The equipment and methods used presently for mixing photographic solutions are the result of changes that have taken place over many years. These improvements are made to keep pace with improved sensitized materials, processing equipment, and environmental regulations.

Substandard solutions make it impossible to operate a processing system within controlled limits. The preparation and use of photographic solutions must be a systematic procedure based on accurate measurement and standardized methods. High-quality standards can be achieved only when the conditions of exposure, chemistry, and processing are all within operating tolerances. Then results are predictable.

In certain critical processing areas, chemical analysis is used to check solutions before and during use. This helps us to keep chemicals up to standard. Analysis of the components of processing solutions is often used as a basis for discarding solutions or adjusting their composition for future use. These procedures are used to determine proper replenishment rates to promote the efficient use of chemicals. Quality assurance procedures are implemented, so solutions are mixed, used, and discarded properly.

Chemical Analysis

Much of the chemical analysis performed is accomplished by using common sense. Practices, such as cleanliness, accuracy, and proper preparation of solutions, should be observed at all times. Contamination can ruin a developer which, in turn, could ruin your images. Things, such as dirt and dust, in the solution can cause artifacts on the film.

Before equipment is used for mixing and measuring chemicals, you should clean it thoroughly. Use a good-quality medium-strength detergent to wash glassware and other vessels. Be sure that all washed items are rinsed well with clean tap water. Keep the entire area clean to prevent contamination and ensure accuracy.

Some of the more common chemical analysis methods you will use are as follows:

Determining pH

Measuring specific gravity

Sensitometric testing

Although there are several reasons for certifying photographic solutions, the principal objective is to get desirable, predictable, and consistent results.

Control of a photographic process requires that the chemical activity of the solutions be maintained. Chemical analysis procedures used to indicate chemical activity are valuable only when the samples analyzed are representative of the processing solutions. The analysis of a sample taken improperly may do more harm than good in controlling the process.

Processing solutions may be clouded or contain floating particles, dispersed oil droplets, or a precipitate. Even a clear solution may have different concentration levels in various parts of a processing tank; for instance, the chemical composition at the point where the film enters may not be the same as in other parts of the tank. To compensate for this, you must use a prescribed procedure, so all the samples are taken from the same location, such as from a point near the tank overflow. These samples should be representative of the solution at an established sampling point, so successive analyses will indicate variations in the chemical composition.

Before sampling large batches of newly mixed photographic solutions, allow sufficient time for all the chemicals to dissolve properly, generally about 30 minutes to an hour.

You should draw a sample 1 inch below the surface of the solution with a pipet. In general, a sample bottle should not be shaken, and it should be allowed to stand for 10 minutes after the sample is taken from the processing solution. This wait allows large particles to settle or turbidity (caused by aeration) to clear.

Chemical Certification

The chemical certification of processing solutions seldom requires a complete chemical analysis. Ordinarily, determining the pH and the specific gravity of the solution is enough, particularly when these tests are followed up with valid sensitometric tests.

CERTIFYING THE pH OF SOLUTIONS.-pH is one of the first tests made of a photographic solution. The pH of a solution is the negative logarithm of the hydrogen ion concentration. The electrical potential between the glass electrode of a pH meter and the solution surrounding it indicates the hydrogen ion concentration.

The pH value of a solution changes with temperature; therefore, to obtain a reproducible pH value, you must standardize the temperature. Generally, you make pH measurements at the same temperature that the photographic process is operated. A change of 10F produces a pH change of approximately 0.10 in a processing solution at a pH of 10.5 and a change of approximately 0.20 at a pH of 13.0. Thus temperature control is more important at higher pH values. To properly use, calibrate, and adjust the meter consult the instruction manual or the particular pH meter you are using.

When certifying the pH of a solution, you can take the pH reading of only one sample or you can read several samples. Of course, the more samples you read, the more reliable your certification. Usually, you must determine the pH value of more than one sample. After four samples have been read, the meter should be cross-checked, then standardized.

Also, remember that no more than 15 minutes should elapse between any pH meter standardization or cross-check.

When using the multiple sample procedure, take the pH for each sample and average the values. Check the average value against the standard pH for that solution. When the pH is within limits, enter the value in the appropriate location on the certification sheet. When the pH is not within limits, further investigation is necessary.

CERTIFYING THE SPECIFIC GRAVITY OF SOLUTIONS.-Many applications of specific gravity are used in certifying a solution; for example, the actual dilution of a developer is obtained by comparing a fresh developer to a used one. Another application may be to compare a fresh fixing bath to a used one. Since the silver content of a fixer increases with use, it is logical to assume that a change in specific gravity will occur. Through experience, you will be able to establish upper- and lower-control limits for specific gravity of the various solutions needing such a check.

Specific gravity tolerances are provided by the manufacturer of photographic processing solutions. The specific gravity must be taken at the temperature recommended by the manufacturer, because temperature affects the specific gravity of a solution. Specific gravity standards for black-and-white processing solutions are set at 70F, because many of these solutions are used in 70F surroundings.

By themselves, the specific gravity readings you take are not enough to tell what is wrong. They simply indicate a change. So, further testing is required. When a developer is being replenished properly, specific gravity remains constant. If the flow of replenisher stops, a change of specific gravity will become evident. Replenisher flowmeters should indicate such a problem, but a backup check of specific gravity and pH, along with control strips, are recommended as well.

SENSITOMETRIC CERTIFICATION OF SOLUTIONS.-Once the developer has been mixed, a sensitometric strip should be processed to check the developer solution. Assuming the pH and specific gravity measurements of the developer are within tolerance, this test can certainly validate the activity of the developer.

When you are using a new developing solution, time-gamma or time-contrast index charts (as appropriate) must be developed for the variety of films to be processed in it. Given time, temperature, and agitation for each particular film in the developer, you will be able to see the results on these time charts.

After the developer solution has been proven, you should process a sensitometric strip with each processing run. Each sensitometric strip should be read with a densitometer and plotted on the appropriate process-monitoring chart. As a minimum, you should process and plot a control strip after start-up and before shut down or at the beginning and end of each work shift. Once the densities are recorded and plotted, an accurate graphic representation of the activity of the process is provided.