lrared Analvsis of Solids by Potass omide Pelllet Technique TSWN ana n. LCC
J M I I ~
ratory, Dow Corning Corp., MidlarId, Mich.
.
.
, . .reennique or preL ne porassiuni ~romzae pressed pelier paring insoluble samples for infrared examination is comparatively new, and has not been thoroughly studied. In this work, an investigation of optimum pressing oonditions was carried out. It was found that for I-inch diameter pellets, a total force of 20 tons applied for 20 minutes gave permanently clear disks. Fairly satisfactory results, however, could be obtained by pressing 3 to 5 minutes at slightly higher pressures. A simple evaouable die is described in which the pellet is Dressed directlv into a holder which fits into the me=-
qualitative and quanti-
ROBABLY the most notew thy development of recent year s ,i .rmnin. fn- inc,.a.a. t h e taohninsrr nf n v o n ,",, .a
tn
andylysis has been the introduction of the potassium bromide pressed plate method (f-8). I n this method the sample is ground with powdered potassium bromide, the mixture is placed in a die which is then evacuated, and the die is compressed. (Evacustion eliminates occluded air which nould expand when the pressure is released and produce an undesirable milkiness in the pellet.) The resulting clear or translucent pellet gives an infrared Or mulling spectrum free from interfering hands Of Went'. With proper precautions, the method can he used to carry out quantitative analysis. Scheidt et ai. ( 6 )and Anderson (1,$1 have described dies suitable for making potasrium bromide pellets. Scheidt's apparatus . . . cmtruet. Anderson's method
.
. . .- .
Figure 2.
.
Unassembled Die
produces very small pellets and while his equipment has the advantage of simplicity minute-sample requirements, it requires modification of the optical path of the spectrometer aith loss of energy, The purpose of this paper is to describe a simple die and pellet holder which give very satisfactory results, including high energy transmission, and yet require only 2 to 10 mg. of sample. The 1. inch-diameter pellet is pressed directly into n holder designed to fit into the cell holder f, a Baird I,,,,., double.beam in. frared spectrometer; this eliminates the of the pellet before the spectrum is run. The design can he easily adapted to other speCtrOmetor8. A simple technique of grinding the sample with potassium bromide and a volatile solvent often gives BUeCtra. camvarable in uualitv . . to solution spec:tra. APPARATlJS
A die was oanstructed which could be used in im unmodified 1 hrirlra,>lilahnrainrri --.-...., ..,_.I _..-.--. Tha -.."Ai-.e, shown in I+..- -, n^.l
~VLIQQ
sistg of five parts: base, pellet holder, sleeve, phnger, and n v.i.n~ m ~ ~ mime. To ensure vacuum tiehtness durinc ~o evraniat,mn ~ ~ . ~ r . .n _t b prksing, 0 ring seals are >sed between the base and pellet holder, the pellet holder and sleeve, and on the shaft of the plunger. Air is evacuated from the cavity through an '/&ch hole through the side of the sleeve, using a rubber hose connection t o a mechanical vacuum pump. When the die is compressed slightly. the surina exerts enough vressure to eive a mod seal. The shaft on the phnger and thesmall inset in &e haseare made of hardened Atlantic No. 33 tool steel; the faces on these two pieces are flat and highly polished. Figure 2 is a photograph of the unassemhled die. The Dress is a 30-ton Wahabash hvdranlir press, Model 30-11. ~~~
CAVITY FOR 0 RING
~
~~~
~
TECIINIQUE
Powdered potassium bromide used in making pellets was obtained by grinding discarded cell windows in % dry box. The powder was not sieved, since subsequent grinding with the sample reduced the particle size satisfactorily. The powder was dried in B vacuum oven, and kept in a desiccator containing phosphorus pentoxide until used. Reagent grade potassium bromide showed small amount8 of potassium nitrate and watpr, 1765
ANALYTICAL CHEMISTRY
1766 and gave translucent pellets instoad of the clear wafers which resulted from u i n g discarded window material. Random-siaed crystalline potassium bromide is also satisfactory and may be obtained from the Harshaw Chemical Co., Cleveland, Ohio. The technique used in running a sample is as follows: About 4 mg. of sample and 1.2 to 1.5 grams of potassium bromide me ground in an agate mortar along with 1 ml. of a volatile solvent (such as acetone, benzene, diethyl ether, etc.). The slurry is ground until the solvent has evaporated, whereupon the mixture is ready for loading into the die.
7/, , I 2 &
, ,
I
,
, , ,
,
1 1 0 30
20
3
2
Figure 3.
11 5 6 7 WAVE LENGTH, P
Transmittance
9
8
IO
Tons Pressure
VS.
A study was made to determine optimum remure and pressing time. For this study, the potassium bromiie was ground dry in an agate mortar, and loaded into the cavity as reproducibly as possible. Pressure tried ranged from 13.5 to 22 tons in total force, and pressing time from 3 to 40 minutes. Clear pellets were obtained in some instances for short pressing times; however, these pellets became translucent upon standing, whereas pellets pressed for longer periods of time remained transparent. Sati8factory pellets, one inch in diameter, were obtained for a pressing time of 20 minutes and total force of 20.5 tons; longer pressing
thicknessbf the pellet, uniformity of d~ststrihutio
