Effect of pH on the microhardness of renal calculi

The effects of synthetic urine environments of pH 4, 6, and 9.5 on the microhardness of renal calculi have been investigated. Tests were made, using both Vickers and Knoop indenters, on three compositions of calculi: 100% calcium oxalate monohydrate (whewellite), 100% uric acid, and 98% magnesium ammonium phosphate hexahydrate (struvite) mixed with 2% carbonate apatite. Whewellite calculi hardness was lowered, relative to (dry) values by 45-55% when saturated with a solution of pH 9.5. Exposure to lower pH conditions was not as effective in lowering hardness in this case. Struvite calculi hardness was lowered by 41-52% compared to the dry hardness and uric acid calculi hardness decreased by 25-36%, compared to dry hardnesses. For uric acid stones the reduction in hardness did not depend on pH within the range of pH values investigated. For struvite stones, acid pH conditions appear to give an increased softening, compared to other pH values.