Table of Contents
Brick Pier Support
It is essential that the balance, used for analytical work, should be mounted on some firm support; as, not only does the accuracy of the weighings depend upon the freedom of the instrument from vibration and jar, but also the life of the balance itself. It is equally important, also, from the point of rapidity, since any disturbance of the swings makes a new trial necessary. The author has seen balances mounted so poorly, that walking across the floor, while the beam was oscillating, would cause a perceptible tremor of the pointer, and the slamming of a door, anywhere in the building, during the “swingings,” was enough to throw the apparatus out of adjustment. The most satisfactory support for the balance is, of course, a masonry pier from the ground. The usual form consists of two brick piers, about two feet part and a brick and a half thick, on which rests a slate or stone slab. Almost every college laboratory has its balances so mounted, and, in some instances, the piers run up from the ground to the second and third floors. Concrete may, of course, be used in place of the brick piers. The stone slab is usually placed high enough from the floor, in the college laboratory, to permit of the student standing while weighing. In the mill laboratory, however, it will be found much better not to make the height so great; and to do the weighing while sitting down. In technical laboratories it is usual to weigh a large number of samples or crucibles at once, so that the operator would find it very fatiguing to stand throughout the operation. Where the laboratory is located on the first floor of the building, the mounting of the balance is comparatively simple, as all it will be necessary to do is to erect a couple of brick or concrete piers and to lay a slab of slate across these. The piers need not extend down into the ground for more than a foot, nor is it necessary to make them thicker than eight or ten inches. The concrete for the piers is made of one part Portland cement, three of sand and from four to six of broken stone or gravel. It should be tamped into the wooden moulds. In place of the slate slab and piers, the all concrete table, Fig. 31, described a little further on, may be used.
Where a cellar comes below the laboratory it will probably be better to build one solid piece of masonry, 24 x 18 inches, from the ground up, and bolt to this a slate slab, 28 x 28 inches; so that it projects ten inches in front of the pier. Or else a pier 30 x 18 inches may be built from the ground to the floor, and, on either end of this, two narrow piers, 18 x 6 inches erected. A slab of slate, 36 x 20 inches, is then to be laid over these. Slate slabs for this purpose need not be thicker than 1½ to 2 inches. Where slate or stone slabs can not be obtained, a top of two-inch seasoned and neatly dressed board may be made to serve instead.
As the board is not heavy enough to stay down of its own weight, it must be bolted to the pier. This may be done by bolting the battens to the pier and then screwing the top to the former. The screws should be sunk about an inch below the level of the surface and the holes then stopped up with a round peg and glue, and the whole smoothed off with a plane and sandpaper. Or the top may be bolted directly on to the pier, and the bolts so arranged as to come out of the way, under the balance.
Iron Column Support
Figure 30 shows a simple way of mounting a balance solidly. It consists of a wooden top fastened to two battens, which are in turn bolted to four iron columns, which are imbedded in concrete in the ground. The columns are made from 2 or 3-inch wrought iron pipe by flattening and bending. Any blacksmith can make them. The concrete piers are only large enough to give the necessary stability and need not come above the ground. If made 12 x 18 x 36 inches, they will be of ample size. The battens are first bolted to the columns, the heads of the bolts being sunk below the upper surface of the battens ; the latter being trued up so as to make the table top level, by the use of metal liners. The top, which should be made of well seasoned 2-inch lumber, is then fastened to the battens by means of screws, from below. In order to get the columns in proper position in the concrete, it will be found simplest to make a rough frame and bolt the columns to this, so that their tops are all on a level, and then to pour the concrete into the moulds and
around the lower ends of the pipes. The table top may be filled and varnished and the pipe columns should be painted with aluminum paint—such as is used for radiators, etc.—or with black asphalt varnish. Where a plank top is used for a balance table, particularly if this is made of white pine or other soft wood, the legs of the balance case are apt to sink into the wood, and it is therefore well to rest the legs upon small pieces of metal, such as one-cent pieces.
Solid Concrete Support
Figure 31 shows the arrangement for mounting the balances in the laboratories of the Dexter Portland Cement Company. The laboratories are on the second floor, and the balance room is built above a large, fireproof, concrete vault, used for the storage of the books, papers, etc., of the office below. On the roof of this vault a concrete table for the balances has been built. Its construction is evident from the illustration. The concrete was made of a mixture of one part Portland cement and four parts limestone screenings. The upper surface of the table was trowelled, while the concrete was still wet, until it presented a smooth glassy surface. A wooden form was, of course, first made, iron rods were inserted as shown in the cut, and the concrete was poured into this form, and tamped and trowelled as usual. The wooden forms should be fastened together with screws to avoid breaking off the corners and edges of the table, in removing the boards. This concrete table presents a pleasing appearance to the eye and is satisfactory in every way.
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Shelf Support
The rigid mounting of balances in laboratories located on the upper floors of factories and other buildings often presents a problem difficult of solution. If the building is of brick or stone, the best plan will be to rest the balance on a shelf supported from the outer wall by iron or heavy wooden brackets, bolted fast to the former. In factory or mill buildings, much of the shafting is supported by the girders and beams of the floor above, and consequently the latter trembles more or less all the time when the machinery is in operation. Where the laboratory is on such a floor, it is almost an impossibility to make a weighing when the balance merely rests on a table. In such cases, if it is impossible to bolt a shelf on to another wall, the following arrangement may be resorted to, and will deaden the vibrations to a certain extent. First, a very heavy wooden table is made and on this is placed a slab of slate, stone or even metal, resting on six or more solid rubber balls an inch and a half or two inches in diameter. The heavy slab takes up much of the jar. The addition of another slate slab, resting on rubber balls, which in turn rest on the first slab, will still further lessen the jar. As the weight of the slab flattens the rubber balls somewhat, it is not necessary to scoop out depressions in the table or slab in which they may rest. When a table is made use of to support the balance it should be made heavy and substantial.
Location of the Balance Support
The balance table or support should be placed in a good light but not where the sun will ever shine directly upon it. If it must be placed by a window which would permit the sunlight to fall upon it, the former should be provided with a screen or awning to prevent this.
Balance Support Bench
Figure 32 shows a small bench or seat for use with the balance table. Its form is evident from the cut. It is made of 7/8-inch boards and its dimensions are—width, 14 inches; length, 16 inches; height, 18 inches. The small hand hole in the top of the bench is to carry it about by. A table made similar to this only larger and of much heavier lumber will also make a good balance table. In this case the dimensions should be about as follows:—width, 20 inches; length, 30 inches; height, 30 inches.
Balance
It is perhaps well here to say something about the balances themselves. Two general types of balances are on the market in this country. In the first of these types, shown in Fig. 33, the beam itself is graduated to receive the rider
directly, and is made in the form of a very much flattened isosceles triangle with the apex pointing downward. The balances manufactured by Becker’s Sons (Rotterdam), Christian Becker (New York), H. Kohlbush (New York), Henry Troemner (Philadelphia), and Wm. Ainsworth (Denver), all have the characteristics of this type. In the second type, shown in Fig. 34, the rider , moves upon a scale separate from and fastened to the beam, and the latter is in the form of an isosceles triangle, much more acute than in the former type, and with the apex of the triangle pointing up. Sartorius and Staudinger, Arthur H. Thomas Co., Agents, Philadelphia, balances are representative of this type. Balances of first type are the more used, however, in mill, furnace, and smelter laboratories, in this country. Enough money
should always be put in a balance to get a good one. This does not mean a showy one, but one which will do accurate work under the trying conditions of technical analysis. The short beam balances will be found accurate enough for technical work and they are much more rapid. Nothing is gained by having an analytical balance which will be sensitive to less than 0.1 milligram with a load of 100 grams, since this is a degree of accuracy seldom reached by the other operations of a technical analysis. For assay work, however, button balances, sensitive to 0.01 milligram are desirable. A balance with an aluminum beam is to be preferred to one made of brass, since the lighter the beam the greater sensibility the balance possesses. The knife edges should be preferably of agate, and the planes always of agate.
The arrangement of the arrest of beam and pans should be such that when the balance is at rest, the knife edges and planes are not in contact; and if the pan arrest moves in the same arc as the beam itself it is a point in favor of the balance, as the dulling of the knife edges by dragging them across the planes, which happens when the weight in one pan is two or three grams heavier than the other, is avoided. The balance should be preferably mounted on a glass plate.
Assay Balances
These are of two kinds—button balances and pulp balances. The former are used for weighing the buttons of gold and silver obtained by cupellation and the latter are for weighing the samples of ore. Button balances are made much more sensitive than analytical balances and are intended for weighing only a few milligrams. The pans are much smaller, are concave and removable and the beam is much lighter. They are now usually made with the rider attachment. Troemner and Ainsworth both make assay balances sensible to 1-500 of a milligram. For ordinary purposes a balance sensible to 1-50 of a milligram will be sufficient. All the bearings of this, however, should be of agate. Small platinum weights, 1 milligram and up, are used. F. W. Thompson, of Denver, Colo., makes an assay balance, in which all of the small weights (72 milligrams) are in the form of riders. These are so suspended from a carrier that they may any or all of them be dropped over a projecting arm on the wire suspending the right hand pan. This does away with handling the small weights.
Any balance sensible to two or three milligrams and having a capacity of two or three hundred grams will answer as a pulp balance.
Weights
The weights must, of course, be accurate, and a new balance and weights should always be tested. It is not necessary for ordinary technical analysis that the arms of the balance shall be of equal length, nor that the weights should be compared with the standards in Washington, but the balance should be sensitive to at least 0.2 milligrams with a load of 50 grams on each pan, and still better, to 0.1 milligram with a load of 200 grams. The latter requirement is enough for all analytical purposes. It is also essential that the weights should be relatively correct with regard to each other; that is, the 10-gram weight must be equivalent to exactly ten times the weight of the 1-gram piece, etc.
Torsion Balances
In every laboratory there will be needed a balance capable of weighing quickly and roughly 200 or 300 grams of salts to be used in making up solutions. The balances with the pans above the beam will be found the most convenient of these. The torsion balances are made to cover a wide range of work:—for example these can be obtained with pans 4 inches in diameter, a capacity of 500 grams and a sensibility of 5 milligrams, or with 6-inch pans, a capacity of 5 kilograms and a sensibility of ½ gram. The latter may be obtained with beam graduated to ½ gram. The “Harvard Trip Balance” is also convenient. This has 6-inch porcelain plates in place of pans and a graduated beam (1-10 gram to 10 grams). It is sensible to 1-10 gram and has a capacity of 1 kilogram. “Troemner’s New Laboratory Scale” is also well suited to this work. It has 6-inch pans, a capacity of 200 grams and is sensitive to 1-20 gram. The weights are kept on a projecting shelf at the base of the balance. The Harvard and torsion balances will need a set of cheap weights ranging from 200 grams to 1 gram.
Pans
In most technical laboratories, it is usual to weigh out an exact amount of a substance; and for this purpose counterpoised watch-glasses are usually used. That is, two watch-glasses balanced against each other. These can be bought of any dealer in balances or chemical supplies; but, if the chemist desires, he can make a pair himself from ordinary watch-glasses, by selecting from his stock the two which agree most closely in weight, and grinding and filing until they balance. If they are to be left on the pans when crucibles are weighed, the final adjustment of this may be done with the aid of the screws on the end of the balance beam.
Where a sample has to be transferred to a flask, it is necessary, if a watch-glass is used, to brush the sample from this on to a piece of glazed paper, and then from this in turn to the flask. To avoid the double operation and chance of loss of the substances, it will be found best to substitute a pair of counterpoised celluloid pans for the watch-glasses. These are made by cutting out squares of thin clear celluloid, with rounded corners, large enough to completely cover the balance pans. These can be curved between the fingers, to fit the mouth of the flask, and the sample brushed from them directly into the flask. Aluminum foil may also be used for these pans. If a watch-glass is used it should completely cover the pan, so as to avoid danger of the material dropping on the pan and being included in the weight but not in the sample.
To transfer the sample to the pan and remove the excess, a small spatula is used. This should be ground down on an emery wheel or grind-stone so it tapers to a rounded point of about 1/8-inch width. When it is to be used for iron and steel analysis, it should be magnetized by rubbing across the poles of a bar magnet or dynamo. In weighing pig iron samples, however, care should be used not to do anything more than get the final adjustment to the weight by the use of its magnetic properties, as the non-magnetic particles of the sample contain a larger proportion of the metalloids than do the magnetic ones, and an undue proportion of the former may be left.
To brush the sample from the watch-glass or celluloid pans, use a flat sable or camel’s hair brush, ¾ to 1 inch wide. To transfer the sample, rap the glass or pan gently with the brush handle, until most of the material falls into the dish or flask, and then brush in the few particles which remain attached.
In water analysis, alkali determinations, etc., where a residue of deliquescent salts, left from evaporation, has to be weighed in a platinum dish, it will be found almost an impossibility to get an exact weight owing to the absorption of moisture from the air by the contents of the dish. A useful adjunct of the balance will therefore be an aluminum box, with cover, large enough to hold the dish. Such boxes are sold for holding soap, salves, etc., and while the covers do not fit tight enough to make them air-tight, they do fit tight enough to keep the moisture of the air from coming in contact with the contents of the dish during the short time necessary to make a weighing. The box may be counterpoised by a small piece of brass, filed down to balance it; or the dish and box may be weighed together. In use the box should be dried in the desiccator while the dish is cooling. When the dish and contents are cool, they are to be placed in the box, the cover fitted on and the weight immediately taken. If an aluminum box can not be obtained a seamless tin box such as is used for salve or blacking may be employed. It is much heavier than the aluminum box, however, and in large sizes so much so as to prevent it being used.
Where hygroscopic substances have to be weighed, weighing bottles will be needed. These may be purchased with light blown glass stoppers and in a variety of forms, preferably with flat bottoms and straight sides. When samples of ore, etc., are to be weighed, a small bottle 25 mm. in diameter and 40 mm. high will be found convenient. If the sides of the bottle are straight the sample can be brushed from the bottle with a round camel’s hair brush. These little bottles can also be obtained in a squat form, as wide as 70 mm. and as low as 30 mm. Such weighing bottles are well adapted to weighing the residues from evaporation and drying, both operations being conducted in them. They should be cooled in the desiccator with stopper out. In weighing the bottles the stopper should always be removed for a second, just before weighing, to allow the pressure to equalize. Counterpoised filter papers are usually supposed to be weighed between watch-glasses held together with clips. A weighing bottle 70 mm. high and 30 mm. in diameter will be found much better, as it is lighter. This size will take an cm. filter paper. Hygroscopic precipitates may be weighed, crucible and all, in such a bottle. To give an idea of the lightness of these little blown glass-stoppered weighing bottles, one which the author has, measuring 50 x 30 mm. and holding 30 c. c. weighs only 18 grams.