The Microchemical Balance

CHAPTER 2

The Microchemical Balance

The microchemical balance* is generally accepted as the instrument used for making the weighings in connection with quantitative organic microanalysis! and the first one was built by Kuhlmann 5 1 for Pregl 7 3 as listed in Table 1. Since then, a number of companie s 2 ' 9 ' 1 6 - 5 3 ' 5 6 ' 5 9 , 6 2 ' 6 9 ' 7 1 ' 8 4 " 8 6 have produced these instruments. They are similar, but smaller, in construction to good analytical balances and often possess the same type of extras: weight carriers, optical projection, damping devices for reducing the weighing time, etc. The essential differences between the various types of balances are in the capacities and

T A B L E 5

T Y P E S OF BALANCES, T H E I R CAPACITIES, AND CLAIMED SENSITIVITIES

Capacity Claimed sensitivity

Analytical 200 gram 0.1 and 0.05 mg. (some 0.025 mg.)

Assay 1 gram 0.02 mg.

Semimicrochemical 100 gram 0.01 and 0.005 mg.

Microchemical 20 and 30 gram 0.001 mg.

Microbalance Usually a few mg. Some down to 0.001 and

(Microgram balance) (some up to several tenth gram)

0.005 μg.

the sensitivities (and precisions or reproducible sensitivities) as shown in Table 5. (See Chapter 3 for Determination of Sensitivity and Precision. Also "Additional Information" for this chapter.)

Theoretically, 1 2 if the precision (or reproducible sensitivity) of a balance is

* Please see references 1, 5, 10, 11, 12, 20, 21 , 29, 30, 34, 35, 37, 45 , 49 , 65 , 66, 68, 73, 74, 76, 7 8 - 8 1 , 87, 88, 93 .

t This is the analysts' basic tool and the caliber of work performed is dependent upon its condition. For this reason, the author strongly advocates that each analyst be provided with his (or her) own balance. With proper care, a balance will give excellent results for twenty years or more before reconditioning is necessary. In addition, the arrangement of one balance per person is a necessity for those doing carbon-hydrogen determinations. Any other arrangement places the analyst at a distinct disadvantage from the standpoints of both quantity and quality.

30

31 Essential Parts

0.001 mg., it is possible to work with samples which weigh as little as 0.3 mg., but if the precision is only 0.020 mg., the sample must weigh at least 6.0 mg. The practical weights, however, are much greater. 6 7 Table 6 shows the relationships which exist between these variables1 8- 6 6 and from it can be seen that the microchemical balance is suitable for samples in the size range used in the procedures in the following chapters.

T A B L E 6

SIZE OF SAMPLE REQUIRED W H E N USING BALANCES WITH VARIOUS PRECISIONS

Weight of sample required

Precision of bal Theoretical minimum Practical average ance (mg.) (mg.) ( m g )

0.001 0.3-0.4 3-5 0.002 0.6-0.8 4 - 6 0.005 1.5-2.0 5-8 0.010 3.0-4.0 6-10 0.020 6.0-8.0 8-12

Microchemical balances have a reputed sensitivity of 0.001 mg., but the precision of the best instruments is of the order of 0.003 mg . 7 6 at no load. At the maximum load of 20 grams (30 in some cases) the sensitivity and precision of a good balance drops off about 5 % . 5 * A good semimicrochemical bala n c e 2 , 9 ' 5 6 ' 8 5 can be used if the sample size is increased slightly, but the author approves of such procedures for emergency measures only. Although some of the m i c r o b a l a n c e s 2 3 ' 2 5 ' 2 6 ' 4 4 ' 5 2 ' 6 3 , 6 4 ' 7 0 , 8 2 (microgram balances) are more sensitive than microchemical balances the former are not suitable because of the lower capacities.

It is taken for granted that the beginner in microanalysis is familiar with the details of construction of, as well as has had experience with, analytical balances. Therefore, microchemical balances will be discussed rather generally here, with emphasis only on certain details.

Essential Ports

The essential parts of the balances are the beam, stirrups, and pans or hangers. A particular balance might have particular features regarding the parts, but the following general information will acquaint the reader with basic information which will cover all cases. Additional information regarding several different makes and types of balances follows this basic material.

* Two pan type.

2. The Microchemical Balance 32

BEAM

The beams of modern balances are usually prepared from special alloys (aluminum), bronze, or platinum plated metal, approximately four or five inches in length. This is in direct contrast to the tiny beam of the original Kuhlmann 5 1 balance which was 70 mm. in length. Attached to the beam is a central knife edge usually made of agate, sapphire, or other extremely hard material such as boron-carbide. In addition, there are end knife edges for each of the pans or hangers (two for the Ainsworth,2 Becker,9 Bunge, 1 6 etc., types and one for the constant load types represented by the Met t l e r 5 6 ) . Attached to

FIG. 11. Essential parts of the Bunge microchemical balance, ( a ) The beam, ( b ) The stirrups ( two) , ( c ) The pans ( two) , (d ) The pointer, ( e ) The scale or reticle, ( f ) The rider, ( g ) The image of the reticle projected on a screen, (h ) Zero point adjuster, not attached, used only when balance is first assembled, ( j ) Knife edges ( three) .

the beam, on the two pan type, is a pointer [Fig. 1 1 ( d ) ] which swings in front of a fixed scale, usually for the purpose of making rough readings. A separate small scale, or reticle [Fig. 1 1 ( e ) ] , is sometimes attached to the pointer, above the point. As the pointer swings, this reticle is viewed either through a telescope or microscope (see Fig. 9 ) or the image of the reticle is projected on a screen for magnification [see Figs. 8, 1 1 ( g ) , 12, 15, 16, 17, and 2 0 ] . Toward the top of the pointer rod or on the beam, is a small weight (either in the form of a threaded knurled knob or sliding weight with a locking bolt) used for adjusting the sensitivity. Raising the weight raises the center of gravity and increases the sensitivity, while lowering the weight lowers the center of gravity and decreases the sensitivity. This adjustment should be

33 Essential Parts

made, only by an expert, if the sensitivity does not fall within the range of 95 to 105 (see Chapter 3 ) . ( I f the center of gravity is raised too much, an unstable pendulum system results and the balance becomes erratic.) The knife edges are either held in place by a set of tension bolts (Fig. 11) or are swaged into place (Figs. 13 and 1 4 ) . When tension bolts are used, they never should be tampered with as the setting of these are most vital to the sensitivity and precision of the balance. I f any of the knife edges become chipped, even though only detected microscopically, the edges must be reground by the manufacturer. The central knife edge, in operation, rests upon the flat surface or

FIG . 12. Bunge microchemical balance, model 25 M P N .

plane at the top of the column of the balance, while the end knife edges are in contact with the planes of the stirrups as explained below. All of the planes are prepared of the same material as the knife edges.

STIRRUPS

As mentioned in the above paragraph, during operation the end knife edges are in contact with the stirrups, or suspensions [Figs. 1 1 ( b ) and 1 4 } , and that the flat surfaces or planes are prepared of the same material as the knife edges. The metal portions are prepared from various metals, often gold or rhodium plated. When the flat surfaces or planes on the stirrups or column become worn so that microscopic grooves are present, the sensitivity and precision are greatly affected, just as when the knife edges become chipped, and the balance must be returned to the manufacturer for reconditioning.


PANS

The pans or hangers [Fig. 1 1 ( c ) ] are suspended from the stirrups. These are equipped with hooks for the purpose of supporting some of the objects to be weighed.

RIDER AND RIDER CARRIER

Most balances are equipped with notched beams [Figs. 1 1 ( a ) , 13, and 1 4 ] , and, therefore, require riders. A number of shapes are used , 2 ' 9 ' 4 9 ' 7 4 each with the purpose of obtaining perfect rider placement which is absolutely necessary or the balance will have poor p r ec i s i on . 2 ' 9 , 4 9 ' 6 5 ' 6 6 ' 7 4 Riders are generally made of aluminum, bronze, or in some instances, of quartz. Their shapes are usually either wishbone [Fig. 1 1 ( f ) ] , U-, or rod-shaped. The rider carrier, of course,

FIG. 1 3 . Beam of Ainsworth microchemical balance.

must be designed to fit the rider. Some of these, as shown in Figs. 15 and 16 are equipped with a magnifier as an aid in manipulating. Figure 14 shows the carrier on an Ainsworth balance equipped with a rod-shaped rider.

CASES AND RELEASING MECHANISMS

Balance cases are prepared from mahogany or metal, the present trend being definitely towards metal. Some have only front doors (Fig. 1 5 ) , some have only side doors (Figs. 12 and 1 7 ) , while some have both (Figs. 16 and 2 0 ) . The beam- and stirrup-, and pan-releasing mechanisms on some balances operate simultaneously from a single control (Figs. 12, 17, and 1 9 ) , while others use separate mechanisms similar to those found on the ordinary analytical balance (Fig. 1 5 ) .

A number of excellent balances are commercially available, but space here does not permit their detailed description. In addition, the author prefers

35 Essential Parts

to adhere to the policy adopted throughout this book of going into detail only in regard to apparatus and procedures with which he is familiar. Therefore, descriptive material is included for Ainsworth, Becker, Bunge, and Mettler balances. Instruments of the first three named have been used by the author

FIG. 1 4 . Enlarged view to show details of construction of stirrup and rider carrier of Ainsworth microchemical balance.


FIG . 15. Ainsworth microchemical balance. Model shown is FHM having keyboard operated weight carrier employing 0.5 mg. rider giving beam 0 to 1 mg. range. Model FH is similar, but without keyboard operated weight carrier and employing 5 mg. rider giving beam 0 to 10 mg. range. (Note: The author suggests discarding the equilibrium or zero adjusting riders furnished with this balance and making the adjustment by means of the knurled screw bolts on the beam.)

FIG. 16. Becker microchemical balance, model EM 1.

37 Essential Parts

for periods ranging from fifteen to twenty-two years, although not necessarily the models shown in Figs. 12, 15, and 16, which obviously are the current ones. The description of these individual balances should not be interpreted by the reader to mean their endorsement over all other makes. Likewise, the omission of descriptions of other makes of balances should not be interpreted

FIG . 17. Mettler microchemical balance, model M-5.

as an indication of their inferiority. Before selecting a balance, the analyst should take into consideration his (or her) particular needs regarding weight carriers, damping,* price, etc.

Figures 12, 15, 16, and 17 show the Bunge (model 25 M P N ) , Ainsworth (model F H M ) , Becker (model E M - 1 ) , and Mettler (model M-5) microchemical balances, respectively. Table 7 shows the capacities, sensitivities, lengths and compositions of beams, number of rider notches, weights of riders, materials from which knife edges and planes are made, types of optical systems,

* Some manufacturers contend that with damping a certain amount of precision is sacrificed for the sake of speed.2-9

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39 Essential Parts


damping, weight carriers, cases, etc., of the above-mentioned balances. The Ainsworth, Becker, and Bunge balances are of the two pan type while the Mettler is of the one pan, constant load variety. With the former, weights are added to the right side to counterbalance the load on the left. With this type, the sensitivity decreases with the load, usually about 5 % 5 at full load ( 2 0 or 30 grams). With the Mettler balance, the weights, which are on the same side

FIG. 18. Diagram showing principle of construction of the constant load Mettler microchemical balance (damped). ( 1 ) Pan brake. ( 2 ) Pan. ( 3 ) Sets of weights. ( 4 ) Sapphire knife edge. ( 5 ) Stirrup for hangers and weights. ( 6 ) Movable weight for adjustment of sensitivity. ( 7 ) Movable weight for adjustment of zero point. ( 8 ) Main knife edge (sapphire). ( 9 ) Counterweight. ( 1 0 ) Air damper. ( 1 1 ) Engraved optical scale. ( 1 2 ) Lifting device. ( 1 3 ) Arrest lever.

of the main* knife edge as the pan, are removed to compensate for the load added (substitution). Since there is a constant load, the sensitivity remains constant. Figure 18 is a diagram of the Mettler instrument.

In closing the discussion on balances, it should be remembered that many factors affect their performance and the analyst should consult the manufacturer immediately in case of dissatisfaction.

* The constant load balances have two knife edges instead of the customary three.

41 Mounting the Balance

Mounting the Balance

The table upon which the balance should be placed was described in Chapter 1. When the inertia block-isolator type is used, balance feet made of metal, glass, or resin are placed on top of the glass plate with small pieces of thin plastic tape between the bases of the feet and the plate to prevent slipping. (Any rubber or other compressible material attached to the balance feet should be removed as this detracts from the system.) The balance is placed on the feet and made level by means of the adjusting thumb screws* until the bubble(s) in the spirit level ( s ) , or the plumb at the end of the plumb line, is centered. The balance is allowed to remain undisturbed for some time before using—a day or two if brought in from the outside in cold weather.

I f the rigid combination type of table is used, four large regular rubber stoppers (at least two inches in diameter) are placed on the table, and on top of these is placed a piece of 14 inch plate glass which is slightly larger than the base of the balance. On the glass are placed balance feet made of metal, glass, or resin from which any soft spongy rubber or easily compressible material has been removed. The rest of the mounting is performed as above. Depending upon the type of vibrations encountered, other mountings, such as lead or tightly compressed paper, might prove more effective than the above. However, nothing should be used which can be depressed without immediately returning to its original position or the precision of the balance will be affected. Regardless of the mounting, the same extent of vibration isolation cannot be obtained with the rigid type table that is possible with the inertia block-isolator type and this fact must be accepted.

Assembling ond Cleaning the Balance

Balance manufacturers always furnish detailed instructions3 for assembling (and cleaning) their balances, but a brief description of the operations in connection with the simpler types, such as that shown in Fig. 16 is in order here to serve as an example and will serve as a guide when dealing with the more complicated models. Since a microchemical balance is a very delicate instrument, its assembly should be done with extreme care and, if possible, by someone with previous experience. Because the cleaning of a balance includes both dismantling and reassembling, its description will cover both the setting up of a new instrument and the servicing of an old one. After a balance has been assembled and is operating satisfactorily, it is wise to accept the advice given

* Two or three screws, if the balance case has three legs, and four screws, if the balance has four legs.


by the saying, "let sleeping dogs lie." If the atmosphere in the balance room is kept comparatively dust-free, cleaning need be done only once or twice per year, unless dust particles can actually be seen on the beam and stirrups. On the other hand, the pans should be frequently removed and dusted with a clean camel's-hair brush. This can be done by either of two methods. First, it should be made certain that the arresting mechanism for the beam is locked. The pans or hangers are then removed with the aid of a clean, dry chamois skin,* dusted and replaced. Or, if preferred, the pans need not be removed, but can be held by means of the chamois skin or a pair of bone-tipped forceps to keep them from swinging, while being brushed. No part of the balance should ever be touched with unprotected fingers. Likewise, the base plate of the balance should be kept free from dust by cleaning almost daily with a camel's-hair brush as this will help keep the working parts dust-free. Occasionally, the base plate should be wiped off with a clean cloth dampened with alcohol. The disassembling, cleaning and reassembling of the balance should be done in the following manner: The rider is first removed from the beam and allowed to remain on the carrier which is pulled to the extreme right. If the rider shows dust particles it should be gently cleaned with a small camel's-hair brush. After making certain that the beam and stirrup arrestments are in the locked position, the pans or hangers are removed and dusted with a camel's-hair brush and, if necessary, a chamois skin. They should then be placed on clean chamois skins on a table. The stirrups are next removed using a piece of chamois skin. (Caution: Again make certain that the beam arrestment is in the locked position). They are gently dusted with the camel's-hair brush and set beside the pans or hangers. Finally, the beam is removed using chamois skin and cleaned by brushing the beam, notches, and all three knife edges. It is then set beside the other parts, all of which are continually protected by resting on chamois skin. The flat surface or plane on the column on which the central knife edge of the beam rests during operation is then gently brushed. Occasionally, if the balance has become very dusty it may be necessary to wipe all the knife edges on the beam and all the flat surfaces or planes on the stirrups and column with chamois, f After each knife edge and flat surface is cleaned, it should be examined with a magnifying glass to make certain that no brush hair or lint particle still adheres. Then all beam and stirrup arrestment supports are cleaned with the camel's-hair brush, or if necessary with the chamois skin.

The balance is now ready for reassembling and all parts are replaced in the reversed order from which they were removed. Chamois skin is used at all

* Chamois skins are best cleaned by repeated washing with acetone or by washing with soap, water, and dilute ammonia, rinsed with water and air dried,

t In extreme cases, acetone may be used on the parts.

43 Additional Information for Chapter 2

times to protect the parts from the fingers. First the beam is replaced. (Caution: Arresting mechanism locked.) Next, the stirrups are replaced, making sure that they are put in their respective places. (The left-hand one is marked with a single dot, the right-hand one with two dots.) The pans or hangers are then hung onto their respective stirrups. They are marked in the same manner as the latter (one dot for the left, two dots for the right). Finally, the rider is replaced on the beam. After cleaning, the balance should be allowed to rest overnight before being used. It should then be tested for sensitivity as described in Chapter 3 (Weighing). If the sensitivity is lower than before cleaning, or if the balance does not swing freely, it should be dismantled and recleaned as it probably contains brush hairs or lint which interfere with the knife edges.

ADDITIONAL INFORMATION FOR CHAPTER 2

Additional Bunge microchemical balances, of the same capacity and sensitivity, are available in mahogany cases with air damping built into the base plates and reading devices which consist of a telescope without objective. The

FIG. 1 9 . Bunge microchemical balance, model 2 5 D K O .

FIG. 21. Mettler microbalance, model M-5/SA (lower section).

45 Additional Information for Chapter 2

ocular is fitted with a microscale. Either of two types of scales are available, namely a regular type with readings to both sides of the zero line and the other of entirely different design described by Zimmermann. 9 4 To avoid any possible errors, the scale bears no markings on the left side, with the exception of a single graduation mark " E . 0.1 mg." which is used in the sensitivity test only. Two metal case models are available in hexagonal cases, one with the same conventional beam (model 2 5 D K L ) used on all other Bunge models and the other designed for use as a riderless balance, with an additional fractional weight loader for weights from 1-10 mg. Both metal case models have fractional weight loaders ( 1 0 - 9 9 0 mg.) and a special collimation reading device for viewing either of the above types of scales. Figure 19 shows a photograph of the new, riderless balance, model 25DKO.

Another riderless,* air-damped balance with weight carrier, is available from Oertling, their model 147. It has a capacity of 20 grams and a sensitivity of 0.002 mg. per division, direct reading, using optical projection. The reticle is divided 0 - 1 0 0 0 in 500 divisions, the range being 1.0 mg. The case is mahogany with a separate glass-enclosed beam compartment. The beam, which is 5 inches in length is made of nickel chromium alloy, and the knife edges, which are held in place by tension screws, are agate (Fig. 2 0 ) .

A slightly modified Mettler instrument, known as model M-5 /SA, was designed for handling small samples, with means of loading the sample outside the weighing chamber. This model is used as a microbalance (Fig. 2 1 ) .

The Cahn electrobalance 1 7- 6 1 is a portable microbalance designed for weighing small samples. Current through a coil applies a torque to the beam and this current is so adjusted to make the torque equal and opposite to that caused by the sample. Since the beam is always returned to the same position, the torque is proportional to the current.

A new ultramicro balance is now available from Mettler, their model U M 7. It has a microscope reading system and a claimed reproducibility of ± 0.1 μg.

A new automatic recording vacuum semimicrochemical balance is available from Ainsworth. It has a capacity of 100 grams, sensitivity of 0.01 mg., and a range of automatic weight operation of 400 mg. It will weigh samples in air or inert gases, at atmospheric or reduced pressures, at room or higher temperatures, on the balance pan or suspended below the balance in a furnace for thermogravimetry or differential thermal weighing. It has remote control, built-in weights. The recorder charts weight, or weight and temperature against time.

* A rider type is also available.


T A B L E 8

ADDITIONAL INFORMATION ON R E F E R E N C E S * RELATED TO C H A P T E R 2

Following the plan used in Chapter 1, this table lists additional references to which the author wishes to call the attention of the reader. (See statement at top of Table 4 of Chapter 1, regarding completeness of this material.)

Books Belcher and Godbert, 10, 11 Clark, E. P., 18 Clark, S. J . , 20 Emich, 25, 26 Furman, 29 Grant, 34, 35 Niederl and Niederl, 65, 66 Niederl and Sozzi, 68 Pettersson, 70 Pregl, 73 Roth, 78-81 Steyermark, 88

Microchemical balances Brown, 15 Corwin, 21 Furter, 30 Hodsman, 38-40 Hull, 41 Kuck and Loewenstein, 49 Macurdy, Alber, Benedetti-Pichler, Car-

michael, Corwin, Fowler, Huffman, Kirk, and Lashof, 54

Martin, 55 Mitsui, 57, 58 Pfundt, 72 Ramberg, 74 Tuttle and Brown, 90 Wilson, 92

Microbalances Alber, 4 Ambrosino, 6 Asbury, Belcher, and West, 7, 8 Bradley, 13 Cahn, 17 Czanderna and Honig, 22

Microbalances (Conf.) Donau, 23 Edwards and Baldwin, 24 Emich, 25, 26 Gorbach, 31, 32 Graham, 33 Hales and Turner, 36 Ingram, 42, 43 Kirk, Craig, Gullberg, and Boyer, 44 Komârek, 46 Korenman and Fertel'meister, 47, 48 Kuck, Altieri and Towne, 50 Muller, G., 60 Miiller, R. H., 61 Pettersson, 70 Richards, 75 Rodder, 77 Sarakhov, 83 Wiesenberger, 91

Electronic balances Cahn, 17 Clark, 19 Feuer, 27, 28

Assay balance Bromund and Benedetti-Pichler, 14

Semimicrochemical balances Ainsworth, 2 Becker, 9 Clark, E. P., 18 Seederer-Kohlbusch, 85 Stock and Fill, 89

Analytical balance Niederl, Niederl, Nagel and Benedetti-

Pichler, 67

* The numbers which appear after each entry in this table refer to the literature citations in the reference list at the end of the chapter.

47 References

REFERENCES

1. Ainsworth, A. W. , Ind. Eng. Chem., Anal. Ed., 11, 572 ( 1 9 3 9 ) . 2. Ainsworth, Wm., & Sons, Inc., Denver, Colorado. 3. Ainsworth, Wm., & Sons, Inc., "Installation, Care and Use of Ainsworth Bal

ances," Denver, Colorado, 1943. 4. Alber, H. K., Ind. Eng. Chem., Anal. Ed., 13, 656 ( 1 9 4 1 ) . 5. Alber, H. K., Personal communication ( 1 9 4 0 ) . 6. Ambrosino, C , Chim. e ind. (Milan), 33, 775 ( 1 9 5 1 ) . 7. Asbury, H., Belcher, R., and West, T. S., Mikrochim. Acta, p. 598 ( 1 9 5 6 ) . 8. Asbury, H., Belcher, R. and West, T. S., Mikrochim. Acta, p. 1075 ( 1 9 5 6 ) . 9. Becker, Christian, Clifton, New Jersey.

10. Belcher, R., and Godbert, A. L., "Semi-micro Quantitative Organic Analysis," Longmans, Green, London and New York, 1945.

11. Belcher, R., and Godbert, A. L., "Semi-micro Quantitative Organic Analysis," 2nd ed., Longmans, Green, London, 1954.

12. Benedetti-Pichler, Α. Α., and Paulson, R. Α., Mikrochemie ver. Mikrochim. Acta, 27, 339 ( 1 9 3 9 ) .

13. Bradley, R. S., / . Sci. Instr., 30, 84 ( 1 9 5 3 ) . 14. Bromund, W . H., and Benedetti-Pichler, Α. Α., Mikrochemie ver. Mikrochim. Acta,

38, 505 ( 1 9 5 1 ) . 15. Brown, L. E., Anal. Chem., 23, 388 ( 1 9 5 1 ) . 16. Bunge, Paul, Hamburg, Germany. 17. Cahn Instrument Co., Downy, California. 18. Clark, E. P., "Semimicro Quantitative Organic Analysis," Academic Press, New York,

1943. 19. Clark, J . W. , Rev. Sci. Instr., 18, 915 ( 1 9 4 7 ) . 20. Clark, S. J . , "Quantitative Methods of Organic Microanalysis," Butterworths, Lon

don, 1956. 21 . Corwin, A. H., Personal communication; meeting of Metropol. Microchem. Soc ,

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The Microchemical Balance