5mM, 2mM and 1mM. Test the pH of each of these concentrations. Then with each concentration make a 0.06% MS-222 solution and test the pH again.

Observations: The pH of 1M Tris was 8.13. The 500 and 50mM dilutions buffered the pH of the 0.06% MS-222 solution above a pH of 8. The 5 and 2mM dilutions kept the pH of the 0.06% MS-222 solution closer to neutral. The 5mM concentration brought the pH of the 0.06% MS-222 solution closest to the initial pH of the conditioned tap water. The 1mM Tris solution buffered the 0.06% MS-222 solution only slightly above that of un-buffered conditioned tap water.

Tris Buffer Concentration (mM)	pH	pH in 0.06% MS-Soln.
1000	8.13	N/A
500	8.14	8.14
50	8.13	8.06
5	8	7.29
2	7.88	6.63
1	7.8	6.33
Tap	7.56	6.18
1% MS-222	1.55	N/A

Conclusions: The 5mM and 2mM dilutions of the Tris buffer seem to be more effective at bringing the pH of the 0.06% MS-222 solution close to neutral than the phosphate buffer. The 5mM dilution seems to work the best keeping the pH just below that of pure conditioned tap water.

Contributed by Leona C. Hazlewood, Steven Kazianis, D. Trono and Ronald Walter. Experiment performed by Leona C. Hazlewood, Xiphophorus Genetic Stock Center.

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