The Electrolyte

The oxidation or reduction attending t h e electrographic solution is brought about entirely b y t h e potential differences maintained a t t h e specimen surface and not b y t h e chemical activity of t h e impregnating solution. This solution, therefore, functions primarily to provide ionic conduction through t h e printing medium a n d t o establish anodic or cathodic conditions conducive t o t h e smooth, efficient transfer of speci­

men material. W i t h these requirements fairly easily satisfied, t h e choice of impregnant is determined largely b y t h e secondary effects desired, such as t h e precipitation of t h e transferred ions as colored insoluble salts or their fixation as colorless, temporarily insoluble products to be t r a n s ­ formed and developed subsequent t o printing.

T h e salt concentration in t h e electrolyte should not be too high.

T h e n t h e desired voltage drop m a y be established without excessive current. Heating and gassing are also avoided and better control of t h e whole process is obtained. Two t e n t h s to five t e n t h s molar solutions produce good prints. I t is convenient t o use equivalent concentrations of t h e various electrolytes. These m a y be interchanged without large changes in resistance a n d t h e circuit will require a minimum of adjustment.

T h e n u m b e r of electrolytes containing color-forming ions suitable for direct electrographic printing is rather limited. Table I lists those

TABLE I

Reagent Ag Cu Cd Zn Pb Bi Fe Ni Co Mn Iodide Lt. yel. — Yel. Yel.

Sulfide Blk. Blk. Yel. Wh. Blk. Blk. Blk. Blk. Blk. Flesh Chromate Red — Yel. Yel. Yel. Yel. — — — — Ferrocyanide Wh. Red Wh. Wh. Wh. — Blue Apple — —

green

Ferricyanide — ^— — ^— — — ^{Blue —} — —

Nitrite Yel. —

which have been used, together with t h e colored products formed with commonly encountered metals. T h e greater n u m b e r of electrographic procedures employ organic and special reagents in conjunction with a general electrolyte.

I n certain cases, when electrolysis is accompanied by formation of an

ELECTROGRAPHY AND ELECTRO-SPOT TESTING 183 insoluble product, precipitation m a y be so rapid a n d t h e product so impervious t h a t t h e specimen surface is quickly coated with a barrier film a n d little or no transfer can occur. Such films m a y be so t h i n as t o be invisible. Lead, for example, is quickly rendered inactive in a sulfate electrolyte and practically no transfer t o t h e paper occurs. Passivation sometimes m a y be employed t o a d v a n t a g e where it is desired t o repress t h e solution of one component of a surface t o t h e enhancement of t h e solution of another. T h u s pinholes in chromium plate on brass m a y be studied advantageously with a phosphoric acid electrolyte which pas-si v a t es t h e chromium a n d favors t h e solution of t h e copper.

W h e n indirect print development is used, it is i m p o r t a n t t h a t diffusion be prevented during t h e interim operations. W i t h gelatin paper, this diffusion is very slow, a n d for most purposes, it can be neglected. P r i n t s m a d e on open textured paper, however, suffer m u c h loss of detail unless the transferred materials are converted immediately t o a nondiffusible product. Although relatively insoluble, such products m u s t be capable of reaction with developing reagents. I t is i m p o r t a n t also t h a t t h e precipitated product be formed within t h e body of t h e paper rather t h a n on the specimen surface, where it would quickly block further solution. For metals, these requirements are generally m e t b y fixation as basic salts, carbonates, or other insoluble salts of weak acids. W i t h such products, precipitation is an easily reversible process, controlled by p H adjustment, and no special problem is created in t h e subsequent development. T h e dependence of precipitation on p H also provides a mechanism by which it can be deferred until t h e ions have reached the desired depth in the printing medium. This is understandable when it is recalled t h a t the region about t h e anode becomes increasingly acidic during electrolysis, while t h e cathodic region grows alkaline. T h e acidic and alkaline zones t e n d t o spread respectively from t h e anode and cathode, so t h a t when a neutral salt is used, t h e p H will v a r y con-tinuously through t h e printing p a d from acidic low values at t h e anode to alkaline high values a t t h e cathode. Basic or weak acid salts cannot be precipitated in the strongly acid anode region, b u t as t h e metal ions move into t h e paper, t h e y will reach a zone where t h e p H is high enough t o permit this precipitation. T h e p H gradient through t h e printing p a d will not be uniform, mainly because of t h e difference in t h e migration rates of hydrogen and hydroxyl ions. T h e p H distribution will depend also on t h e electrolyte a n d on t h e a m o u n t of electrolysis. F o r neutral salts such as sodium nitrate, with t h e average transfer, using #576 a n d

#601 papers, t h e acid zone extends two-thirds t o three-quarters of t h e way through t h e pad. T h u s it is likely t h a t no fixation at all will t a k e place in t h e t h i n printing paper which is used on a thick backing sheet.

PAPER ELECTROLYTE

C.S. & S. #575 0.2M N a N 0⁸ C.S. & S. #601

UNBUFFERED ELECTROLYTE ON UNSIZED PAPER

Excessive diffusion resulting in bad blurring Diffusion extends into backing paper

PAPER ELECTROLYTE

C.S. & S. #575 0.2M N a N 03

C.S. & S. #601 0.5M N a²C 0³

BUFFERED ELECTROLYTE ON UNSIZED PAPER

Fixation of copper by precipitation as basic salt results in clear print detail

PAPER ELECTROLYTE

Ε. K. Co. Imbibition (gela- 0.2M N a N 03

tin) paper

UNBUFFERED ELECTROLYTE ON GELATIN COATED PAPER

This print shows marked retarding of diffusion by gelatin medium.

FIG. 9. Developed ferrocyanide electrographs of bronze coin showing control of diffusion by buffering the electrolyte and using gelatin printing medium. Prints developed in 5% potassium ferrocyanide solution.

184

ELECTROGRAPHY AND ELECTRO-SPOT TESTING 185

Metal ions will pass t h r o u g h t h e p a d until t h e y reach t h e deep alkaline layers of t h e backing paper, close t o t h e cathode, and some lateral dif­

fusion m a y t a k e place on t h e way. T h e r e t h e y will precipitate as basic nitrates or hydroxides. If, on t h e other h a n d , t h e electrolyte is buffered with sodium carbonate, extension of t h e acid zone into t h e paper can occur only as t h e buffer is used u p . B y experimental adjustment of t h e concentration of t h e buffer, t h e precipitation can be m a d e t o occur at t h e desired depth in t h e printing paper. F o r a C.S. & S. #576 a n d #601 paper pad a n d a 30-second printing, 0.5 Μ sodium carbonate with 0.1 Μ sodium nitrate or chloride provides satisfactory solution a n d fixation for most metals. Fig. 9 shows prints m a d e with buffered a n d unbuffered electrolytes as well as in gelatin paper.

Solution of silver, copper, a n d lead is best obtained with t h e nitrate ion, while iron, aluminum, a n d nickel sometimes show passive tendencies with nitrate a n d dissolve b e t t e r in t h e presence of t h e chloride ion.

Amphoteric metals such as bismuth, antimony, a n d tin, whose salts hydrolyze readily, require a strongly acid condition a t t h e anode. T h e y react best t o chloride ion with little or no buffer.