Robert P. Clark

Thermoanalytical investigation of calcium chromate

Thermoanalytical methods have been used to study calcium chromate (CaCrO/sub 4/) samples. Thermogravimetry studies indicate that TG can be an effective quality control tool in screening out samples which have an assay value below 97.0% CaCrO/sub 4/. Thermal decomposition temperatures determined by TG in vacuum and by DTA in air could not be successfully used for screening purposes. More complete analyses using TG in an argon atmosphere gave good results for CaCo/sub 3/ and H/sub 2/O content as well as for total CaCrO/sub 4/. Reliable measurement of Ca(OH)/sub 2/ was not achieved. Mass spectrometry was used to identify gaseous products as a function of temperature, thereby making interpretation of TG curves more certain. Thermogravimetry, differential scanning calorimetry, nuclear magnetic resonance, and mass spectrometry have been used in an effort to explain the unusual weight loss observed between 400 and 600/sup 0/C for many CaCrO/sub 4/ samples. Ca(OH)/sub 2/ decomposition is not the primary cause of this weight loss, as originally suspected, but instead the loss appears to be due to volatilization of H/sub 2/O trapped in the CaCrO/sub 4/ crystal. 17 figures.

Phase diagrams for the binary systems CaCl2-KCl and CaCl2-CaCrO4

Abstract Phase diagrams have been determined using differential thermal analysis for the binary systems CaCl 2 -KCl and CaCl 2 -CaCrO 4 . CaCl 2 -KCl phase diagrams have been previously reported but results were not consistent. No prior studies have been reported for the CaCl 2 -CaCrO 4 system. In the CaCl 2 -KCl binary system two eutectics have been located at 24.0 mole % KCl (m.p. 615°C) and 74.3 mole % KCl (m.p. 594°C). A double salt of composition CaKCl3 melting congruently at 741°C has been found. The CaCl 2 -CaCrO 4 system is a simple eutectic system with the eutectic occurring at 23.4 mole % CaCrO 4 and melting at 660°C.

Thermal decomposition of zinc chromates

Abstract A study of the thermal decomposition in air of zinc chromates from several sources was conducted using thermogravimetry, chemical analysis, emission spectroscopy and X-ray diffraction analysis. Four zinc chromate samples were obtained from commercial suppliers and four others were prepared in this laboratory. The zinc chromates were identified as various materials depending on the method of preparation. Compounds either identified or postulated included 4ZnCrO4·K2O ·3H2O(I), 2ZnO  CrO3  H2(II), 4ZnO  CrO3·3H2O(III). ZnCrO4(IV), 5ZnO  5CrO3·Na2O  3H20(V) and 5ZnO·4CrQ3  Na2O·5H20(VI). Upon heating to 9OO°C, compound I underwent a four step decomposition to form ZnO, ZnCr2O4 and K2CrO4. Compounds II and III each decomposed in two steps to form ZnO and ZnCr2O4. The thermogram for compound IV showed only a single step with the final products also being ZnO and ZnCr2O4. The decomposition of compounds V and VI each involved two steps with the formation of ZnO, ZnCr2O4 and Na2CrO4. Chemical analyses gave assay values for the zinc chromates ranging from 40.2 to 96.9% ZnCrO4.

Heat of reaction determinations in the system CaLiClKClCaCrO4SiO2 using differential scanning calorimetry

Abstract As a result of the observation of heat generation in electrochemical thermal cells of the type −Ca|LiClKCl, SiO2, CaCrO4|Fe+, heats of reaction of the CaLiClKClCaCrO4SiO2 system were investigated using differential scanning calorimetry. Three significant reactions were studied, all involving calcium. Calcium reacts rapidly with LiCl immediately upon melting of the LiClKCl eutectic mixture at 352°C. The exothermic reaction expressed as 2 Ca + 2 LiCl → CaCl2 + CaLi2 has a heat of reaction equal to −1.4 kcal (−17.5 cal/g Ca). Calcium undergoes a rapid and highly exothermic reaction with CaCrO4 which occurs at the melting point of the LiClKClCaCrO4 ternary eutectic mixture (342°). Expressing this reaction as 3 Ca + 2 CaCrO4 + 2 LiCl → 2 Ca2CrO4Cl + CaLi2 the heat determined was −212 kcal (−1770 cal/g Ca). Calcium reacts with SiO2 at much higher temperatures (400–600°C) also in a strongly exothermic manner. The Ca/SiO2 reaction is considerably slower than the other two reactions studied. For the reaction expressed as 4 Ca + SiO2 → 2 CaO + Ca2Si the heat of reaction is −106 kcal (−664 cal/g Ca).

The phase diagram for the binary system K2CrO4CaCrO4

Abstract The phase diagram for the binary system K 2 CrO 4 CaCrO 4 has been determined for CaCrO 4 concentrations up to 60 mole%, using the techniques of differential thermal analysis, X-ray diffraction, and drop calorimetry. Essential features of the phase diagram are: the solid-solid phase transition for pure K 2 CrO 4 at 670°C, β-K 2 CrO 4 ⇄ α-K 2 CrO 4 ; a eutectoid reaction at 14 mole% CaCrO 4 and 548°C, β-K 2 CrO 4 ⇄ α-K 2 CrO 4 + K 2 CrO 4 · CaCrO 4 ; a peritectoid event at 50 mole% CaCrO 4 and 640°C, β-K 2 CrO 4 + CaCrO 4 ⇄ K 2 CrO 4 · CaCrO 4 ; and a eutectic reaction at 51 mole% CaCrO 4 and 678°C, L ⇄ β-K 2 CrO 4 + CaCrO 4 . X-ray diffraction studies lead to the determination of the unit cell dimensions for the K 2 CrO 4 · CaCrO 4 double salt, a C -centered monoclinic form with a 0 = 7.615(6) A, b 0 = 22.797(15) A, c 0 = 9.777(9) A, β = 115.45(5)°.

The phase diagram of the system Li2CrO4K2CrO4

Abstract The Li 2 CrO 4 K 2 CrO 4 phase diagram was determined by using differential thermal analyses with confirmation by high-temperature X-ray diffraction. The outstanding features are the formation of a eutectic composition of ∼22.5 mole% K 2 CrO 4 between Li 2 CrO 4 and the double salt, δ-Li 2 CrO 4 · K 2 CrO 4 , at 390°C; the formation of a congruently melting double salt Li 2 CrO 4 · K 2 CrO 4 with three solid state allotropic forms, γ, δ, and e, the d spacings and colors of which are given; and the formation of a eutectic composition at ∼54 mole% K 2 CrO 4 between αK 2 CrO 4 and the double salt, γ-Li 2 CrO 4 · K 2 CrO 4 , at 525°C.

Phase diagram for the ternary system LiClCaCl2CaCrO4

Abstract The phase diagram for the system LiClCaCl 2 CaCrO 4 has been studied using differential thermal analysis. LiClCaCl 2 CaCrO 4 has been shown by X-ray diffraction to be a stable, diagonal section of the Li, Ca//Cl, CrO 4 reciprocal ternary system. The three binary systems are: LiClCaCl 2 which exhibits a double salt (LiCaCl 3 ), which decomposes without melting at 439°C and a eutectic at 36.3 mole % CaCl 2 (m.p. 487°C); CaCl 2 CaCrO 4 which shows a eutectic at 23.4 mole % CaCrO 4 (m.p. 660°C); and LiClCaCrO 4 with a eutectic at 14.3 mole % CaCrO 4 (m.p. 538°C). In the ternary system, a eutectic exists at 63.2 mole % LiCl32.9% CaCl 2 3.9% CaCrO 4 (m.p. 479°C). In addition, a four-phase equilibrium, involving all solid phases, exists at nearly all compositions at 435°C. Isotherms are shown for the liquidus surface (primary crystallization) and for the secondary crystallization surface. Isothermal and vertical sections through the ternary phase diagram are shown.

Instrument for Continuous Measurement of Internal Resistance of Voltaic Cells During Discharge

An instrument for measuring electrolyte conductance or resistance is described. This instrument is unique in that it has the following combination of features: (1) resistance values in the range 1×10−1 to 2×105 Ω can be measured with an accuracy of better than 5%; (2) resistance can be continuously recorded as a function of time; (3) the internal resistance of voltaic cells can be measured during cell discharge; and (4) no special conductivity cells are required. The present instrument is a modified ac Wheatstone bridge. The value of the unbalance signal is monitored and mathematically converted to resistance. Uses for the instrument are discussed and typical results are given.