Apparatus Using Semiconductor Electrodes for the Measurement of Acid Concentrations J. P. McKaveney Garrett Research and Development Division, Occidental Petroleum Corporation, La Verne, Calif 91750
C . J. Byrnes Crucible Materials Research Center, Colt Industries Inc., P. 0. Box 988, Pittsburgh, Pa. 15230
A novel method and apparatus for the measurement of acid concentrations using semiconductor electrodes i s described. Prior chemical or instrumental approaches required the use of a variety of chemicals and or separation techniques to minimize metal ion interference. Germanium, indium antimonide, and silicon have proved satisfactory for acid measurements. The semiconductor serves as the anode of an electrochemical cell operated at a positive potential with respect to a cathode of material inert to the electrolyte (stainless steel, titanium, or platinum). However, silicon was the best all around semiconductor examined, as it does not appear to be influenced by high metal ion concentrations as are germanium and indium antimonide. Ammonium fluoride is a key component of the electrolyte both for preventing film formation at the electrodes as well as increasing electrode sensitivity. Calibration curves have been drawn for hydrochloric, hydrofluoric, sulfuric, nitric, acetic, perchloric, phosphoric, sulfurous, oxalic, citric, tartaric, and sulfanilic acids. The approach was developed to monitor acid concentrations in stainless steel and titanium pickling baths (HBSOI, "0, HFHN03). No discernible metal ion interference has been met when using the silicon electrode.
MEASUREMENT OF ACID concentrations in the presence of hydrolyzable cations has always been difficult. Efforts to use complexing or chelating agents have not produced accurate data as the agents themselves often cause an alkaline or acid reaction following hydrolysis or chelation. Physical methods such as conductivity or specific gravity are usually indirect and are only applicable to single acids such as hydrochloric or sulfuric. In the manufacture of stainless steels, it is not uncommon to use consecutive baths of sulfuric, mixed nitrichydrofluoric, and finally nitric acid alone. An earlier communication ( I ) briefly described the investigation which resulted in the development of a method and apparatus for the measurement of acid concentrations. This paper gives a more detailed account of the experimental work which occurred during the development. The initial program was aimed at the development of a monitor for hydrofluoric acid (HF) after it was found that Turner (2) had published a method for fluoride ion using concentrated nitric acid as an electrolyte. Turner had used N-type silicon as the anode of a n electrochemical cell with platinum as the cathode separated by a potential of 1.56 volts. He indicated that the limiting current produced was proportional to the amount of HF. L. Steinbrecher and coworkers ( 3 ) extended Turner's approach to include a constant source of light visible to the silicon electrode in order to make it more sensitive to lower concentrations of fluoride ion. Both Turner and Steinbrecher reportedly used N-type silicon of 0.7 ohm-cm resistivity for the measurement of fluoride ions. (1) J. P. McKaveney, Anal. Lett., 3, (l), 17-22 (1970). (2) D. R. Turner, ANAL.CHEM., 33,959-960 (1961). (3) L. Steinbrecher et al., U. S. Patent 3,129,148 (1964).
Preliminary Studies with Silicon and Other Semiconductors. It required several months for the investigators to find that a 0.7 ohm-cm N-type silicon anode and platinum cathode would not give satisfactory response to nitric-hydrofluoric mixtures at concentrations required for stable current measurement. Continued use of the electrode caused film formation at the anode surface with a resultant drop in sensitivity. Intrinsic as well as various resistivity N-types of silicon were examined before it was discovered that N-type with a maximum resistivity of 0.05 ohm-cm was necessary. I n earlier work on the measurement of free HF in stainless steel pickling baths, one of the authors ( 4 ) used the bleaching effect of H F on ferric acetylacetonate. This procedure worked well for baths containing cations such as iron, chromium, aluminum, or nickel but failed for baths containing titanium. With titanium pickling H F - H N 0 3 baths, the ferric acetylacetonate was bleached by the TiF4 complex in addition to the free HF so that total fluoride was measured. The N-silicon electrode overcame this problem as it responded only to free H F and not complexed metallic fluorides. Alibaths (up to 25 grams of " 0 2 and quots of mixed "01-HF 5 grams of H F per 100 ml) on dilution to 200 ml with water and containing 0.250 gram of H F produced current readings of 750 PA. The anode area was about 2.5 cm*. Samples with only H F produced nearly the same current for HF as in those with equivalent HF and high concentrations of " 0 3 the original sample aliquot. Baths containing wide ranges of iron, chromium, aluminum, nickel, titanium, and manganese were successfully analyzed for free HF. After solving the problem of measuring free HF, it was reasoned that a semiconductor such as germanium might be suitable for measurement of other acids such as nitric and sulfuric. Efforts were immediately successful with germanium as a sensor for acid measurement, but when metal salts were present an interference was apparent. The use of fluoride salts was necessary to prevent film formation at the electrode surfaces in the presence of metallic ions. The use of oxidizing agents such as hydrogen peroxide gave added sensitivity. However complete freedom from interference by cations found at concentrations in pickling baths was not obtained and other semiconductors (indium antimonide and gallium arsenide) were also examined. Again data were obtained to indicate sensitivity to acid by the semiconductor but dissolved salts at higher concentrations interfered. Complete details on the work with germanium and the other semiconductors will be discussed in a future publication while some comparison data will be covered in the discussion section of this paper. DEVELOPMENT WORK WITH SILICON
After only limited success in developing an electrode from germanium, indium antimonide, or gallium arsenide that (4) J. P. McKaveney, ANAL.CHEM., 40,1276-1279 (1968). ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
1023
z
o
w i t h 0.3009 H202
o
o
~ t
250v
A
,6001 and2209 NH4F YI
2 ooot-
j / /
-I
/
/
1
P,
L
za
Q,
1200
20
800 500
4 00
0
I
I
I
I
I
0.04 0.08 0.12 066 0.20 H 2 S 0 4 ( g r a m r / 2 0 0 ML 1
Figure 1. Influence of oxidizing agent on N-silicon (3.0 cm2) anode response would be all inclusive to cover all ranges of acids and metal salts present in steel pickling baths, attention was directed again to silicon. The work with the fluoride salts using germanium led to the idea of converting the N-type silicon electrode, from a selective hydrofluoric acid sensor to a general acid sensor. As hydrofluoric acid is considered a weak acid (pK, 3.14), it was reasoned that a stoichiometric equivalent or excess of a soluble, noncomplexed fluoride salt would favor the formation of H F in the presence of hydrogen ions, viz: H++ F - e H F An experiment using N-type silicon (0.04 ohm-cm) was performed. The electrode was approximately 1/4 inch in diameter by 1 inch in length with a 1-inch length platinum wire as counter electrode or cathode at a potential of 1.35 volts d.c. The ohmic contact was made to the silicon by the method of Sullivan and Eigler (5) using electroless nickel deposition and soldering to the nickel plating. An electrolyte of 150 ml of 5 grams NaF/liter was used and it contained 1.00 ml of sulfuric acid (3.32%). A current reading of 76 pA was produced. Upon repeating the same test in the presence of 4 drops of hydrogen peroxide, a current of 165 p A was produced, The current reading in the presence of just sodium fluoride and hydrogen peroxide was 9 pA. The data indicated an apparently successful means of measuring sulfuric acid, using a fluoride salt with N-type silicon and an oxidizing agent such as hydrogen peroxide to improve sensitivity. Further testing revealed that ammonium fluoride would have to be substituted for sodium fluoride, as a precipitate believed to be Na3FeFBappeared to be occurring when iron bearing, nitric-hydrofluoric acid solutions were analyzed. The switch to the ammonium salt eliminated the salt precipitation effect which could form a coating on the silicon sensing electrode. Use of Oxidizing Agents. It was also discovered that metallic ions such as iron, chromium, nickel, etc. do not interfere, unless they are present in an oxidation state, which ( 5 ) M. V. Sullivan and J. H. Eigler, J. Electrochem. Soc., 104, 226
(1957). 1024
1.0 2.0 3.0 Grams NHqF/200ML
0
I
4.0
Figure 2. Response of acid to NH4F with N-silicon anode using 0.250 grams of HN03diluted to 200 ml may consume hydrogen ions of the sample in the presence of an oxidizing etchant, e.g., Fez+
+ H z O +~ 2H+
-t
2H20
+ Fe3+
As a result, the use of oxidizing agents such as hydrogen peroxide, potassium dichromate, and potassium permanganate had t o be abandoned when analyzing solutions containing ferrous iron or oxidizable cations. Solutions containing ferric ion can be readily analyzed in the presence of these oxidizing etchants, with a varying iron concentration not affecting the acid measurement. Ammonium persulfate was chosen as an oxidizing etchant, as it did not appear to consume hydrogen ions when used in the presence of ferrous iron. The most promising concentration was 30 grams of (NH& SzOswhen used in conjunction with 80 grams of ammonium fluoride per 500 ml of electrolyte. Although several of the oxidizing agents produce increases in response of over 50% in current reading, quite good calibration data can be obtained with only ammonium fluoride in the electrolyte (Figure 1). Fluoride Electrolyte. Another finding that occurred at this stage of investigation was the apparent increased electrode sensitivity for acid when using increased amounts of ammonium fluoride beyond the stoichiometric equivalent in the electrolyte. The earlier development work had arrived by chance at the use of 10 ml of 220 grams of NH4F per liter, in each 200 ml of measuring solution containing acid. However, the pronounced difference in meter response for equivalent amounts of hydrofluoric acid, with and without the electrolyte, drew attention to the anomaly. Figure 2 shows a plot using 0.250 gram of nitric acid with varying NH4Fin the electrolyte all diluted to 200 ml. The stoichiometric equivalent for the H F produced is 450 p A compared to the 2200 FA obtained using excess electrolyte. Figure 3 indicates a typical calibration curve for hydrofluoric acid and the effect of an added electrolyte. The curve without electrolyte was prepared for the measurement of H F in mixed HF-HNOI baths. Temperature Effect. Like most analytical sensors, temperature also affects the response of semiconductor electrodes. One can overcome this 'problem by preparing a calibration curve for the temperature at which the majority
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
-
1
1
Table I. Temperature Effect on N-Silicon Acid Response Concn, grams/200 Current, pA Acid mla 15 "C 20 "C 25 "C 30 "C 211 238 265 180 0.0400 &SO4 196 219 174 148 "03 0.0380 1062 1300 885 0.1598 765 HzSO4 646 775 949 558 "03 0.1500 a Solution containined 2.20 grams of NHIF per 200 ml.
I
-
5000-
4 000v)
z
-
0
3000-
-
0 0
of measurements will be made and daily analyzing a standard of known acid concentration to correct for temperature variations. Other possibilities are to use dilution water from a constant temperature bath in making measurements or to build a temperature compensator into the semiconductor circuit. The easiest approach for the present authors was the use of constant temperature water for dilution of the acid sample. The data of Table I indicate the effect of temperature for sulfuric and nitric acids on an anode of N-silicon (about 3 cm2area). Normality Relationship. The early work with HNO,-HF acid mixtures indicated that when using an electrolyte of ammonium fluoride the response of the silicon electrode appeared to be related to the normality of the respective acids. Accordingly a series of tests were performed using different acids and relating the current response to the normality of the respective acids. The data are shown in Table 11. Only the normality values for HNOI and HCL are based on standardized solutions. The values for &Sod, H3P04, and H F are calculated from dilution of concentrated acids. Response of Organic Acids. While the primary aim had been to develop a method or instrument for the determination of sulfuric, nitric, and hydrofluoric acids, it became apparent that the technique could also be applied to other acids. Accordingly, calibration data were also obtained for hydrochloric, phosphoric, acetic, sulfurous, and p3rchloric acids. Table I of Reference ( I ) indicated some information from those calibrations. Table I11 of this investigation, indicates data for a number of organic acids obtained with a silicon
I
I
L
0
roooC
I
/ w i Et1ehc t2,2gNH4F{ r olyte
o
o
without
Elect o ro lyt
Grams HF/200 ML Figure 3. Response of N-silicon anode (5.90 cm2) to hydrofluoric acid anode having an area of about 5.9 cm2. The data of Reference ( I ) referred to an anode of about 4.7 cm2. Acetic and sulfuric acids are also included in this table for comparison purposes, as the electrode areas were slightly different. The calibration data of Reference (1) used a modified electrolyte with an oxidizer and were also plotted for direct reference to the original acid content of the sample. EXPERIMENTAL
Development of Acid Concentration Meter. As preliminary data for various acids in the presence of metal salts appeared quite encouraging, it was necessary to design a fairly rugged portable acid sensor that could be used for measuring acids at plant pickling lines. The all important semiconductor anode was the weak link, because of its brittle nature and sensitivity to light. By this time, over two years of in-
Table 11. Response of N-Silicon Electrodea to Acid Normality Condition Normality Current, p A 1.00 ml, 8.86 g HS04/100ml to 200 mIb 0.00904 1250 "0s 20.00 rnl, 0. lOON HN03to 200 ml 0.01000 1400 HCL 20.00 ml, 0 . lOON HCL to 200 ml 0.01000 1400 H3P04' 1.00 ml, 28.72 g H3P04/100 ml to 200 ml 0.01465 2125 HF l.00m1, 5.77gHF/100mlto200ml 0.01442 2085 a Area of silicon electrode about 5.9 cm2. * Based on each 200 ml containing 2.20 grams NH4F at 25.3 "C. Normality of Hap04 calculated for single strong hydrogen ion. Acid
pA/O.OlON 1393 1400 1400 1450 1446
Table 111. N-Silicon Anode (5.9 cmz) Response to Organic Acids Slope Acid Acetic Citric
Formula HCzHsOz H,Ce"0?
K (ionization) Molarity 8.82 x 1 0 - 3 1.75 x 10-5 8.7 x 10-4 2.38 x 10-3 1.8 x 10-5 4.0 X 10-6 Oxalic H2Cz04 6.5 x lo-* 3.97 x 10-3 6.1 X Sulfanilic HSOaC6HrNHz 5.9 x 10-4 5.23 x 10-4 Tartaric HzC4H4Oe 9.6 x 10-4 3.33 x 10-3 2.9 X x 10-1 4.52 x 10-3 Sulfuric 4 (Reference) 1.2 x 10-2 Based on 200 rnl of solution and containing 2.20 grams of NH4F at 25 OC.
Grams acid per 200 ml 0.1058 0.0914
Current, pA 475
0.0714
1,000
1259
0.0181 0.1001
69 885
1319 1329
0.0885
1,200
1327
pA/0.010
500
normal acid 538 1050
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
1025
t
1.30V LAPPROX.1
I
-'I + POLYVINYL CWLO ELECTRODE CAP MICRO$MMETER
UNTER ELECTRODE TUBE INLESS STEEL,PUTINUM,ETC.I
GERMANIUM OR SILICON ELECT
SOLUTION UEASUREO
Figure 4. Acid combination electrode and circuit
vestigation had occurred with hydrofluoric acid sensors designed along the lines suggested by Turner. It had become apparent that a single combination electrode of the immersion type would be preferred. After several trials, a design similar to that shown as part of Figure 4 was used. It consisted of a 1-inch rod of N-type silicon (0.25 inch in diameter) placed concentrically and recessed within a 1-inch 0.d. 304 type, stainless steel tube ('I8 inch id.). Both the silicon anode and stainless steel cathode were attached to a polyvinyl chloride cap. About 1 inch of the cathode and inch of the anode were exposed by immersion in a solution to be measured. The cathode had small holes (a/is inch in diameter) drilled around the top outer circumference about inch apart. This would prevent air locks of solution and at the same time provide a dark surrounding for the anode. A photograph of the assembled instruments is shown in Figure 5. Although the instrument could analyze any type of acid, each one built was calibrated for either sulfuric or nitric (also HNOrHF) acids. These are the principal acids used for pickling at steel mills, rolling stainless steel, and nf the titanium alloy sheets. The current .... m n Do_ ~_. .......instnlment ........... is from about 0.001N to 0.050N acid (following dilution)
using anodes from 3 to 6 cm2 in area. The battery and associated variable resistors for controlling the current attenuation are housed within a steel casing (8 X 8 X 5 inches), The steel casing is also sealed to minimize corrosion from the atmosphere found near many acid pickling tanks. Auxiliary Apparatus. A constant temperature water bath (heating) of 5 gallon capacity for the dilution water is required. It should be capable of holding temperature at 30 & 0.5 "C for prolonged periods assuming ambient temperature to be about 25 "C. Also needed is a 1.00-mI Eppendorf micropipet with plastic tips and pushbutton sample filling and release. Reagent. Ammonium fluoride electrolyte, 220 gramspiter, is prepared by dissolving 220 grams of NHnF in water and diluting to 1 liter. It is then stored in a polyethylene bottle. Procedure for Acid Analysis. To a 200-ml plastic volumetric flask, add 10 ml of electrolyte and dilute with constant temperature water to about 100 ml. Follow with a 1.00-ml sample of acid (1.0 to 30.0 grams of acid per 100 ml) from a micropipet. Dilute to volume with constant temperature water and mix the contents of the flask. Transfer the contents of the flask to a 400-ml plastic beaker, add a plastic encapsulated stirring bar, and place the beaker on an automatic stirring apparatus. Regulate the stirring rate to medium speed. Immerse the electrode assembly in the stirred bath so that the tubular cathode is about one-half inch above the rotating stirring bar and the top circumference holes of the stainless steel cathode are beneath the surface of the solution. Turn on the instrument and rotate the range selector to obtain a meter reading which is on scale. After about 1 minute, obtain the ammeter reading and multiply by the proper range factor. Record the value obtained. Refer to a calibration curve to obtain the acid concentration. For acids with ionization constant values greater than 1 X concentration can be read directly in normality while to 1 X lo-@)the confor weak acids (K values 1 X centration is best read in grams per volume. It should be remembered that hydrofluoric acid, when present with other acids, can be determined directly with the silicon electrode version of the instrument. The electrolyte salt is omitted and the aliquot size increased 5 to 10 times because of reduced sensitivity for hydrofluoric acid in the absence of electrolyte. This will involve a separate calibration for mixed acid solutions which are analyzed for H F without electrolyte. The germanium anode version of the instrument has been found suitable for the measurement of acid in very dilute solutions, as is encountered in metal rinse waters and mine acid waters. RESULTS AND DISCIUSSION
- . . . . . - . . __ . . Sample Applications. lable IV indicates
data obtained with the silicon electrode for various sulfuric, nitric, and mixed nitric-hydrofluoric acid baths. Comparison data for
Table IV. Data for Various Acids with the Silicon Electrode sample 7L 6G1T 1GZT 8
6G3T 4G3T 20
1L-3T 4L2T 6L2T
1026
HF,
HsSOI,
si100 ml
si100 ml
3.2 7 ...3
16.4 15.7 6.40 13.3 7.9 10.0
1.05
11.3 5.1
0.30 2.15
Acid (Chem): gjl00 ml 3.3 6.9 16.1 16.0 6.5 13.4 7.9 10.2(HNOs) l.lO(HF) 11.2 ("01) 0.33 (HF') 5 . 1 ("02) 2.32(HF')
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
Fe
g/100 ml Cr
0.2
...
3.2 4.7 6.1 1.1 0.5
0.4 1.5 1.9 4.2
Ni
...
... ... ... ... ... ...
0.7
0.6
0.6 1.6
0.3 0.1
...
...
... ... ...
...
the chemical method used to determine H2S04and “01 (6) as the method for H F (4) are shown as well as some data for the iron, chromium, and nickel contents. The results are the average of three or more analyses per sample. Values except for H F are not reported beyond the first decimal since the 1.00-ml pipets used for sample analysis are not N.B.S. certified. A calibrated 5.00-ml pipet was used for the H F values. Data were also obtained for “03-HF baths used for titanium pickling but are not shown since the metal content is not ordinarily determined. However, electrode values for acid agreed in all cases with chemical values within sampling errors. To check repeatability and applicability to other pickling acids, a sample containing 13.60 grams of HCL and 2.78 grams of iron as FeClz per 100 ml. was analyzed 10 times. The statistics gave an average value of 13.37 i 0.20 grams of HCL per 100 ml and the coefficient of variation calculated to 1.50%. The 13.60 acid value used as a basis for accuracy was obtained by the procedure described in Reference (6). In the stainless steel and titanium metal bearing samples analyzed in this investigation, nearly all baths contained major amounts of Fe, Cr, Ni, and Ti with lesser amounts of Mn, Mo, V, and Si. The concentrations of these metals in the final sample aliquot would require very little complexing by fluoride where fluoride complexes are possible. Normally NiZ+,Fez+, and Mn2+ do not form fluoride complexes under the conditions of analysis. In mixed HN03-HF baths, elements such as Cra+, Fe3+, Ala+, etc. should already contain complexed fluoride. This would leave sulfuric baths which can contain Cr3+ (also noncomplexed Fe2+ and Ni2+) and nitric baths containing principally Fe3+ and Cr 3+. Fortunately these baths very seldom contain greater than 2.0 grams per 100 ml of Fe3+ or Cr3+. One-milliliter aliquots for analysis would thus contain only about 0.02 gram of each element requiring about 0.006 gram and 0.012 gram of fluoride for complexing respectively by Fe3+and Cr3+[see Reference (4)]. The 1.10 grams of fluoride ion present in each diluted aliquot would still be in such large excess that electrode response would not be appreciably affected. If the electrode should be used in applications where low acid concentrations and high quantities of elements which complex or form precipitates with fluoride such as ferric iron or rare earths are encountered, a preliminary sample treatment may be necessary. Sufficient N H 4 F would have to be added to complex the ferric iron and precipitate the rare earths prior to the addition of electrolyte for sample analysis. Table I1 indicates the normality relationship which exists for acids having ionization constants greater than that of H F (7.2 X 10-3. This fortunately encompasses all of the usual strong acids and those weak acids with ionization constants greater than HF. The data indicate that phosphoric acid can be measured as a strong acid if it is treated as a single Kz = 6.2 X lo+, and hydrogen ion species (K1 = 7.5 X K3 = 4.8 X The data of Table I11 indicate the response of the electrode to water soluble organic acids. The normality calculation is based on acetic as monobasic, citric as dibasic (the third hydrogen apparently does not respond), oxalic as dibasic, sulfanilic as monobasic, tartaric as dibasic, and sulfuric as dibasic. It is evident that sulfanilic, tartaric, and sulfuric all behave as strong acids with respect to the silicon electrode while citric and oxalic acids respond as slightly weaker acids
and acetic as being very weak. If the handbook values are correct, it is not completely understandable. Why should citric acid with K ion values of the first two protons being very similar to tartaric be a much weaker acid than tartaric in reference to silicon? Also both citric and tartaric acids in having their second K ion values very close to that of acetic acid respond as much stronger acids. It may be a matter of citrate and oxalic having much higher complexing ability for silicon than acetate. Aspects of Semiconductor Theory and Response. In a companion paper to his original proposal of using N-silicon as a fluoride monitor, Turner (7) indicated the semiconductor electrochemistry involved the formation of electron holes at the surface in an etching solution. In the present situation where very low resistivity material (0.05 ohm-cm) had to be used to obtain reproducible etching over an extended period, the silicon appears to be functioning as a degenerate semiconductor. As such, the silicon dissolution does not appear to be dependent on the supply of electron holes at the surface. Recently Gereth and Cowher (8) have explained this phenomena for germanium; they suggest the dissolution process to be governed by a conduction band mechanism and therefore no holes are required. Regarding electrode material and life, all of the silicon used had the 111 crystal orientation and electrodes prepared over three years ago are still functioning in daily analytical applications. The etching required for a single determination is microscopic so that unless the electrode is used as part of a continuous process monitor, electrode wear should not be a problem. Output current is a direct function of the electrode area as well as applied potential. By using smaller area electrodes higher acid concentrations for a given current reading can be measured. Also smaller electrodes will tend to have a lower over-all temperature effect. In the work on the fluoride electrolyte, the necessity of using a large excess of NHIF in acid measurement was indicated in Figure 2. Beyond 2.0 grams of NH4F per 200 ml, the law of diminishing returns takes effect for the electrode sizes and acid concentrations covered. Recently two 1.00-ml aliquots of 17.72 grams of HzSO4 per 100 ml were diluted to 200 ml with electrolyte. One diluted sample contained 2.20 grams of NH4F while the other contained 4.40 grams of NH4F. The former aliquot produced a current response of 3900 PA and the latter 4600. Thus a 100% increase in the quantity of electrolyte only resulted in a current increase of 17 %. Table V indicates data obtained with some silicon, germanium, and indium antimonide electrodes using dilute as well as fairly strong solutions of HzOZ,NHdF, and (NH& SzOs with HzS04. The earlier work had used fairly dilute solutions, but the silicon data indicated that stronger solutions improved sensitivity and eliminated metal ion interference. It is obvious from the data that germanium, while responding with sensitivity to acid, is also influenced by other electrolyte chemicals. This explains the earlier observation that increasing metal salts appeared to contribute to the current. Also with germanium, oxidizing agents appear to produce greater current sensitivity on an equivalence basis than fluoride salts, although fluoride salts are necessary to minimize coating of the electrodes. In the case of indium antimonide, fluoride salts rank next to acid in their sensitivity with oxidizing agents, being least sensitive on an equivalence basis. It is apparent that the
(6) A. Moskowitz, J. Dasher, and H. Jamison, Jr., ANAL.CHEM., 32, 1362-1364 (1960).
(7) D. R. Turner, J. Electrochem. Soc., 108, 561-563 (1961). (8) R.Gereth and M. E. Cowher, ibid.,115, 645-649 (1968).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
1027
Table V. Electrode Response to Electrolytes Electrode Ge Ge Ge Ge Ge In-Sb In-Sb In-Sb In-Sb Si Si Si Si Si
Type N-0.04 ohm-cm N-0.04 ohm-cm N-0.04 ohm-cm N-0.04 ohm-cm N-0.04 ohm-cm N-0,004 ohm-cm N-0.004 ohm-cm N-0 .004 ohm-cm N-0.004 ohm-cm N-0.03 ohm-cm N-0.03 ohm-cm N-0.03 ohm-cm N-0 .03 ohm-cm N-0.03 ohm-cm
Area, cm2 4.7 4.7 4.7 4.7 4.7 1.0 1.0
1.o 1.o 3.2 3.2 3.2 3.2 3.2
HzOz, g 0
0.300
0 0
0
(9) H. Gerischer, "Advances in Electrochemistry and ElectroChemical Engineering," Vol. 1, Interscience Publishers, New York, N. Y.,pp 139-232, 1961.
0 0 0
g
HzS04, g
0.010
1.60
0.010
0.600 0
1.60 1.60
0.600 0.600
0 0
0 0 0
0 0 0 0 0
2.20 2.20 2.20 2.20
0 0
0
0.100
0.300 0.300
"aF,
0
0.600 0 0.600
0 0
g
0.010 0 0
0 0 0 0 0 0 0
concentration of fluoride salts would have to be reduced considerably when using indium antimonide as an acid sensor. The data of Table V indicate that silicon is the best allaround acid sensor because of its freedom from either electrolyte or metal ion current contribution in the absence of acid. Also the sensitivity of the oxidizing agents examined or fluoride salts occurs only in the presence of acid when using silicon. The various sensitivity effects of these semiconductor materials are not presently explainable on the basis of the classical electrode theory. As the experimental work progressed, it became apparent that many factors influence electrode response in addition to electrode area and electrical potential. Electrode resistivity and type (N, P, or Intrinsic) are critical, as well as dislocation density (as observed from etch pits with a microscope) and crystal orientation. There was also some evidence that the elemental dopant for a given resistivity and type influenced current response. Gerischer (9) discusses
1028
("dz&Os,
0.088
0.100 0.100
Dilution, ml 200.0 200.0 200.0 200.0 200.0 500.0
500.0 500.0 500.0 200.0 200.0 200.0 200.0 200.0
Current, pA 200 650 340 290 530 4,550 4,300 252 870 1,090 640 10
11 5
some aspects of the relationship between the physical parameters of a semiconductor and its response in dilute etching media. ACKNOWLEDGMENT
The authors thank Colt Industries Crucible Materials Research Center and Hach Chemical Company for permission to release data contained in this paper. Hach Chemical has been assigned the pending patent applications and is presently manufacturing a commercial version of the instrument described in this work. One of the authors (J. P. McKaveney) is also indebted to the Garrett Research and Development Division of Occidental Petroleum Corporation for permission to further this investigation and prepare this paper.
RECEIVED for review March 23,1970. Accepted May 27,1970. Presented at 21st Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 5, 1970.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
