Colorimetric Assays

Colorimetric Assays are assays in which the colour imparted to a solution by some compound of the metal to be determined is taken advantage of; the depth of colour depending on the quantity of metal present. They are generally used for the determination of such small quantities as are too minute to be weighed. The method of working is as follows :—A measured portion of the assay solution (generally 2/3, ½, 1/3, or ¼ of the whole), coloured by the substance to be estimated, is placed in a white glass cylinder standing on a sheet of white paper or glazed porcelain. Into an exactly similar cylinder is placed the same amount of re-agents, &c., as the portion of the assay solution contains, and then water is added until the solutions are of nearly equal bulk. Next, a standard solution of the metal being estimated is run in from a burette, the mixture being stirred after each addition until the colour approaches that of the assay. The bulk of the two solutions is equalised by adding water. Then more standard solution is added until the tints are very nearly alike. Next, the amount added is read off from the burette, still more is poured in until the colour is slightly darker than that of the assay, and the burette read off again. The mean of the readings is taken, and gives the quantity of metal added. It equals the quantity of metal in the portion of the assay. If this portion was one-half of the whole, multiply by two; if one-third, multiply by three, and so on. When the quantity of metal in very dilute solutions is to be determined, it is sometimes necessary to concentrate the solutions by boiling them down before applying the re-agent which produces the coloured compound. Such concentration does not affect the calculations.

This method is used to assay copper by color.

  • 8 cc’s or nitric acid is placed on 5 grams of sample and heated until all action has ceased. (Brown fumes have ceased rising from the beaker).
  • Potassium Chlorate is added to the solution to decompose the sulphides and the solution is again heated  for a short period.
  • 100 cc’s of water and 20 cc’s of ammonia are added to this.
  • Filter the solution by decant action into a coloured bottle. Only 100 cc’s are required.

Copper in the sample is indicated by the blue tinge caused through  the action of the a ammonia on the copper nitrate. The colour is then compared with the standard colour for copper.

  • One quarter of the original reading must be added.

Notes on Colorimetric Analysis Of Copper Method:

    1. Copper Nitrate CuNO2 use formed by the action of nitric acid on copper.
    2. Potassium Chlorate KC103 must be added if there is any sulphide in the sample.
    3. Copper ammonium nitrate a deep blue color is formed by the action ammonium hydroxide on copper nitrate.
    4. 1 cc. Of copper nitrate and 15 cc’s of of ammonium hydroxide diluted to 100 cc’s in standard bottle represents 0.01 grams of copper in the solution.

Heine’s “ blue test ” for copper, as described by the authorities generally, calls for a set of standard colors; and there has been some discussion concerning the relative superiority, for this purpose, of sulphate and nitrate solutions. The whole matter evidently hinges on the preservation of the standard colors in well-stoppered bottles. The apparatus described by G. L. Heath cannot be much improved, when very accurate readings are required.

The following method was devised for the purpose of doing away with preserved sets of standard colors, by making a fresh standard for each batch of assays. The solutions are prepared in the usual way, each ammoniacal solution being filtered into its separate bottle, and then filled up to the containing-mark and thoroughly mixed. A similar color-bottle, with an “ S ” etched upon it, to distinguish it from the other bottles, which are numbered to correspond with the samples, is kept to run the standard. About 150 c.c. of water is put into it, and then the amount of acid (sulphuric or nitric) present in each determination, followed by 30 c.c. of ammonia (s. g. 0.90), which should make the mixture strongly ammoniacal.

The liquid is now made up almost to the containing-mark (200 c.c.), say within 1 c.c. The lowest assay is selected first and placed alongside the standard, which, at the beginning, contains no copper. A copper-solution is then dropped into the standard from a burette ; and after each addition the bottle is well shaken and compared with the assay-sample. This operation is repeated until the two shades match exactly, when the burette is read and the assay-result is calculated. The assay next in order of depth of color is now taken and treated in the same way, and so on, until the batch is completed. It will be found that the volume of the deepest colors is from 1 to 2 c.c. less than the standard, which has increased by successive additions of copper-solution. This is corrected by adding the necessary amount of water to the assay, just before the last one or two drops of copper-solution are added to the standard.

Method of Assaying Copper by Color 

The copper-solution contains 5 grammes c. p. copper per 2000 c.c., and is made by dissolving the metal in a small quantity of nitric acid and diluting, so that 1 c.c. = 0.0025 gramme Cu. It is convenient to have a syphon-attachment, for filling the burette with this solution.

Tailings Assay

One gramme is digested on a hot plate with from 3 to 5c.c. of HNO3 to 3 c.c. of HCl, and 5 c.c. of H2SO4. Rapid heating will soon decompose the mineral; and the treatment should be continued until the sulphur globules which form are quite yellow. Add about 30 c.c. of water; then an excess of ammonia-water (s. g. 0.90). Mix thoroughly and filter hot, through a S. and S. folded filter No. 588 or a very rapid paper. Wash the iron precipitate twice with 1/10 ammonia-water ; then dissolve off again into the original vessel with 5 c.c of 1 to 1 H2SO4 and hot water. By lifting the filter and its contents out of the funnel, then opening it out and washing back the precipitate with a jet into the original vessel, the operation can be performed in less than a minute, and with very little water. The solution is now reprecipitated with ammonia, and the filtrate is combined with that obtained from the first precipitation. One final washing is enough (1/10 ammonia) for any material not running over 1.5 per cent, of copper.

Slag Assay

One gramme is boiled in a dish with 15 c.c. of water; then 5 c.c. of HNO3 is added along with 5 c.c. of 1 to 1 H2SO4. The digestion is carried on until the slag is thoroughly decomposed, and any sulphur globules are yellow. It is not necessary to dehydrate the silica; and as decomposition is immediate, there is rarely any delay at this point. The assay is now treated in the way as described above for the tailings- assay; only, care should be taken to avoid using too large quantities of water in washing and transferring precipitates. A third precipitation of the iron has always resulted in a filtrate free from copper when working on blast-furnace slags; but in analyzing high-grade slags, some copper may remain in the second precipitate.

The standard is prepared as already described, and the colors are carefully matched. When one gramme is taken, each c.c. of copper-solution is equivalent to 0.25 per cent, of copper.

The following table shows the results of a batch of assays occurring in the daily work of a large copper-plant, and illustrates what may be expected from the colorimetric assay in a busy office. The samples are varied, consisting of mill-tailings, slags and lean ores. The results are not so close as those given by Mr. Heath in his paper; but he says that his electrolytic assay was made on the solution which he had used for his colorimetric assay. The electrolytic assays tabulated below are separate determinations on a separate weighing of the sample, the copper being precipitated from the acid solution of the ore without any previous separation of iron by ammonia.

comparative-results-of-colorimetric-and-electrolytic-assays

Green tints generally result in a low reading. Organic matter is the principal cause; but considerable percentages of arsenic will also interfere, and produce tints which it is impossible to compare. Small percentages of arsenic have no effect.

The advantages of the above method over the usual method of using sets of standard colors are:

  1. In a small office space is valuable, and a set of standard colors will take up room which may be used for something else.
  2. An exact match is made with each assay.
  3. The assays are read from a fresh standard, and not from a bottle which may have been made up months before.

It was found that Mr. G. L. Heath’s bottle was expensive, and easily broken. A bottle which is inexpensive, stands the wear of the assay-office of a works, and, at the same time, gives excellent results, is a square bottle, No. 5675 of Eimer and Amend’s catalogue, 2 in. in diameter, and 4¾ in. high from bottom to neck, upon which any required markings for contained volumes can be readily etched, according to the desire of the assayer.

The results tabulated above were obtained with this bottle.

In presenting some notes on the “Heine’s Blue Test” Mr. Smith expresses a preference for fresh standards and a cheaper bottle; but this is rather a reversion to the old method, since I devised in 1897 the newer method of preparation and preservation of permanent standards, which were not used with any success in the original method.

Many busy western assayers no doubt value a method for preparing a permanent set of standards in air-tight glass-stoppered bottles, whose use brings the method as near perfection as one could desire as regards rapidity of color-reading and avoidance of unnecessary work.

Mr. Smith notes that his color-readings were compared with an electrolytic assay of another ore-sample, and that my electrolytic assays were made on the “ blue test ” solution from colorimetric assay.

He omits, however, reference to the fact, showing the advantage of my procedure, that the next column in table of results in the original paper showed the copper as (determined by battery-assay) remaining in the precipitate of iron oxide and silica from the colorimetric assay.

The sum of the electrolytic tests of the blue solution and the residues gives the true copper-contents of the original ore without introducing the element of error of sampling, as Mr. Smith has done by testing two samples instead of one in each experiment.

The experiments, in my paper referred to, indicated more than this, as they showed by the method of tabulation, etc., described, how nearly one may read the color of a known amount of copper in a solution, and how near that ammoniacal solution comes to containing the total copper originally present in the test-sample.

As in the color-method for carbon in steel and the cyanide-titration of copper, there must be for close work a careful adherence to uniform conditions and agreement of tint or shade between standards and assays.

A set of unchangeable standards containing a definite quantity of ammonia ought to permit closer work, since the tint varies with excess of ammonia from purple to clear blue and even greenish blue.

In further discussion of the question—If standards may be made so that they are good for months afterwards, is there any object in wasting time to make up fresh ones each day ? I would say that it is true that bottles made from selected tubing with tightly-ground stoppers may be a little expensive for a private laboratory; but the price (about $1 each) ought not to be objectionable to a company when much is gained by their use.

A set of 24 uniform bottles, in which the 200 c.c. marks do not vary more than 1/8 inch from each other, will allow 12 bottles for standards and 12 extras, and should last for many years.

If standards of permanent nature are desired, both a strongly ammoniacal copper sulphate solution and containing-bottles with tight glass stoppers are indispensable, according to our experience. If a chemist prefers the old make-shift, involving fresh standards every day, there is nothing more that I can add to this discussion.

In commenting upon my modification of the color-test, Mr. Heath considers it a reversion to the old method, and objects to the waste of time in making a fresh standard. The old method requires a fresh standard to each assay-solution; on the other hand, in the method described by me, one standard will do for a batch of samples—as large as may arise in the routine-work of the chemist. At the same time it only takes about one minute to fill the standard tube with the requisite amount of water, acid and ammonia, and the titration does not take any longer than the comparing of solutions with permanent standards. Having used both methods for several years, I do not hesitate to say that I find my method as rapid as Mr. Heath’s, and for reasons stated in my paper prefer to use it. Mr. Heath was the first chemist to make permanent standards possible, and his bottle is as perfect as can be made. If, however, fresh standards can be run with one or several assays at once, and as rapidly as with standard colors, then there is no reason why a chemist should go to the trouble of preparing the latter. There is no reason why the same uniform conditions cannot be observed in making the fresh standard as well as by any other method, so that Mr. Heath’s remarks on this point are unnecessary.

With reference to my electrolytic assays being made on a second weighing of ore, it seems to me preferable to do this in checking assays, as the errors in weighing will then be reduced. Averages are always more reliable than single assays. My experience with the bottles made from selected tubing has been rather unfortunate, they being so easily broken by the laboratory attendant. If a cheaper bottle can be used, and at the same time the results are accurate and obtained rapidly, it is unfair to class the device as a make-shift. Simple apparatus is always effective if suited to the work required.