The international photographer (Jan-Dec 1937)

PHOTOGRAPHER the hydroxyl ions decrease, and vice versa, so that knowing the hydrogen ion concentration, we likewise know the hydroxyl ion concentration. When solution of a material has a hydrogen ion concentration equal to one ten-millionth of a gram per liter, it is said to be neutral. When solution of a material contains more than one tenmillionth of a gram of hydrogen ions per liter, it is said to be acid, and when it contains less it is said to be alkaline. By definition, then, a substance which furnishes a preponderance of hydrogen ions when placed in solution is called an acid, and a material which furnishes a deficiency of hydrogen ions (corresponding with a preponderance of hydroxyl ions) in solution is called an alkali. The range of hydrogen ion concentrations encountered industrially runs from ten grams per liter (approximately) to less than one hundred-trillionth of a gram per liter, or over a quadrillion fold. In early analytical work, chemists were forced to express acidities in unwieldly terms such as 4.63X103 and 8.7X10-11. Addition, subtraction, and multiplication of such terms was laborious, and the final magnitudes of the values were difficult to grasp and interpret. Hence, introduction of the logarithmic pH system by Sorenson was therefore a welcome contribution, for by its use, we are able to employ a simple series of numbers from 0 to 14 to express a range of hydrogen ion concentration of one hundred trillion fold. Accompanying this article is a chart graphically showing pH values encountered in motion picture processing solutions. The use of the logarithmic pH system for the representation of hydrogen ion concentrations is not only simpler, but it gives a much truer picture of the actual state of affairs. An increment in hydrogen ions of only one tenmillionth of a gram per liter may affect a reaction just as profoundly in the neighborhood of neutrality, as an increment of a tenth of a gram of hydrogen ions will affect a reaction in a solution already strongly acid. Earlier methods for the measurement of acidities were based upon titration. In this procedure, a measured quantity oi the sample whose acidity is to be determined is titrated (balanced) against a standard solution of known strength, using an indicator dye which changes color at the point of equivalence to establish the "end-point," or balance point. Ratio of the volume of the sample to the volume of standard alkali required to balance is an indication of the total acid present. However, it soon became apparent that the total acid present in a solution is not the significant factor. Two solutions having the same total acidity by titration often behave very differently in their actual acid properties. When we drink lem (Laboratory— Cont.) APRIL, 1937—19 pH RMGES IN FILM PROCESSING — U 11 10 • 9 ■Water from J Hollywood main$l NEUTRALITY — Sodium hydroxide developers Sodium carbonate developers > — Borax developers I — Zeolite Softened -waters > — Alum.treated waters NEUTRALITY Hypo solutions Basic dyeSolutions 6 Chemical toning" Solutions D Gelatin, emulsions dry fastest -Sound track, deformation least Emulsions resist abrasion "\ ____# — Iodine bleaches onade we drink a solution containing 10% citric acid and find it palatable, but it is doubtful that any of us would be foolhardy enough to drink 10% hydrochloric acid; yet both solutions contain the same amount of total acid. We know that a hypo solution acidified with the correct proportion of acetic acid remains stable and free from sulphur precipitation. The same hvpo formula, with sulphuric acid substituted for the acetic acid, will soon decompose and deposit sulphur. In both of the above instances it is active acidity, rather than total acidity that is important. When an acid substance is placed in solution, it may yield a greater or lesser number of hydrogen ions for a given amount of total acid. It is the number of hydrogen ions liberated which determines the active acidity, and a substance which furnishes a high active acidity is called a "strong" acid; a substance which furnishes a low active acidity is called a "weak" acid. Listed below are a series of solutions of different acids. All of the solutions have the same total acidity, and from the accompanying hydrogen ion concentrations (active acidities), and the corresponding pH values, may be seen the vast difference in the actual strengths of the acid solutions. Acid H-Ion Concentration pH Hydrochloric 0.08 gms/L. 1.1 Sulfuric 0.063 1.2 Orthophosphoric 0.031 1.5 Citric 0.0063 2.2 Acetic 0.001 3.0 Carbonic 0.0001 4.0 Boric 0.0000063 5.2 Similarly, titration of alkaline solutions to give results in terms of total alkalinity seldom yielded data which could be correlated with the actual ob