The Beckman G pH meter was the first commercial instrument to take use extensive electronic components in a chemical instrument. The electronics of the Beckman G are covered in some depth in the Model G's history page, so this page will focus primarily on the glass electrode. The glass electrode was a remarkable device in its own right, particularly after improvements made on the design by Beckman's group at National Technical Laboratories.
Today there are a vast number of types and models of electrodes available. Models exist for almost every temperature range, pressure, and application imaginable, as well as for many other ions besides hydrogen.
Glass electrodes were used in the Beckman Model G because they were impervious to most chemical interferences. Specifically, glass electrodes were impervious to sulfur dioxide, SO2, a common preservative that was used in citrus juices. The electrochemical activity of sulfur dioxide made the common method for pH determination of the time, the hydrogen electrode/ reference electrode cell, useless.
Glass electrodes work by creating a voltage which can be measured to calculate pH. A glass electrode consists of a glass membrane that is placed in a sample. Inside the electrode is a buffer of known pH. Depending on the pH of the sample solution, hydrogen ions will naturally migrate across the glass membrane to the area of lower concentration (in this case,the solution with higher pH). As protons migrate across the membrane, a voltage builds up that can be measured. Because glass has a high electrical resistance, very little current is generated to be measured. (Electronics terms can be confusing if no one has explained them. If you are lost, included below is a "Very Brief, Very Simple Introduction to Electronics.")
To overcome the difficulty of measuring a very small current, previous attempts had tried increasing the surface area of the glass electrode and making the glass smaller. Neither of these techniques worked well for regular use. Arnold Beckman saw the problem differently and recommended a dual-stage amplification system to multiply the current from the glass electrode. Beckman's amplification system used two vacuum tube amplifiers in series. In a dual stage system, the minuscule current is multiplied by the first tube, then that amplified current is multiplied by the second, greatly increasing the size of the current. With Beckman's amplification system, even the smallest current was amplified enough to be detected by an ordinary current meter, or galvanometer, which is exactly what took place in the Model G.
To collect data on a Model G, the operator had to rotate a "pH dial" while watching the galvanometer, or null balance meter. The pH dial altered a current designed to balance the voltage f the current from the electrode while the null balance meter displayed how much current was flowing through the system. When the "null balance meter" on the Model G read zero and was stable, that indicated that the current from the sample exactly canceled out current from the Model G. The current from the Model G was controlled by the pH dial, so when the null-balance meter stopped at zero the pH of the sample was displayed by the reading on the pH dial. Compared to the elaborate processes involved in using a hydrogen electrode, the Model G was amazingly simple and fast.
The next revision of the glass electrode sealed the buffer compartment from the rest of the electrode, eliminating the depth errors that had plagued all commercial electrodes until that point. The introduction of sealed glass electrodes opened a whole new market for Beckman Instruments, and remains one of their strongest product lines.
Very Brief, Very Simple Introduction to Electronics
To use the classic analogy, think of electricity as water. Just as water flows, so must electricity. In flowing water, three characteristics are important: the volume of water, the water pressure, and how much resistance the water has to flow against. In electrical terms, current, measured in amperes, (amps for short) is the volume of water; more current equals more electricity. Continuing the analogy, voltage is water pressure; more volts equals more "potential," basically how hard the electricity is being pushed or wants to push something else. Voltage is measured in volts. Finally, the resistance of water can be thought of as how hard the water must work to go uphill or downhill. Obviously, going uphill equals greater resistance. Electrical resistance, measured in ohms, measures how hard it is for electricity to flow, and increases as it gets more difficult for current to go through a circuit.