October 15, 1931
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Thiosulfate Titrations of Small Amounts of Iron in Glass Sands' Lewis B. Skinner FIRSTNATIONAL BANKBUILDING, DENVER,COLO.
A description of a rapid and accurate method for N T R Y I N G various developing. B o r a x (19 iron determination, using ordinary laboratory equipprocedures for the comgrams of NazBz07.10Hz0 per ment, is given. mercial removal of iron liter) p r o t e c t s the solution, Sand low in alumina is run to dryness with hydrofrom glass sand, it was found but requires too much time to fluoric acid in a silver dish; the residue is fused a few that published methods for discharge eventually the color moments with sodium hydroxide and chlorate; the the d e t e r m i n a t i o n of iron which returns a number of melt is disintegrated by water and transferred to a oxide on this material (1, 3) times. Benzoate of soda (7 beaker; the solution is acidified by hydrochloric acid and any g r a v i m e t r i c ones grams per liter) permits of and a little nitric and boiled; ferric hydroxide is prewhich could be devised were splendid titrations, but does cipitated by an excess of ammonia, filtered off, and dist o o t e d i o u s , so it was denot protect the thiosulfate solved in a little dilute hydrochloric acid; the solution s o l u t i o n effectively. Boric cided t o a t t e m p t to work is evaporated to a small volume, potassium iodide and up a v o l u m e t r i c s c h e m e acid (10 grams per liter) prostarch are added, and it is titrated by a standard thionot demanding complicated tects the solution, but causes sulfate. e q u i p m e n t a n d manipulalong-drawn-out end points. When alumina is high relatively, sodium chlorate is tions. All investigatory work Benzoic acid (2.5 grams per omitted in the fusion, the melt is disintegrated in done will not be detailed for liter) furnishes rapid and acwater, sodium sulfide is added, the sulfide of iron is the list of experiments would curate end points and prefiltered off, dissolved, and oxidized, and ferric hydroxide be too long, but some items serves the solution fairly well is precipitated by ammonia and is treated further as will be noted. and may be used. Boric acid shown above. Numerous t i t r a t i o n s a t (5 grams per liter) protects various temperatures by stanthe solution better than bennous chloride, using pitassium thiocyanate as an internal in- zoic acid, but is a little more tedious in securing final end dicator in a ferric chloride solution, were made. The action points. Lundell, in a private communication to the author, on small amounts of ferric chloride by cold stannous chloride suggested sterilizing. This was done by boiling a solution proved too sluggish and indefinite, so boiling throughout the of the desired quantity of thiosulfate for about 1 hour, coolperiod of titration was done. This affected the thiocyanate ing, diluting to the required volume, using freshly boiled water, and putting this into a glass stoppered bottle which so as to provide widely varying end points. Attempts to compare quantitatively the colors produced had been treated by hydrochloric acid and washed by boiled by ferric sulfocyanate in mixtures of amylic alcohol and ether water. This solution seems to keep quite well without the were not successful on material of this kind, owing to the necessity for special precautions in handling. extraordinary care required to avoid contamination, the deliTo determine the effect of the presence of variable amounts cate manipulation found necessary, and the magnification by of hydrochloric acid, titrations were made on weak acidified the dilutions required of variations in small-quantity weigh- ferric chloride solution by a thiosulfate solution, with the ings, separations, and measurings. results as shown in Table I. Portions of weak ferric chloride solutions containing free Table I-Effect of Variable Amounts of Hydrochloric Acid in hydrochloric acid, to each of which 150 mg. of potassium Protecting Thiosulfate Solution STD. . THIOSU~.- ._._ ~ iodide were added, were titrated cold by a weak standard soluFATE REQUIRED tion of sodium thiosulfate, adding starch solution near the HC1 (SP.OR. STD.THIOSULFOR HC1 STD.FeCla 1.18) FATE WITHOUT FeCla finish. Although this amount of iodine would seem excessive, TAKEN PRESENT REQUIRED ADDED it was found to be insufficient for rapid titrations, and thereMl. MI. Ml. MI. fore trials of various greater quantities were made. Excesses over 600 mg. for each titration did not prove beneficial. However, provided sufficient potassium iodide is added, additional amounts do not affect final end points. Varying the volumes of solutions being titrated by greater or lesser amounts of the standard thiosulfate, due to higher I n these titrations, the greater the amount of acid present and lower irons, likewise does not affect titrations, except to the longer the time required to reach an end point eventually. prolong the time of titration a t greater dilutions. Upon allowing a weak unprotected thiosulfate solution to The larger quantities of hydrochloric acid added provide stand, it was found that it decomposed quite rapidly, requiring more straw color a t the finish. Since there is a necessity for having some free hydrochloric almost daily standardization. Neither sodium hydroxide nor sodium carbonate proved effective as preventatives. acid present, it was decided to add 1 ml. (1.18 grams) for all Yoshida (4) recommends the use of either carbon di- subsequent titrations, the varying amounts of iron in unknown sulfide, disodium phosphate, or borax for sterilizing this solu- determinations taking up such a small proportion of the tion against bacterial action which causes its decomposition 1 ml. as to be of no moment. Since blanks will be run, the effect of the hydrochloric used will be accounted for anyway. into sulfate and sulfur. Carbon disulfide does not protect the weak thiosulfate Preparation of Solutions Used solution required. Disodium phosphate (7 grams of NanPOTASSIUM IODIDE SOLUTION-Dissolve 120 grams of HPO, per liter) gives an end point which is far too slow in potassium iodide in water and dilute t o 200 ml. 1 Received March 31, 1931.
I
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ANALYTICAL EDITION
STARCHSOLUTION-LOW (9) recommends the following: Make a cold saturated solution of commercial sodium chloride in distilled water and filter it. To 500 ml. of this solution add 100 ml. of glacial acetic acid and 3 grams of starch. Mix cold. Boil until nearly clear, about 2 minutes. Add a little water to replace that lost by boiling, perhaps 25 ml. A true solution of all the starch is thus obtained. No filtering or settling is required, and the solution may be cooled and used a t once. It keeps indefinitely and gives sharper end points than the ordinary starch liquor.
Believing it advisable to adhere to standardized conditions, it is felt that pipetting 1 ml. of this solution into each determination is preferred to using varied quantities of starch solution prepared from time to time. STANDARD THIOSULFATE SOLUTION-Dissolve 1.55 grams of c. P. sodium thiosulfate (Na&3,0,.5H,O) crystals in distilled water and dilute to 1 liter, preserving the solution by sterilizing as described. To standardize, accurately weigh on a gold button balance sensitive to 0.01 mg., portions of 15 to 16 mg. of thoroughly cleaned iron wire into 150-ml. beakers. An analytical balance does not weigh closely enough to depend upon two or three 15- to 16-mg. portions for standardizing, so an accurately measured 10-inch (25.4-cm.) length of No. 30 gage iron wire may be weighed thereon and be cut precisely into five parts 2 inches (5.08 cm.) long and each treated as noted in the following. This will prove more accurate usually than taking aliquot parts of a dilute solution of iron chloride. The portions will not check so closely between themselves as will those weighed on a button balance, but should be closer than those from separate weighings on an analytical scale, even though done accumulatively. Cover each beaker with a watchglass; dissolve in 2 or 3 ml. of 1 to 3 nitric acid; boil out nitrous fumes; add a coupIe of small crystals of chlorate to destroy carbonaceous matter; continue heating a minute; dilute to about 100 ml.; add ammonia in excess; bring to a boil; filter on a 9-em. paper; wash the precipitate five times by hot water to remove nitrates; discard the filtrate; remove the bulk of the ferric hydroxide from the paper into a 180-ml. “copper” flask by a wash-bottle jet; place the flask beneath the funnel and paper formerly containing the precipitate; dissolve the hydroxide remaining on the paper by means of a hot mixture of exactly 1 ml. of strong hydrochloric acid and 5 to 10 ml. of water; and wash the paper free from ferric chloride. Look for iron behind the fold. Avoid allowing the ferric nitrate solution to go to dryness and thereby decompose the compound (shown by red nitrous fumes), but should this happen, take up with nitric acid, (not hydrochloric acid), and note whether or not all the residue is dissolved. Swirl the contents of the copper flask so as to wash down all the hydroxide into the solution, and boil down to about 10 ml., this concentration being advisable for taking specks into solution. Sometimes such baked-on specks escape observation and are only noted when titrations are approaching completion, in which case, pour the solution into another flask, add 0.1 to 0.2 ml. of strong hydrochloric acid to the original flask, run the few drops of acid around the wet surfaces of the glass onto the spots while heating gently, return the original solution and complete the titration. Cool the flask and contents, wash down the sides, pipet 1 ml. of potassium iodide solution into the liquid, and, to avoid air-oxidation, pass carbon dioxide gas from a limestone and commercial muriatic acid generator through a sodium bicarbonate solution wash bottle at a qinimum rate of four bubbles per second. The glass tube conveying the gas into the titration flask
Vol. 3, No. 4
should be bent a t the upper portion to hook on the rim and a t the lower portion to conform to the inner walI. It should terminate below the bulge of the flask. The gas is so heavy that it is not necessary to stopper the flask or to pass the gas through the solution in it while titrating. While the contents of the flask are being swirled, deliver the standard thiosulfate solution from a 50-ml. buret until the yellow color is almost discharged, pipet 1ml. of starch solution into the flask, wash down the sides, and continue the titration until the solution is colorless. The color may return several times when handling relatively high irons, but should be discharged from time to time by further additions of drops of thiosulfate, or until the solution remains colorless for some 15 minutes. Since the solution is protected from oxidation by carbon dioxide gas, it makes little difference how long a time is taken to remove all color eventually. Ground glass stoppered burets are not convenient and are liable to breakage, it being advisable to have the buret tip deliver below the neck of the flask. Clampson rubber tubing should not be used, because of variations of pressure on the rubber and possible changes of position. Glass rod plugs, about 6 / ~ a inch (0.79 cm.) long, inserted in the rubber coupling between the buret and the glass tip, work admirably and accurately when not shifted by squeezing to permit flow of the liquid. Titrations should be conducted over white paper or other white surfaces and be placed between the observer and a window. The colors produced by iodide of starch range, as free iodine is removed, from greenish black to blue, to purple, to pink, and to straw. When the straw stage is reached and the titration is allowed to stand for a minute or so, the color usually changes back to pink. I n any event, a reading should be taken a t this point and a drop or so of the solution added to determine whether the straw-colored solution may be bleached to one which is colorless. For a blank, place the same volumes of water, hydrochloric acid, and potassium iodide and starch solutions in a flask, and discharge the color by the thiosulfate. Between 0.1 and 0.2 ml. may be required, which may be deducted from the preceding. Multiply the iron taken by its percentage of purity and by 1.4298, and divide by the net milliliters used. The thiosulfate solution should be equivalent to about 0.0005 gram of Fez03 per milliliter. Pinkish or bluish colored flasks tend to mislead an analyst and should not be used. SODIUMHYDROXIDE SOLUTION-Dissolve 500 grams of fresh, dry, fused c. P. caustic soda (NaOH) in distilled water and dilute to 1 liter. Do not store this in unprotected glass bottles since it will attack the glass. Ceresin-coated glass bottles are recommended. SODIUMSULFIDESOLUTION-Dissolve 200 grams of fresh c. P. sodium sulfide (NazS.9Hz0) in distilled water and dilute to 1liter. SODIUM SULFIDE WASHWATER-DiSS01Ve 5 grams of Na&9Hz0 in 1 liter of water. 50 grams of SODIUMCHLORATESoLuTIoN-Dissolve NaC103 in 1 liter of water. Method for Sands Low in Alumina
(Sands containing approximately 0.25 per cent aluminum oxide.) Blanks should be run, measuring the liquid constituents carefully and weighing the solids. The aggregate of the iron in the reagents used is sufficient to vitiate results. (a) Weigh 10 grams of 100-mesh glass sand into a 250-ml. silver dish.
October 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
(b) Add 75 ml. of 48 per cent c. P. hydrofluoric acid (no sulfuric), and evaporate to dryness. (c) Cool, add 10 ml. of sodium hydroxide solution and 1ml. of sodium chlorate solution, run the mixture around the sides of the dish, and evaporate to dryness. (d) Heat over a flame to incipient redness in order to cause quiet fusion, and run the mass slowly around the sides of the dish while still being heated. Do not stop heating when effervescence, due to dehydration, ceases. ( e ) Cool, add about 100 ml. of hot water, warm gently a minute or so to disintegrate the mass, transfer to a 250-ml. beaker, scrub the dish, and then remove stains by rubbing it with a few drops of hydrochloric acid. (f) Carefully add 10 ml. of hydrochloric acid and 1 ml. of nitric acid, bring to a boil, and allow to boil awhile until the iron hydroxide is dissolved. Cool somewhat, add ammonia in excess, again bring to a boil, allow to settle a few minutes, filter through a common 9-cm. paper, wash five times by hot water, and discard the filtrate. (9) By means of a wash-bottle jet, transfer the bulk of the precipitate from the paper into the beaker in which the precipitation was made, place the beaker beneath the funnel, and slowly pour around the paper a boiling mixture of 5 to 10 m1. of water and exactly 1 ml. of hydrochloric acid. As may be told by the color, this will dissolve the iron hydroxide, but may leave other constituents. However, do not be misled by a yellowish residue of titanium peroxide (TiOa) which is sometimes formed. Remove the beaker and substitute a 180-ml. copper flask beneath the funnel to catch any drops from the funnel stem. It is not necessary to wash the paper. (h) Bring the contents of the beaker to a boil, continue boiling a few minutes until the iron is dissolved completely, running the solution up the sides of the beaker, filter through the paper previously used into the copper flask, wash five times by hot water, discard the precipitate, boil the contents of the flask down to about 10 ml., run this liquid up on the sides of the flask to dissolve dried-out specks of iron chloride, cool, wash down the sides, add 1 ml. of potassium iodide solution, and titrate in an atmosphere of carbon dioxide by the standard thiosulfate solution, as described for the standardization by iron wire. Deduct the number of milliliters used by a blank, multiply the Fez03 value in grams per milliliter by the number of milliliters used, and, since a 10-gram charge was used, move the decimal point one place to the right for per cent of FeaOa. Method for Sands Relatively High in Alumina
For sands containing several per cent of alumina, follow paragraphs a, b, and c, with the exception that the 10 ml. of sodium hydroxide solution recommended in (c), being insufficient, should be increased possibly to as much as 20 ml. to provide a liquid fusion. The addition of 1 ml. of sodium chlorate is omitted, there being no necessity for oxidizing carbonaceous matter and no wishing for converting ferrous iron to ferric, since a sodium sulfide separation is to be made. Continue as in paragraphs (d) and (e) and then pass to (i). (i) To avoid subsequent tearing of the filter paper by the excessive causticity required to make a liquid fusion, neutralize a part of the alkalinity by a few milliliters of hydrochloric acid, and then to the still alkaline solution add 10 ml. of sodium sulfide solution. Bring to a boil and permit to stand until the supernatant liquor is yellowish, instead of greenish. Preferably, allow this precipitation to stand overnight because of the slowness with which it becomes complete. ( j ) Filter, using a 9-cm. common paper, and wash several times by warm, sodium sulfide water. The filtrate should be yellow. Any greenish color should cause suspicion that some iron has passed into the filtrate. Discard the filtrate.
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( k ) By means of a fine jet, wash the bulk of the precipitate from the paper into the beaker in which the precipitation was made, and place it beneath the funnel. The black coloration left on the paper should be dissolved by a boiling mixture of 3 ml. of strong hydrochloric acid and 7 ml. of water poured around the edges and onto the dark spots of the paper. The iron sulfide will be dissolved even though a t times there may be some black silver sulfide coloration left, which may be ignored. Wash the paper in hot water. (1) Boil the solution until there is no odor of hydrogen sulfide, then add 1 ml. of nitric acid and evaporate to dryness to dehydrate silicic acid. Cool, add 5 ml. of water, 5 ml. of hydrochloric acid, and 1 ml. of nitric acid, bring to a boil, dilute to 100 ml., boil again, filter through a common 9-cm. paper, wash, breaking up the coagulated particles of silica, and discard the precipitate. (m) Add an excess of ammonia to the filtrate, bring to a boil, allow to settle, and collect the ferric hydroxide on a common 9-cm. paper, washing by hot water. Discard the filtrate. (n) Wash the precipitate from the paper into a copper flask and proceed as described before in preparation for titrating. Comments (a) The silver dishes may be 4 inches (10.16 cm.) wide and 2
inches (5.08 cm.) deep with rather flat bottoms. The metal may be about l/32 inch (0.079 cm.) thick. They weigh about 4 troy ounces (0.124 kg.) each. Platinum should not be used for the sodium hydroxide fusion. Pure silver is unacted upon by hydrofluoric acid and only slightly by dilute hydrochloric under the conditions prescribed, so the dishes will last some time. Ordinary or sterling silver, containing copper, should not be used, since any dissolved copper may precipitate in part with the iron hydroxide and raise the titration. The U. S. Government Mints a t Philadelphia, Denver, and San Francisco, and the New York Assay Office produce electrolytic silver by the Moebius process, having a fineness of from 999.5 to 999.75 and containing no iron. Some manufacturing jewelers and silversmiths are able to secure such material from the mints and provide sheets for those who wish them. Manufacturers of electric fixtures may form these sheets into dishes. Unless the latter are familiar with the working of silver, they should be informed that silver should be annealed from time to time while being worked. This may be done by heating to low redness, say by a gasoline torch. These are relatively inexpensive. Chemical supply houses may be able to supply them, but the author has not found them stocked. ( b ) Hydrofluoric acid, c. P. grade, is obtainable only in 1-pound (0.45-kg.) ceresin bottles. If a number of determinations are to be made, it is advisable to pour the contents of several into a large glass pitcher or some similar vessel which has been thoroughly dried and then coated with melted ceresin, the material of which the hydrofluoric acid bottle itself is made. Pour the melted wax into the vessel, carefully heat it, and allow the excess to run around the sides and drain out. The blended acid may be returned to the individual ceresin bottles. A blank from one is representative. There is no volatilization of iron in the absence of sulfuric acid, ordinarily used in a hydrofluoric acid rundown, as proved by evaporating "knowns." ( c ) In order to account for the iron in fused caustic soda by running blanks, it is considered preferable to measure exact quantities of a solution rather than to attempt to weigh equal portions of this deliquescent substance. Were the pellets which are on the market of reasonably uniform weights, they would be highly desirable in avoiding the necessity for evaporating water, but they vary too much to enable one to count a certain number into each determination and blank. Although the chlorate by itself would attack the silver pronouncedly, the sodium hydroxide present yevents this to some extent. Sodium chlorate melts a t 248 C. and potassium chlorate at 368" C., so the sodic salt is preferable. Sodium hydroxide melts a t 318" C. Sodium peroxide is undesirable as an oxidizer, since the reagent is too impure. It oxidizes silver and the combination produces a mess. ( d ) The fusion with caustic soda and chlorate completely decomposes the hydrofluoric acid residue and eliminates organic matter, most of shich is derived from the ceresin bottles. Should any organic matter remain, incomplete precipitation of the iron hydroxide would occur and permit of low results.
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ANALYTICAL EDITION
(e) Silver is somewhat soluble in strong hydrochloric acid, and therefore the rubbing with it should be done quickly. (f)The residue in hydrochloric acid solution should be inspected for dark undecomposed particles, but all the sands tried have been thoroughly decomposed without the need of a pyrosulfate fusion. If there is an odor of chlorine, it is probable that all organic matter has been destroyed. The ammonium hydrate precipitate will contain ordinarily not only iron hydroxide, but silica and the hydroxides of aluminum, zirconium, and titanium, and possibly slight amounts of vanadium and copper. Should the latter two be present, they are not ordinarily in sufficient amounts to affect the thiosulfate titration. (g) The “copper” flask is of glass, and its shape has been developed for ease of handling in making copper and other titrations in the West. After use, it may become coated inside by a white film. This may be removed by boiling a 50 per cent caustic soda solution in it and washing. (h) The filtering of the dilute hydrochloric acid solution leaves much of the impurities, except aluminum, behind on the paper, The titration is not affected by what passes through. (i) Although it is an annoying separation to make, sodium sulfide precipitation is used because no manipulation which has been noted or which was tried provides for a sufficiently complete precipitation of iron hydroxide by caustic soda. Further, the greater the excess, the more the iron found in the filtrate. It has occurred to the author t o add sufficient ferric chloride solution to the sodium hydroxide reagent to saturate it and provide a slight excess of ferric hydroxide, to allow the latter to settle, and to pipet clear supernatant liquor for fusions, for the purpose of doing away with the objectionable use of sodium sulfide and permitting filtering the sodium hydroxide solution almost immediately, but time t o try it out and check it has not been available. The residue containing the iron hydroxide should be dissolved and run t o dryness to dehydrate silicic acid,
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filtered, and the solution treated as described for precipitation by ammonia in paragraph ( m ) of Method. ( j ) ,Slight amounts of vanadium seem to have the effect of carrying iron into the filtrate, thereby causing the green-colored solution. Long standing only will permit of complete precipitation and a yellowish filtrate. A single sodium sulfide precipitation will not remove iron sulfide from aluminum, but does free it of enough to avoid the nuisance of having an excessive amount present in the iron hydroxide precipitate formed later (paragraph ( m )of Method). ( k ) The iron sulfide is readily dissolved. There should be no more than a film of sulfides left on the filter paper anyway, provided the precipitate has not been allowed to become dry before removal by the jet. ( I ) , When alumina is relatively high, it is difficult to remove all the silica by a hydrofluoric acid rundown, but the caustic soda fusion will take care of this. The gelatinous silicic acid formed by acidification is objectionable later on in the filtering of iron hydroxide and in the titrating by thiosulfate, so it is best t o dehydrate it a t this point and filter it off. (m) No particular time of standing for complete precipitation by ammonia is necessary, provided the solution is brought to a boil. (n) Were a sodium sulfide separation not made to free a small amount of iron from a relatively large quantity of aluminum, and were silica not removed, this titration would be prolonged so greatly, owing to entanglement, that uncertainty would be created as to when an end point is reached. Literature Cited (1) Bur. Standards, “Certificate Analyses, Standard Sample No. 81.” (2) Low, A. H., “Technical Methods of Ore Analysis,” 9th ed., p. 80, Wiley, 1922. (3) Lundell, G. E. F., and Rnowles, H. B.,Jr., J . Am. Cerum. SOC.,11, 119 (1928). (4) Yoshida, I., J . Ckem. SOC.Jupan, 45, 26 (1927).
Residual-Current Measurements in Control of Metal Solution in Milk’*’ H. T. Gebhardta and H. H. Sommer UNIVERSITY OF WISCONSIN, MADISON, WIS~
The solution of copper in milk occurs through t h e action of local elements in which the cathode consists of passive areas. The corrosion elements which are active in copper solution in milk m a y be studied by means of model elements, using copper anodes, a n d platinum or gold cathodes, or by using bright a n d oxidized copper electrodes. For copper, the current gives a qualitative measure of the solution rate u n d e r various conditions. From the c u r r e n t the amount of copper dissolving in milk m a y be calculated w i t h f a i r approximation to the gravimetrically determined values.
The solution of nickel in all probability m a y be s t u d i e d
in a similar m a n n e r as copper. For other metals the curr e n t m e a s u r e m e n t s give a qualitative indication of the probable solution rate, but the correct interpretation of the observed c u r r e n t requires more investigation. The principle of the oxidimeter of Toedt, w h i c h was used m a i n l y to m e a s u r e the corrosion r a t e of iron and steel, m a y also be used successfully for measuring the solution rate of copper. Therefore, residual-current measurements m a y be used in t h e s t u d y of copper solution in practice.
. . .. , . . .. .. .. HE problem of metal contamination in dairy products is important, as it involves not only the durability of dairy equipment, but also the flavor and keeping quality of the products. I n recent years investigators have studied this problem, with the result that manufacturers of equipment have introduced new alloys designed to be more resistant to corrosion. The industry in general
T
1 Received April 16, 1931. Presented before the Division of Agricultural and Food Chemistry at the 81st Meeting of the American Chemical Society, Indianapolis, Ind., March 30 to April 3, 1931. 2 Published with the permission of the Director of the Wisconsin Agricultural Experiment Station. The work was supported in part by a grant from the Special Research Fund of the University of Wisconsin. a Present address, Control Laboratory, NestlB’s Milk Products, Inc., Marysville, Ohio.
is in an expectant mood for more information on the problem. The corrosiveneps of many metals under various conditions has been investigated in laboratory corrosion tests, but useful data can be obtained only if in the laboratory experiment the corrosion factors are carefully controlled. Such tests require elaborate apparatus and consume appreciable time (8). On the other hand, laboratory corrosion tests are often criticized (3,9) because the results of such tests are not directly applicable in practice. Corrosion conditions in practice are difficult to simulate in the laboratory. Various compromising methods have been suggested to meet the conditions of special practical problems, Unfortunately
