Chemicals should always be mixed in cylindrical containers made of suitable materials, The size of the mixing container should be suitable for the amount of solution to be prepared. A small batch of solution should not be mixed in a large vessel that uses mechanical agitation because large amounts of air may be introduced, and splashing may occur. So, the mixing container, and for that matter, scales and graduates, should be sized to the quantities and volumes of solutions required.

GRADUATES

Graduates are used to measure liquids. Graduates are made in various sizes, calibrations, and of various materials. The units of measure of graduates are calibrated in the U.S. liquid measurement system of ounces, quarts, and gallons, and in the metric liquid measurement system of cubic centimeters, milliliters, and liters.

Glass is most commonly used for making graduates because it is NOT affected by most chemicals. Glass is also transparent and reasonably durable. Graduates are also made from plastic and stainless steel. When using graduates made of plastic, do not try to measure strong acids, such as sulfuric acid, which could cause severe damage. You must also be sure that the material the graduate is made of does not react with any of your photographic chemicals.

For accuracy in measuring liquids, graduates should be proportional in size to the quantity of solution being measured; for example, an 8-ounce graduate should be used instead of a 32-ounce graduate to measure 2 or 3 ounces.

When measuring a liquid in a glass graduate, hold it at eye level and pour the solution into it until the surface of the liquid reaches the correct mark. You will notice a curved surface on the top of the solution. This curved surface is called the meniscus. The correct amount is indicated by the lower of two visible lines of the meniscus (fig. 9-1). These two lines can be seen easily through the side of a glass graduate when it is held correctly. With an opaque graduate, such as stainless steel, the two lines can be seen by looking down into the graduate from an angle. Stop pouring the solution when the lower line of the liquid reaches the calibration mark. Major divisions are indicated by numbers on the graduate. Subdivisions are shown by calibration lines only. You must determine the value of the individual subdivisions; for example, the marked or numbered lines may indicate ounces and read in series of 10. When there is only one calibration line between each graduation of 10, then the value of the calibration line is 5.

THERMOMETERS

All chemical action takes place faster at high temperatures than at low temperatures. In the photographic process, when you mix or use a solution, you must know its temperature.

Thermometers are used to measure the temperature of the solution and may be made of glass or metal. The average glass thermometer consists of a bulb, containing either mercury or colored alcohol, attached to a capillary tube. This tube may be calibrated or it may be secured to a graduated scale. When you are reading a

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Figure 9-1. Read tbe lower line of the meniscus when measuring liquids

thermometer, your eyes should be level with the top of the liquid column in the capillary tube; otherwise, the reading may be off as much as 2 or 3 degrees. This error is due to the refraction of the cylindrical magnifier that is built into the capillary tube.

Most Navy photographic labs have metal, dial type of thermometers made of corrosion-resistant steel. They have a long, thin metal stem, or rod, with a circular dial indicator at the top. The action of this thermometer is remarkably fast, and the dial is easy to read.

The accuracy of all lab thermometers should be checked regularly against one of known accuracy, such as a Kodak process thermometer.