ANALYTICAL CHEMISTRY
1004 tion of the formaldehyde, the pH drops until about 4.4is reached after 10 minutes. Read the buret containing the 0.2N sodium hydroxide solution, then titrate until the pH is 9. The volume of 0.2N sodium hydroxide used after the addition of the formaldehyde is equivalent to the nitrogen present. blank correction (usually between 0.15 and 0.25 ml.) should be determined and applied. This procedure has not been applied to compounds which are knon-n to be refractory, such as, heterocyclic and azo compounds, and nitro derivatives. It is satisfactory for organic compounds in which the nitrogen is known to be converted, quantitatively, into an ammonium salt by the usual Kjeldahl procedure. RESULTS
-4sample of ammonium sulfate containing 21,2294 nitrogen was analyzed four times. The mean result was 21.26% nitrogen and the mean deviation was 0.01%. Nitrogen in Nonrefractory Samples. Pure organic compounds and student unknowns containing nitrogen, which can be converted completely into an ammonium salt, were analyzed for nitrogen. The results of four replicate analyses in each case were as follows:
a
Organic Mean hIean Comp Sitrogen, % Value, 70 Dev, % 10 38 0 02 Acetanilide 10 36 8 11 0.01 Sulfanilic acid 8 08 Benzidine 15 20 15 2 1 0.01 Std. nitrogena 11.27 0.04 Sample 1 11 24 9.87 0.04 Sample 2 9 86 Student unknowns from Standard Sample Co., Ames, I o x a .
DISCUSSION
Elimination of the interference by selenium is accomplished by precipitation of selenium with sulfurous acid. The excess sulfurous acid is removed by boiling. Completeness of removal is determined by testing with dilute potassium permanganate solution To prevent the precipitated selenium from redissolving upon heating, a relatively large volume of Rater must be added folloi$ing the digestion. Fifty milliliters were adequate. The optimum p H for the addition of formaldehyde was found to be 6.8 to 6.9. Lower pH values caused the results to be too low. After the addition of the formaldehyde, it is necessary t o allow the pH to fall to the loF-iest value, which requires about 10 minutes. SUMMARk
The nondistillation Kjeldahl method, using selenium as a catalyst, is an accurate procedure. This procedure eliminntes the use of a concentrated solution of sodium bromide. LITERATURE CITED
Artmann, R., and Skrabal, A., 2.anal. Chem., 45, 5 (1907). (2) Haanappl, T. A. G., Pharm. Weekblad, 75, 510 (1938). (3) Marcali, K., and Rieman, W., IND.ENG.CHEM.,ANAL ED., 18, (1)
709 (1946). (4) (5)
Rupp, E., and Rossler, E., Arch. Pharm.. 243, 104 (1905). Willard, H. H., and Cake, W. E., J . Am. Chem. SOC.,42, 2646 (1920).
RECEIVED for review September 3,
1954.
Accepted November 26, 1954.
Application of Zero Grid Current Vacuum Tube Voltmeter to Measurement of pH with the Glass Electrode SAMUEL NATELSON Department of Biochemistry, Rockford M e m o r i a l Hospital, Rockford,
A vacuum tube voltmeter was developed to measure minute voltages through high impedances in order to measure ionic concentrations of certain ions through various types of glass and plastic sheets. Its application to pH measurement with the glass electrode is described.
A
SENSITIVE instrument developed to measure minute voltages through high impedances has been applied to measurement of pH with the glass electrode. The problems to be overcome were those of grid current, drift, noise, and low gain associated with vacuum tubes of low grid current. An absolute, stable reference point from which potentin1 could be measured was essential. Conventional vacuum tube voltmeter circuits tested did not satisfy requirements (1-6, 8-12). The circuit described takes advantage of the fact that potential, in an electron tube, is first negative with respect to the grounded cathode and finally positive, as one approaches the plate, going through a particular point of inversion where the voltage is zero with respect to ground. The location of this point, aside from the characteristics of a particular tube, varies with the temperature of the cathode, the voltage on the plate, and, when the tube has a screen grid, the voltage on the screen grid. Thus the location of this point may be continuously changed by varying the current through the filament, the voltage on the plate, or the voltage on the screen grid. When the field is adjusted so that the control grid is located a t this point of ground potential, no current will flow when a resistance connects the grid to ground. The effect will be that
111.
of zero grid current, and no potential will be placed on the grid as a result of the current flowing through the resistance. Such a circuit will measure minute voltages from sources with high internal resistance, and is independent of the internal resistance of such source. I t permits the use of inexpensive tubes which could not be used in conventional circuits because of excessive grid current. In conventional circuits the grid is a t a definite negative potential, to which is added, algebraically, the potential to be measured. A small percentage change of filament current or plate voltage, or aging of the component parts will produce changes in the base line. I n the circuit described, the reference point for measurement is absolute and can be readily checked, as it is the ground level of the instrument. Thus the zero adjustment in the potentiometer in Figure 1 will remain at the same point from day to day if constant temperature is maintained. The vacuum tube circuit indicates ground and the null point only. Thus the performance of the instrument, as to linearity, is the performance of the potentiometer circuit used to buck out the electromotive force measured. In Figure 1, electromotive force placed on the control grid, from the source measured, will upset the balance and cause grid current to flow, which is increased as the electromotive force increases. For this reason the instrument is most sensitive around the null point, This is an advantage, in that when large voltages are placed on the grid the needle of the galvanometer will swing wide but choke up as it moves large distances from the null, when high resistances are in the grid to ground circuit. The instrument may be used, as a direct-reading instrument, only over a limited range with sources of high internal resistance.
V O L U M E 27, NO. 6, J U N E 1 9 5 5
IO05 ments of the resistance values for R1, R2, R5, P2, and R6, with suitable glass electrodes, pH may be measured from 0 to 14 pH units. B1 may he replaced by two or more batteries in series by adjusting the resistances of the circuit. In this way the instrument becomes useful for potentiometric titrations over a wide raunee for ~
~
~~
~~
~~
.....
Switch S4. ^Switch S4 i s ceramic, nonshorting of lever-type design. This is advantageous in order to avoid jarring of the I
POTENTIOMETER
Figure 1.
Schematic diagram of zero wid current PH meter
8 , El. 81. 8 2 . 83.
C. E2.
G.
PI.
P2. P3. R1. R2.
Glsas electrode with saturated KCI bridze or d o m e 1 doe-
trode Mercury battery, Mdlory R M 12 or RM 42 30-volt. Burgess K20 or W2OPI mr Everyready 430-E 250-npf. ceramic condenser, Centrnlab Glass electrode Galvanometer, G. M. Laboratories, Chioago, 0.06 pa./mm.. 1.I"" ..."hms ......-, No ..-.S,n--F."i -. -. 200 ohms 2 watts Mallow M200P 2000 oh& 10 turns, Helipot or G. W. BOTE,Micropot 20,000 ohms. Helipot, 10 turns 360 ohms wire-wound 0.5 watt 5% 200 ohms: 2 watts. M&y M260P
than 10% ofits value. Tube Circuit. The 1U6 tube was chosen because i t has low filament current (25 ma.) with sensitivity of the same order as that observed with higher filament current in other tubes. Actual filament current in the tube in Figure 1 is 18 ma. The numbers listed for the 1U6 tube parts in Fi&e 1are the tube pin numbers. As used in Figure 1, most tubes will have an amplification factor of 5 to 7 &8 measured by a v&cuum tube voltmeter inserted in place of the galvanometer. For the more commonly available 1L6 tube, R9 is omitted, R 8 is 17 ohms, and B2 is mercury battery RM42.
R3,R4,R5.R6500ohms.0.15wstt,IRCWWlOJ,I% R7. 1000 megohms Victoreen R8. 22 ohma 0.5 w'ntt 107 17 ohms if 1L6 tube is used
R9. R10.
R11. R12. R13. S1, S2, 83. s4
15,OOO~hm~. 0.15 &stt,'fRC WWlOJ, 1%(omit if iL6 tube is used) 5000 ohms, 0.15 watt. IRC P?WlOJ, 1% 3000 ohms 0.15 watt, IRC P?WlOJ, 1% 20.000 oh168 Helipot. 10 tu1'ns 4700 ohms, &re-wound. 1 WI Ltt, 10% All one 3 PDT lever-type switoh looking, Switchcraft 300QL Ceramic lever-type,ayiteh. Centralab, Milwaukee; No. 010309, 1 pole, 3-positmn. ~ p n n greturn to center
Elimination of grid current in this circuit permits the measurement of electromotive force through very high impedances, thus permitting the use of two glass electrodes in pH measurements (Figure 3). Leakage of current from the grid pin seal to ground will decrease the sensitivity of the instrument but not change the nullpaint. A wide variation may be made in the circuit design, tube used, and construction of the potentiometer, using the principle of maintaining the grid voltage a t ground so as t o avoid development of voltage on the grid from the internal resistance of the source. The instrument described below, and shown in Figures 2 and 5, was designed for measuring the PH of biological fluids and has been in routine use for more than a DETAILS OF DESIGN OF prl
Potentiometer. The potentiometer circuit 19 Shown m Figure 1. Wide variation in its design is permitted, as long as P2 is calibrated for 59.1 my. for one 360" turn a t 25" C. The current flowing through this potentiometer when S1 is closed is approximately 1 ma. With this drain B1 has a life of 3600 hours. In actual test, for 500 hours continuously, variation in voltage was less than 0.3% or 0.18-mv. drop from 59.1 mv. This corresponds t o approximately 0.004pH unit. For the purposes of this instrument this cell is aatisfaetorv. As this instrument is ndt affected, over a wide range, by increased resistance between ground and grid, the potentiometer circuit is not limited to low resistance values m with conventional circuits. Current drain has been reduced t o 100 pa. by increasing resistances of the potentiometer tenfold, with perfectly satisfactory results. A standard cell such 8.8 the Weston or Clark may be used with long life. To increase standard cell lifeSl may he closed only when S4 is switched to position 3. In this circuit the temperature control changes the spread for 1 pH unit from approximately 54.5 to 63.5 mv. per pH unit or equivalent t o a temperature range of from approximately 4" to 40"for 270" rotation. This dial is calibrated 8.8 described belaw: In the instrument shown, pH from 2 to 12 is measured for a 3600 turn. With a 14-turn helical potentiometer and suitable adjust
i n one vanaxion f f i wasvmaQiem~teaaoi ~ ffiuGhangmgthe voltage an the screen grid changes the sensitivity of the tube, because i t changes the tube conductivity between 2 and 3. R12 is used t o compensate far any drop in voltage from B3 and 60 keep screen arid and plate voltages constant. B2 i s t h e me
