Film Development Washing Water Quality 'Calcium and Magnesium Ion Scale Deposition (Limescale & Water Spots)': Practical Use of EDTA Chelating Agent and Water Softening Resin

Why can't ordinary lens cleaning cloth wipe off the water drop marks on the film after drying?

Tap water evaporation concentration and salt precipitation:
In areas with hard tap water, the total dissolved solids (TDS) of tap water is often as high as 300~600 ppm, and is rich in calcium bicarbonate (\text{Ca(HCO}_3)_2) and magnesium sulfate;
When the film is hung to dry naturally, the water droplets attached to the surface of the film will shrink and gather due to surface tension;
As water molecules evaporate into the dry air, the concentration of mineral salts in the remaining water droplets soars in a geometric progression, eventually reaching a supersaturated state, and extremely tough calcium carbonate (calcite microcrystals) and calcium sulfate crystals crystallize at the outer edge of the water droplets;
Gelatin Polymer Network Embedding:
What’s even more fatal is: the precipitated calcium salt microcrystals are directly embedded with the incompletely hardened animal gelatin polypeptide chains on the surface of the film;
The annular white ring formed after drying is poured on the surface of the film like cement. Dry wiping will not only fail to remove it, but will also directly scratch and draw the emulsion.

To prevent the precipitation of calcium and magnesium ions, the most thorough chemical weapon is Chelating Agents:

Disodium ethylenediaminetetraacetate (EDTA-2Na/EDTA-4Na):
The molecular structure of EDTA is like a crab with six large claws;
When encountering free (\text{Ca}^{2+}) and (\text{Mg}^{2+}) ions in water, the two nitrogen atoms and four carboxyl oxygen atoms of EDTA will tightly wrap the metal ions in the center of the molecule from six spatial directions, forming an extremely stable and completely water-soluble six-dentate chelate ring complex (Chelate Complex);
The inactivated calcium and magnesium ions can no longer combine with carbonate to form a precipitate. When the film comes out of the water, it is crystal clear and has no attachment.
Professional Formulations for Application:
Many world-class pre-packaged developers (e.g. Kodak Xtol, D-76) or fixers are pre-added with 1g/L - 2g/L EDTA, precisely to prevent photographers from creating chemical white fog when developing with hard water.

Using the Ilford water-saving washing method (fill and shake 5 times and drain $\rightarrow$ Fill and shake 10 times and drain $\rightarrow$ Fill and shake 20 times and drain), the thiosulfate fixing residue can be efficiently discharged using normal temperature tap water;

After washing with tap water, never hang it up directly! **
Prepare a bottle of extremely pure medical distilled water or Watsons distilled water (TDS = 0 ppm);
Completely immerse the negative film core in 500ml of pure distilled water and let it soak for 1 minute to completely replace and expel the tap water hard ions adsorbed inside the gelatin;

Add Kodak Photo-Flo 200 or Ilford Ilfotol to distilled water;
Strictly follow the dilution ratio of 1:200 (only 2.5ml is added to 500ml of distilled water, that is, about 2-3 drops);
Serious Warning: Many novices mistakenly believe that the more wetting agent added, the better. Excessive wetting agent will leave a layer of greasy and sticky surfactant residue on the surface of the film, attracting air dust and forming oily fog spots that are difficult to clean!

Remove the film directly from the water bath and hang it vertically in a dust-free drying box or bathroom;
Clamp the weight clamp at the bottom of the film at a 45-degree angle, guiding the remaining water droplets on the surface to quickly gather into a line and slide down to the ground along a thin edge, without leaving a circular water mark in the center of the picture.

Associated Entities & Verification Exits