Dilute acid favors the reduction of oxyluciferin. If one saturates an oxyluciferin solution with CO_{2} or adds a little dilute acetic acid, HCl, HNO_{3} or H_{2}SO_{4}, to it, a certain amount of reduction will occur. No reduction occurs if the solution is saturated with pure hydrogen, even if allowed to stand 24 hours. The action of the acid begins when the solution of oxyluciferin, ordinarily slightly alkaline (PH = 9), is made neutral (PH = 7.1) as indicated in Table 9. The action of the acid must be on the oxyluciferin, as no luciferin or other enzymes destroyed on boiling are present. TABLE 9 _Effect of Acid on Reduction of Oxyluciferin_ ============================================================================= Solution | P_{H}|Luminescence| Remarks | | with | | | luciferase | -------------------------------+------+------------+------------------------- 20 c.c. Oxyluciferin alone | 9.01 | Negative | 20 c.c. Oxyluciferin + .05 c.c.| 8.8 | Negative | 5 per cent. acetic acid | | | 20 c.c. Oxyluciferin + .15 c.c.| 7.1 | Fair | 5 per cent. acetic acid | | | 20 c.c. Oxyluciferin + .30 c.c.| 5.9 | Good |Acid forms precipitate in 5 per cent. acetic acid | | | this oxyluciferin sol. 20 c.c. Oxyluciferin + .50 c.c.| | Good |Acid forms precipitate in 5 per cent. acetic acid | | | this oxyluciferin sol. 20 c.c. Oxyluciferin + .75 c.c.| | Good [10] |Acid forms precipitate in 5 per cent. acetic acid | | | this oxyluciferin sol. -------------------------------+------+------------+------------------------- [10] Light disappears quickly because of the effect of the acidity on the luciferase.