NOTES ON A PRESSURIZED SYSTEM FOR PRODUCING MAGNESIUM BICARBONATE SOLUTIONS
DESPITE THE ADVANTAGES in rate provided by a pressurized system, it was not possible with our equipment to make a magnesium bicarbonate solution clear enough for immediate use, even at concentrations lower than our target of 0.1 M. The persistent cloudiness resulted, perhaps, from some process such as the formation of magnesium carbonate within the system. It was therefore necessary to add an intermediate step—a settling tank—between the preparation of the solution and its use on paper objects. Settling is common to most bulk methods of producing deacidification solutions, and those which start with an excess of magnesium carbonate generate large quantities of sediment. The amount produced by our solutions is much smaller.
We chose a 25-gallon polyethylene settling tank, so that, when demand warrants, three batches of solution can be produced in succession and allowed to settle at the same time. Using 180 grams of magnesium hydroxide powder (slightly more than required) each batch reaches the desired concentration of 0.1 M in ninety minutes. The settling time necessary to eliminate cloudiness varies; two to four hours is typical. The clear solution is then drawn from the tank's spigot, located above the sediment line, and transferred to polyethylene jerry-jugs for storage and use.
Exhaustive studies of the deterioration of magnesium bicarbonate solutions over time are not available. National Archives data10 suggest that solutions in the 0.1 range decrease in concentration less than 1% per day over the first four days. This decrease is usually accompanied by precipitation at the solution-air interface and the solution-solid interface (if the solution stands in contact with the solid from which it is made). Our observation in practice is that surface precipitation first occurs from two to five days after the solution is made. When precipitation is recognized, the solution is thrown out—a rare occurrence at NEDCC due to the volume used. A second titration of the clarified solution is performed when its concentration is in doubt.
Storage in jerry-jugs is consistent with the principle that magnesium bicarbonate solutions should not stand in contact with the solids from which they are made, or with large volumes of air. It has the additional advantage of avoiding the deposition of metal ions which can occur in stainless steel tanks.
The addition of a settling tank was the only modification to our equipment judged cost-effective. For the design of new pressurized systems, however, the following areas might be profitably investigated:
- use of a high-torque, low r.p.m. electric motor to drive the stirrer, which should be quieter and more efficient;
- maximized dispersion of carbon dioxide with a diffuser;
- optimized balance between gas pressure and volume of bubbled gas. Vented gas might be recovered and returned to the input line via a closed loop;
- use of chilled water—perhaps cooled with dry ice;
- use of higher pressures than those provided merely by gas cylinders. Cost of heavier tanks and fittings and safety of personnel must be taken into account.
Many people at the Northeast Document Conservation Center contributed suggestions to this project, particularly Sherelyn Ogden, for whose advice and support I am very grateful.