It is sometimes considered that a fixing bath has reached its limit of safety when the time of fixation of a given emulsion becomes double that required with a fresh bath at the same temperature. 1 As a matter of fact, this method of testing results in carrying the use of a fix ing bath considerably beyond reasonable limits, and would be more suitable for deciding the exhaustion point of the first bath in the case of two-stage fixation.
More accurate indication is obtained by em ploying a method of direct control suggested by Gaedicke in 1906, and recommended by Lumiere and Seyewetz as being satisfactory. A fixing bath should be considered as exhausted (as a single bath or as the second bath in the two-stage method) when a drop of it, placed on blotting or filter paper, turns brown on exposure for some time to moist air and sunlight.
Lastly, a more direct method (Bayer, 1921) is to withdraw 23- oz. (Ioo c.c.) of the bath and to add to this quantity of solution k oz. (lo c.c.) of a 4 per cent solution of potassium iodide ; the bath may be considered as exhausted, so far as the single-solution method is concerned, when a permanent yellow precipitate is formed.
409. Preparation of Fixing Baths. Workers cannot be too strongly recommended to mix correctly the substances used in the preparation of fixing baths.' These solutions are sufficiently stable to allow of being prepared beforehand in fairly large quantities. In order to reduce the bulk due to large quantities of stock solution it is a simple matter to prepare the actual baths when needed for use from more concentrated solutions. 2 Neutral Fixer. A solution of hyposulphite with out any addition is used Hyposulphrte of soda, crystals . 3 to 6 oz.
A (.50 to 300 grm.) Water,to make . . 20 oz.
This solution may be prepared when required for use by diluting a stock solution of 50 per cent or 6o per cent of hyposulphite of soda.
When the solution is prepared in the same vessel in which it is to be used, the hyposulphite is placed in a muslin bag hung over the vessel or attached to one side. The hyposulphite is sprinkled with a little tepid water in order to hasten its solution and to compensate for the lowering of temperature ; the volume is then made up to the desired amount with cold water.
Stock solutions may be made up with cold water when there is no particular hurry.
Acid Fixers. Any of the following formulae may be employed iHyposulphite of soda, crystals 4 to 6 oz.
(zoo to 300 grna.) Bi Soda bisulphite lye 1 . . i oz.
(50 0.0.) I Water, to make . . . 20 oz.
(1,000 c.c.) 1 Hyposulphite of soda, crystals 4 to 6 oz.
(200 to 300 grin.) 1 C Boric acid, cryst. . . . 075 to 263 grs.
(.0 to 30 grm.) Water, to make . 20 oz.
(0,000 c.c.) 1 Ifyposulphite of soda, cryst. 4 to 6 oz.
(200 to 300 grin.) Sulphite of soda, anhydrous 45 gr.
Di (5 grm.) Acetic acid, glacial 2 . 5o min.
(5 c c.) Water, to make . 20 OZ.
(I,000 c.c.) In preparing solution (B), if the solution of the hyposulphite has been made with warm water it must be allowed to cool before adding the bisulphite.
In the case of solution (C), the boric acid should be dissolved separately in about half the quantity of water, which has been previously warmed. This solution must not be added to the solution of hyposulphite until it has almost completely cooled.
In preparing solution (D), the sulphite of soda should be dissolved in about 3 oz. (Too c.c.) of cold water, and the acetic acid should be diluted about five times. The diluted acid is then slowly added with constant stirring to the solution of sulphite. This mixture is finally added in small quantities at a time to the cold solution of hyposulphite.
Alum Fixers. 3 It must suffice to mention two formulae in which chrome alum and ordinary alum respectively are used- For the preparation of solution (E) the chrome alum may be dissolved in about 3 oz. (Ioo c.c.) of hot water ; after cooling, this solu tion is added to the bisulphite, previously diluted with an equal volume of water ; the mixture is then poured into the solution of hyposulphite.
In order to make up the hardening solution employed in the preparation of solution (F), the alum and the sulphite are dissolved separ ately, the alum in about 12 OZ. (600 c.c.) and the sulphite in about 4 oz. (200 c.c.) of warm water ; the acid is poured into the alum solution. and well shaken, and lastly the solution of sul phite is added and the total volume made up to 20 oz. (1,000 c.c.) with water." The hyposulphite solution and the hardening solution should be cold before they are mixed.