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late is large. In this case the conversion of the oxide to sulphate may be employed. The ignited precipitate after cooling is carefully treated by adding 9 molar sulphuric acid dropwise until it is in excess, and the excess of sulphuric acid removed by heating in an air bath (§ 40) at about 300°; the dry precipitate is then ignited moderately (to dull redness for a short time) and weighed, the treatment with acid, etc., being repeated to constant weight.

The calcium oxalate precipitate may also be dissolved through the filter paper by means of dilute sulphuric acid, the filter paper thoroughly washed, and the combined filtrate and washings titrated by means of standard permanganate solution.

Summary of Conditions. The sample taken for analysis may contain up to 300 mg. calcium (= 420 mg. CaO). Ions of the copper, iron and aluminum groups, also strontium ion, must be absent; magnesium, barium, sodium, potassium and ammonium ions are allowable. If magnesium ion is present there should be a sufficient concentration of ammonium ion to buffer the ammonium hydroxide so that the solubility product of magnesium hydroxide shall not be exceeded. The volume of the solution should be about 200 c.c., and should contain enough hydrochloric acid to make its p about 4.0. Precipitation should be effected from boiling solution by adding dropwise a 0.25 molar solution of ammonium oxalate until an amount has been added which is equivalent to the combined amounts of calcium and magnesium present, and then adding dropwise a 3 molar solution of ammonium hydroxide until the solution is alkaline.

201. Limits of Method. Precision. The method is suitable for quantities, expressed as calcium oxide, from 0.0003 to 0.400 g. As to the precision of the method, it may be stated to be good to 2 parts per thousand with quantities in the neighborhood of 0.150 g. of calcium oxide. This precision of course decreases with decreasing amounts of calcium oxide.

202. Exercise No. 18. Determination of Calcium Oxide in a

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Sample of Pure Calcium Carbonate.3. Dissolve a half gram of the sample in a covered beaker by means of the cautious addition of 3 molar hydrochloric acid. Rinse off the watch-glass when the sample has completely dissolved and dilute the solu

A good grade of "Analyzed" reagent will answer admirably for this purpose.

tion to a volume of 200 c.c. Heat to the boiling point and add slightly more than sufficient oxalic acid or 0.25 molar ammonium oxalate solution to precipitate all the calcium. Now add slowly drop by drop 3 molar ammonium hydroxide solution until the solution is alkaline. Allow the precipitate to digest for an hour, filter through a 12 cm. filter paper and wash with 0.01 molar ammonium hydroxide solution until free of chlorides. The moist precipitate of calcium oxalate (wrapped in the filter paper) is placed in a platinum crucible (which has been weighed with its cover) and heated so as to char without inflaming the paper. When the carbon has been made to disappear by stronger heat and free access of air, the crucible is to be covered and ignited with the full flame of a Meker burner for fifteen to twenty minutes. It is a mistaken idea that it takes a long time to reach constant weight at temperatures of 1000° or over; even at the temperature of a large and good Bunsen burner (800°) thirty minutes is ample for any amount of lime that is likely to be met with, all this, of course, with the crucible covered. After ignition, place crucible (with its cover on) in a desiccator for twenty minutes and weigh at once (with cover). Repeat ignition and weighing until consecutive weighings do not differ by more than 0.2 mg. Calculate the percentage of calcium carbonate in the sample.

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203. Determination of Magnesium. — The determination of magnesium is almost invariably based upon the principle of precipitating magnesium ion as magnesium ammonium phosphate by means of di-sodium phosphate from a solution which contains ammonium ion and which is finally made alkaline with ammonium hydroxide. The real difficulty in any such method is the difficulty already discussed under the precipitation of phosphate ion, § 190, namely, of obtaining the precipitate MgNH4PO4·6 H2O which can be weighed as such or ignited to Mg2P2O7, or less preferably of obtaining a mixture of MgNH4PO4·6 H2O and MgNHPO4 H2O, which, while it cannot be weighed for its magnesium content, can be ignited to Mg2P2O7. As the precipitate MgNH PO4-6 H2O can be obtained just as easily in the determination of magnesium as the mixed precipitate of MgNH4PO4·6 H2O and MgNH PO4 H2O and its use carries the same advantages as discussed under the determination of phosphorus (§ 190), the

theory of its formation should be reviewed at this juncture and the principles adapted to the determination of magnesium.

In the precipitation of either of the forms of magnesium ammonium phosphate care must be taken that the solution is free of all forms of silicic acid, likewise all ions that give insoluble phosphates, namely, ions of the copper, iron and aluminum groups, also calcium and barium ion.

Summary of Conditions. The amount of sample should not contain more than 0.100 g. Mg. Calcium, barium, and strontium, also members of the iron group and heavy metals, must be absent. The final volume of the solution should not be over 75 c.c.; ammonium chloride must be present to prevent the precipitation of magnesium hydroxide but its concentration should not be over 0.4 molar; the temperature of the solution should not be over 35°. Assuming prior removal of interfering substances including excessive amounts of ammonium salts as just mentioned, bring the volume of the solution to about 50 c.c., add 5-6 c.c. of 3 molar hydrochloric acid and then make the solution slightly alkaline with 3 molar ammonium hydroxide. If a precipitate of magnesium hydroxide forms, dissolve it by adding 3 molar hydrochloric acid and again make alkaline with ammonia. Repeat this operation until the solution is perfectly clear when ammoniacal. Next make the solution acid to methyl orange by adding 3 molar hydrochloric acid, then add 0.25 molar di-sodium hydrogen phosphate until present in forty per cent excess, after which add dropwise, from a pipette or burette with constant stirring, enough 15 molar ammonium hydroxide to make the solution alkaline, and then enough more to make the final concentration of ammonium hydroxide equal to 1.5 molar. Allow the solution to stand at room temperature (18°-30°) for four hours. The precipitate may then be filtered off on a Gooch crucible, and following the technique

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In the systematic course of analysis, the magnesium will usually be in the filtrate, resulting from the prior separation of calcium or other elements so that it will then be necessary to evaporate the filtrate to the desired volume. As there usually will have been an excessive accumulation of ammonium chloride up to this juncture, it must be gotten rid of before adjusting the solution further for the precipitation of the magnesium ammonium phosphate. This removal of ammonium chloride is best accomplished by adding 60 c.c. 16 M HNO3 and evaporating to incipient dryness on the water bath, repeating the addition of more nitric acid and evaporation to incipient dryness.

51 c.c. 0.1 M Na2HPO4 = 0.0024 g. Mg = 0.004 g. MgO.

of § 191, washed successively with 1.5 M NH4OH, 95% alcohol, and ether, sucked dry at the filter pump for ten minutes, allowed to stand around in the air for fifteen or twenty minutes, or placed in a desiccator for a no greater length of time and then weighed at once as MgNHPO4·6H2O, or it may be washed with only 1.5 molar ammonium hydroxide and then ignited to Mg2P2O7. An alternative procedure is to collect the precipitate on filter paper and ignite in a platinum crucible, the cautions mentioned in § 192 being observed.

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204. Limits of Method. Precision. The method is suitable for quantities, expressed in terms of magnesium, ranging from 0.0004 to 0.120 g. Mg. With respect to the precision it may be said that of all the commonly occurring elements it is probable that the determination of magnesium is attended with the greatest error, and in this regard it is pertinent to quote the authority of Hillebrand who says, "Experience has shown that most analysts are prone to make a very serious plus error in determining moderate amounts of magnesia, such as are found in limestones and Portland cements." All things considered, it is doubtful whether we can claim a precision greater than 5 parts per 1000 for quantities equivalent to, say 0.100 g. of magnesium or thereabouts; this precision falls off rapidly with decreasing amounts of magnesium.

205. Exercise No. 19. Determine the percentage of magnesium and sulphate (SO4) in a sample of crystallized magnesium sulphate MgSO4.7 H2O; or in a standard solution of same as furnished by the instructor.

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206. Exercise No. 20. Determination of Calcium and Magnesium in Dolomitic Limestone. Dolomitic limestone is chosen as an example of a carbonate rock of which the main constituents are calcium carbonate and magnesium carbonate. Carbonate rocks whose main constituents are the two just mentioned are very important technically because they serve as raw material for many branches of chemical technology. Such rocks are classified on the basis of the relative percentages of their content of calcium carbonate and magnesium carbonate about as follows, although the classification is somewhat arbitrary since one group

W. F. Hillebrand, loc. cit., § 13, p. 153.

passes into the other by insensible gradations of the relative proportions of the two constituents:

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Besides the major constituents just mentioned the following minor constituents are almost invariably present in small amounts: the carbonates of iron and manganese, alumina and silica in the form of clay, also iron pyrites (FeS2) and sometimes a little gypsum (CaSO4).

For technical purposes it is seldom considered necessary to analyze the carbonate rocks with the same precise detail that is required for geologic purposes, but rather to make the analyses with regard to the technical utilization of the materials and with determination of only the more important constituents. The following constituents are generally all that are required:

Silica (SiO2)

Combined oxides (Fe2O3, Al2O3, Mn3O4, P2O5)

Lime (CaO)

Magnesia (MgO)

Loss on ignition"

Insoluble matters

Loss on drying at 105°-110°

We will consider here only the determination of the calcium and magnesium. In order to determine these constituents in a carbonate rock it is necessary to remove the silica and the iron and aluminum oxides; if the sample is fairly pure the percentages of these constituents will be small.

Method. Mix one gram of the rock powder with an equal quantity of anhydrous sodium carbonate in a platinum crucible and ignite, at first carefully over a Bunsen burner and finally

7 Loss on ignition. This is determined by igniting 1 gram of the powdered sample in a platinum crucible, gently at first, and then over the Meker burner or blast lamp, until constant weight is obtained. The loss in weight represents moisture and carbon dioxide and is termed "Loss on ignition."

8 Insoluble matter. The ignited sample is transferred to a beaker, and 50 c.c. of water and enough 2 molar hydrochloric acid added to dissolve the sample. The solution is boiled for five minutes and filtered, the precipitate being washed with hot water, ignited and weighed.

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