A Text-book of Assaying: For the Use of Those Connected with Mines Part 52

Sodium 39.38 Pota.s.sium 52.46 Chlorine 60.62 Chlorine 47.54 ----- ----- 100.00 100.00

The percentage of chlorine in the mixed chlorides is calculated. It necessarily falls somewhere between 47.5 and 60.6 per cent., and approaches the one or the other of these numbers as the proportion of the sodium or pota.s.sium preponderates. Each per cent. of chlorine in excess of 47.5 represents 7.63 per cent. of sodium chloride in the mixed chlorides. The percentage of potash and soda in the substance can be calculated in the usual way. Sodium chloride multiplied by 0.5302 gives its equivalent of soda (Na_{2}O), and pota.s.sium chloride multiplied by 0.6317 gives its equivalent of potash (K_{2}O).

The weight of sodium chloride in the mixed chlorides is also calculated thus:--Take the same example for ill.u.s.tration. Multiply the chlorine found by 2.103. This gives--

(0.01422.103) = 0.02987.

From the product deduct the weight of the mixed chlorides found--

Product 0.02987 Mixed chlorides 0.02660 ------- Difference 0.00327

The difference multiplied by 3.6288 gives the weight of sodium chloride in the mixture. In this case it equals 0.0118 gram. The pota.s.sium chloride is indicated by the difference between this and the weight of the mixed chlorides. It equals 0.0148 gram. We have now got--

Sodium chloride 0.0118 gram Pota.s.sium chloride 0.0148 "

from 1 gram of the rock taken. Multiplying these by their factors we have (Soda = 0.01180.5302; Potash 0.01480.6317)--

Soda = 0.625 per cent.

Potash = 0.935 "

~Concentration of the Alkalies.~--With the exception of magnesia, all the other bases are separated from the alkalies in the ordinary course of work without the addition of any re-agent which cannot be removed by simple evaporation and ignition. Consequently, with substances soluble in acids, successive treatment of the solution with sulphuretted hydrogen, ammonia, ammonic sulphide, and ammonic carbonate, filtering, where necessary, will yield a filtrate containing the whole of the alkalies with ammonic salts and, perhaps, magnesia.

The filtrate is evaporated in a small porcelain dish, with the addition of nitric acid towards the finish. It is carried to dryness and ignited.

The residue is taken up with a little water, treated with a few crystals of oxalic acid, and again evaporated and ignited. The alkaline salts are extracted with water, and filtered from the magnesia into a weighed platinum dish. The solution is then evaporated with an excess of hydrochloric acid, ignited at a low red heat, and weighed. The residue consists of the mixed alkaline chlorides.

For substances (such as most silicates and similar bodies) not completely decomposed by acids, Lawrence Smith's method is generally used. This is as follows:--Take from 0.5 to 1 gram of the finely powdered mineral, and mix, by rubbing in the mortar, with an equal weight of ammonium chloride. Then mix with eight times as much pure calcium carbonate, using a part of it to rinse out the mortar. Transfer to a platinum crucible, and heat gently over a Bunsen burner until the ammonic chloride is decomposed (five or ten minutes). Raise the heat to redness, and continue at this temperature for about three quarters of an hour. The crucible must be kept covered. Cool, and turn out the ma.s.s into a 4-inch evaporating dish; wash the crucible and cover with distilled water, and add the was.h.i.+ngs to the dish; dilute to 60 or 80 c.c., and heat to boiling. Filter and wash. Add to the filtrate about 1.5 gram of ammonium carbonate; evaporate to about 40 c.c., and add a little more ammonic carbonate and some ammonia. Filter into a weighed platinum dish, and evaporate to dryness. Heat gently, to drive off the ammonic chloride, and ignite to a little below redness. Cool and weigh.

The residue consists of the mixed alkaline chlorides.

~Separation of the Alkali-Metals from each other.~--Sodium and lithium are separated from the other alkali-metals by taking advantage of the solubility of their chlorides in the presence of platinic chloride; and from one another by the formation of an almost insoluble lithic phosphate on boiling with a solution of sodium phosphate in a slightly alkaline solution. Caesium, rubidium, and pota.s.sium yield precipitates with platinic chloride, which are somewhat soluble, and must be precipitated from concentrated solutions. Caesium and rubidium are separated from pota.s.sium by fractional precipitation with platinum chloride. Their platino-chlorides, being less soluble than that of pota.s.sium, are precipitated first. One hundred parts of boiling water dissolve 5.18 of the pota.s.sium platino-chloride, 0.634 of the rubidium salt, and 0.377 of the corresponding caesium compound. The separation of lithium, caesium, and rubidium is seldom called for, owing to their rarity. The details of the separation of pota.s.sium from sodium are described under _Pota.s.sium_. Ammonia compounds are sharply marked off from the rest by their volatility, and it is always a.s.sumed that they have been removed by ignition; if left in the solution, they would count as pota.s.sium compounds. They will be considered under _Ammonia_.

SODIUM.

Sodium is the commonest of the alkali metals. It is found in nature chiefly combined with chlorine as "common salt" (NaCl). This mineral is the source from which the various compounds of sodium in use are prepared. Sodium occurs abundantly as nitrate (NaNO_{3}) in Chili saltpetre, and as silicate in various minerals, such as albite (or soda-felspar).

It occurs as fluoride in cryolite (Na_{3}AlF_{6}), and as carbonate in natron, &c. Sulphates are also found. Sodium is very widely diffused, few substances being free from it.

The detection of sodium is easy and certain, owing to the strong yellow colour its salts impart to the flame; this, when viewed by the spectroscope, shows a single yellow line.[93] The extreme delicacy of this test limits its value, because of the wide diffusion of sodium salts. It is more satisfactory to separate the chloride, which may be recognised by its taste, flame coloration, fusibility, and negative action with reagents. The chloride dissolved in a few drops of water gives with pota.s.sium metantimoniate, a white precipitate of the corresponding sodium salt.

Sodium salts are dissolved out from most compounds on treatment with water or dilute acids. Insoluble silicates are decomposed and the alkali rendered soluble by Lawrence Smith's method, which has just been described. The separation of the sodium from the mixed chlorides is effected in the following way:--The chlorides are dissolved in a little water and the pota.s.sium separated as platino-chloride. The soluble sodium platino-chloride, with the excess of platinum, is boiled, mixed with sulphuric acid, evaporated to dryness, and ignited. On extracting with water, filtering, evaporating, and igniting, sodium sulphate is left, and is weighed as such.

It is more usual, and quite as satisfactory, to calculate the weight of the sodium chloride by difference from that of the mixed chlorides, by subtracting that of the pota.s.sium chloride, which is separately determined. For example, 1 gram of a rock gave--Mixed chlorides, 0.0266 gram, and 0.0486 gram of pota.s.sic platino-chloride. This last is equivalent to 0.0149 gram of pota.s.sium chloride.

Mixed chlorides found 0.0266 Deduct pota.s.sium chloride 0.0149 ------ Leaves sodium chloride 0.0117

The weight of sodium chloride found, multiplied by 0.5302, gives the weight of the soda (Na_{2}O).

GRAVIMETRIC DETERMINATION.

The solution, which must contain no other metal than sodium, is evaporated in a weighed platinum crucible or dish. Towards the finish an excess, not too great, of sulphuric acid is added, and the evaporation is continued under a loosely fitting cover. The residue is ignited over the blowpipe, a fragment of ammonic carbonate being added towards the end, when fumes of sulphuric acid cease to be evolved. This ensures the removal of the excess of acid. The crucible is cooled in the desiccator, and weighed. The substance is sulphate of soda (Na_{2}SO_{4}), and contains 43.66 per cent. of soda (Na_{2}O), or 32.38 per cent. of sodium (Na).

VOLUMETRIC METHODS.

There are various methods used for the different compounds of sodium.

There is no one method of general application. Thus with "common salt"

the chlorine is determined volumetrically; and the sodium, after deducting for the other impurities, is estimated by difference.

With sodic carbonate and caustic soda, a given weight of the sample is t.i.trated with standard acid, and the equivalent of soda estimated from the alkalinity of the solution.

With sodium sulphate, a modification of the same method is used. To a solution of 3.55 grams of the salt contained in a half-litre flask, 250 c.c. of a solution of baryta water is added. The volume is made up to 500 c.c. with water. The solution is mixed and filtered. Half of the filtrate is measured off, treated with a current of carbonic acid, and then boiled. It is transferred to a half-litre flask, diluted to the mark, shaken up, and filtered. 250 c.c. of the filtrate, representing a quarter of the sample taken, is then t.i.trated with standard acid. The standard acid is made by diluting 250 c.c. of the normal acid to 1 litre. The c.c. of acid used multiplied by 2 gives the percentage. A correction must be made to counteract the effect of impurities in the baryta as well as errors inherent in the process. This is small, and its amount is determined by an experiment with 3.55 grams of pure sodium sulphate.

EXAMINATION OF COMMON SALT.

~Moisture.~--Powder and weigh up 10 grams of the sample into a platinum dish. Dry in a water oven for an hour, and afterwards heat to bare redness over a Bunsen burner. Cool, and weigh. The loss gives the water.

~Chlorine.~--Weigh up two separate lots of 1 gram each; dissolve in 100 c.c. of water, and determine the chlorine by t.i.trating with the standard silver nitrate solution, using chromate of potash as indicator. See _Chlorine_.

~Insoluble Matter.~--Dissolve 10 grams of the salt in water with the help of a little hydrochloric acid. Filter off the sediment, wash, ignite, and weigh. This residue is chiefly sand. Dilute the nitrate to 500 c.c.

~Lime.~--Take 250 c.c. of the filtrate, render ammoniacal and add ammonium oxalate; wash, dry, and ignite the precipitate. Weigh as lime (CaO).

~Magnesia.~--To the filtrate from the lime add phosphate of soda. Allow to stand overnight, filter, wash with dilute ammonia, dry, ignite, and weigh as pyrophosphate.

~Sulphuric Oxide.~--To the remaining 250 c.c. of the filtrate from the "insoluble," add an excess of barium chloride. Collect, wash, dry, ignite, and weigh the barium sulphate.

~Sodium.~--It is estimated by difference.

The following may be taken as an example:--

Moisture 0.35 Insoluble matter 0.40 Lime 0.40 Magnesia 0.05 Sulphuric oxide 0.60 Chlorine 59.60 Sodium 38.60 ------ 100.00

POTa.s.sIUM.

Pota.s.sium occurs in nature as chloride, in the mineral sylvine (KCl), and more abundantly combined with magnesium chloride, in earnallite (KCl.MgCl_{2}.6H_{2}O). It occurs as nitrate in nitre (KNO_{3}), and as silicate in many minerals, such as orthoclase (or potash-felspar) and muscovite (or potash-mica).

Pota.s.sium compounds are detected by the characteristic violet colour they impart to the flame. The presence of sodium salts masks this tint, but the interference can be neutralised by viewing the flame through a piece of blue gla.s.s. Viewed through the spectroscope, it shows a characteristic line in the red and another in the violet. These, however, are not so easy to recognise or obtain as the sodium one.

Concentrated solutions of pota.s.sium salts give a yellow crystalline precipitate with platinum chloride, and a white crystalline one with the acid tartrate of soda. For these tests the solution is best neutral.

These tests are only applicable in the absence of compounds other than those of pota.s.sium and sodium.

GRAVIMETRIC DETERMINATION.

This process serves for its separation from sodium. Take 1 gram of the sample and dissolve it in an evaporating dish with 50 c.c. of water.

Acidify with hydrochloric acid in quant.i.ty sufficient (if the metals are present as chlorides) to make it acid, or, if other acids are present, in at least such quant.i.ty as will provide the equivalent of chlorine.

Add 3 grams of platinum, in solution as platinum chloride, and evaporate on a water-bath to a stiff paste, but not to dryness. Moisten with a few drops of platinic chloride solution without breaking up the paste by stirring. Cover with 20 c.c. of strong alcohol, and wash the crystals as much as possible by rotating the dish. Allow to settle for a few moments, and decant through a filter. Wash in the same way two or three times until the colour of the filtrate shows that the excess of the platinum chloride used is removed. Wash the precipitate on to the filter with a jet of alcohol from the wash-bottle; clean the filter-paper, using as little alcohol as possible. Dry in the water-oven for an hour.