A simple titration (oxidimetry) method using a methylene blue-platinum colloid reagent is effective in determining the concentration of hydrogen gas in an aqueous solution. The method performs as effectively as the more complex and expensive electrochemical method.
Keywords: Hydrogen gas, hydrogen water
Molecular hydrogen is useful for various novel medical and therapeutic applications. In air, hydrogen gas is potentially explosive, whereas, in an aqueous solution, it is safe and convenient to use. Recent biomedical studies have shown that hydrogen is a physiological regulatory factor that has antioxidant, anti-inflammatory, and antiapoptotic protective effects on cells and organs [1-5]. As a result, several aqueous solutions of hydrogen have been developed for use in medical applications as well as in health drinks.
A method for determining the concentration of hydrogen in water is very desirable, particularly if it is simpler and more inexpensive than the current state-of-the-art method involving expensive electrochemical gas sensors. Accordingly, we investigated a simple oxidimetry method that involves a redox reaction of methylene blue (MB) oxidant in the presence of a colloidal platinum (Pt) catalyst. MB is well known to react with an equimolar amount of hydrogen in the presence of Pt or palladium to produce colorless reduced MB (leucomethylene blue, leucoMB), as follows:
MB (blue) + 2H+ + 2e- → leucoMB (colorless)
On the basis of the above equation, we used a volumetric analysis method called oxidimetry to determine the concentration of hydrogen in water by a titration by using the MB-Pt reagent (Figure (Figure1).
Conceptual illustration of the reaction between H2 and MB-Pt. 1 mole of hydrogen molecules reacts with 1 mole of methylene blue (MB) molecules to give 1 mole of reduced MB (leucomethylene blue, leucoMB) molecules.
Preparation of MB-Pt reagent
MB (0.3 g) (Waldeck-Gmbh & Co KG, Munster, Germany) was dissolved in 98% ethanol (98.9 g) to give a solution of MB (99.2 g) in ethanol. An aqueous suspension of 2% colloidal Pt (0.8 g) (Tanaka Kikinzoku Group Company) was added to the solution and the mixture was stirred to give 100 g of MB-Pt reagent (MiZ Company, Kanagawa, Japan). The reagent was distributed in small plastic bottles, from each of which one drop of the reagent (17 mg or 0.02 mL) was drawn.
Preparation of hydrogen-rich water samples
Hydrogen-saturated water (0.8 mM) was prepared by bubbling hydrogen gas through purified water. Three concentrations of hydrogen-rich water (0.3, 0.2, and 0.1 mM) were prepared by diluting hydrogen-saturated water with purified water.
Determination of hydrogen concentrations
Electrochemical determination of the hydrogen concentration was performed using an electrochemical gas sensor (model DHD1-1, DKK-TOA Corporation, Tokyo, Japan).
Oxidimetry determination of the hydrogen concentration was performed by a redox titration. The MB-Pt reagent was added dropwise to 20-mL samples of hydrogen-rich water until the solution changed from blue to colorless (Figure (Figure2).
H2 reduces MB (blue) to leucoMB (colorless). Early during the titration (MB-Pt reagent dropped into hydrogen-rich water), the solution contains more hydrogen than MB. Hence, MB is reduced to leucoMB and the solution is colorless. At the titration endpoint, the solution contains more MB than hydrogen; hence, it turns blue. 1. Drop an MB-Pt into hydrogen-rich water. 2. Hydrogen-rich water is colored by the blue of MB-Pt. 3. Immediately, blue turns colorless by hydrogen.
One drop of the MB-Pt reagent provides 17 mg of the reagent. When the reagent is added to a hydrogen-rich water sample, the molar concentration of hydrogen in a sample is determined as follows:
Moles of H2 per sample = 0.017 × (0.3/100)/319.85 × (number of drops consumed by titration)
From this relationship, we infer that one drop (17 mg) of the reagent contains 0.16 μmol of MB. If the MB-Pt reagent is added dropwise to 20 mL of hydrogen-saturated water (0.8 mmol/L), approximately 100 drops of the reagent are required to reach the titration endpoint (when the solution turns from blue to colorless), as follows:
0.8 mmol/(1000 mL/20 mL)=16μ mol16μmol/0.16μmol/drop =100 drops
However, when we added the MB-Pt reagent to 20 mL of hydrogen water (0.8 mmol/L), only 55 drops of the reagent were required to reach the titration endpoint. The difference between the calculated and actual values is attributed to the evaporation of hydrogen during the measurement time and the quantity difference between MB and hydrogen in water. Thus, we estimate that one drop of the MB-Pt reagent in 20 mL of hydrogen water is actually reduced by 0.29 μmol of hydrogen, as follows:
16μmol/55 drops = 0.29μmol/drop
In other words, if 20 ml of hydrogen water reduces one drop of the MB-Pt reagent, the concentration of dissolved hydrogen (DH) is 14.5 μmol/L or 0.03 mg/L, as follows:
0.29 μmol/drop × (1000 mL/20 mL) = 14.5 μmol/Lor 0.03 mg/L
Similarly, if 20 ml of hydrogen water reduces three drops of the MB-Pt reagent, the concentration of DH is 43.6 μmol/L or 0.09 mg/L.